Abstract

Plant interactions can be defined as the ways plants act upon the growth, fitness, survival and reproduction of other plants, largely by modifying their environment. These interactions can be positive (facilitation) or negative (competition or exploitation). During plantation establishment or natural forest regeneration after a disturbance, high light levels and, sometimes, increased availability of water and nutrients favour the development of opportunistic, fast-growing herbaceous and woody species which capture resources at the expense of crop trees. As a consequence, the growth and survival of crop trees can be dramatically reduced. Although the effects of this competition are well documented, the physical and physiological mechanisms of competition are not. Moreover, the competition process is never constant in time or space. We present a conceptual competition model based on plant growth forms common in global forests, i.e. graminoids, forbs, small shrubs, large shrubs and mid-storey trees, and main-storey trees. Their competitive attributes and successional dynamics are examined. An overview is presented on the way forest vegetation management (FVM) treatments influence these components and outcomes regarding crop tree performance and diversity conservation. Finally, a synthesis of literature yields FVM guidelines for efficiently optimizing crop tree performance and safeguarding diversity. Future research needs to further sustainable FVM are presented.

Introduction

Competition between trees and natural vegetation in newly established plantations and naturally regenerated forests is the major process influencing tree establishment and growth, and drives succession (Bazzaz, 1990; Thompson and Pitt, 2003). Evidence of the benefits of vegetation control for enhanced tree growth is widespread (Wagner et al., 2005), yet there is still a lack of general principles for forest vegetation management (FVM) regarding the strategies to apply to optimize desired tree survival and growth, while conserving floristic diversity. This review will summarize FVM research to lead us to a formulation of general FVM strategies for sustainable forest regeneration and management.

Plants interact by many different mechanisms, including beneficial or detrimental climatic modifications, resource addition or depletion, allelopathic chemical release, and alterations in herbivore, pathogen or mycorrhizae abundance (Tilman, 1988; Grace and Tilman, 1990; Anderson and Sinclair, 1993). Literature on the effects of vegetation on crop seedling survival and growth shows that competition varies greatly depending on crop tree species (Fredericksen et al., 1993; Wagner et al., 1996; Zutter et al., 1997; Mitchell et al., 1999; Kuessner et al., 2000; Reynolds et al., 2002a, b), seedling size (South et al., 1995, 1999; Rose and Ketchum, 2003), vegetation composition (Cain, 1999; Coll et al., 2003; Miller et al., 2003a), site characteristics (Lautenschlager, 1999; Powers and Reynolds, 1999) and silvicultural treatment (Gemmel et al., 1996; Haywood et al., 1997). Wagner et al. (1999) also pointed out that competitive interaction between seedlings and surrounding vegetation is a dynamic process with strong temporal variations over the first years after seedling establishment, which are determined by the pattern of seedling and vegetation development (Miller et al., 2003a, b).

The large body of global literature examining the competitive effects of surrounding vegetation on tree seedling development covers a wide range of plant and tree species, site conditions and silvicultural contexts. This synthesis is based on the concept that it is possible to reduce this complexity by grouping plant species into a limited number of types, principally determined by growth form as well as taxonomy, life history, morphological and/or physiological plant traits as proposed by Goldberg (1996). The concept of plant functional type has concerned community ecologists for decades (Grime, 1979; Gitay and Noble, 1997; Lavorel and Garnier, 2002) and the relationship between plant traits and competitive ability has received much attention. However, most of the existing studies on these relationships were performed in plant communities other than forests, such as wetlands, grasslands or pastures (Goldberg, 1996; Rösch et al., 1997; Dietz et al., 1998; Wardle et al., 1998), while we will focus this review on vegetative competition in forest plantations and naturally regenerated stands.

The specific objectives of this review are:

  • 1 to identify the main mechanisms of resource competition among plants, and analyse the relationships between various plant traits and competitive ability.

  • 2 to define a limited number of groups of species with similar competitive ability towards tree seedlings based on experimental data and the existing literature. Species will be grouped according to their competitive magnitude and the temporal pattern of the competitive effects on crop tree growth and survival. Each group will be characterized by a series of plant traits known to be related to competitive ability under specific environmental conditions.

  • 3 to assess the effects of the main silvicultural treatments applied in young forest stands (e.g. herbicide applications, mechanical treatments and shelterwood cuts) on the composition and the development of the different species in the groups defined earlier, and the consequences on the competitive effects of the surrounding vegetation.

  • 4 from all this review and knowledge, to examine the practical consequences for sustainable FVM as well as the needs for future research.

General mechanisms of resource competition among plants

Resource competition occurs when plant individuals utilize the same pool of growth-limiting resources. All plants require light, water and inorganic nutrients, and the combined demand of plant individuals for a resource may exceed the immediate supply, leading to a reduction in the survivorship, growth and/or reproduction of the competing individuals (Begon et al., 1990).

The ability of individual plants to compete is related to the efficiency to which they acquire and use resources, which in forestry is highly related to their hierarchy within a forest stand. There are two ways in which individual plants can effectively compete: by rapidly depleting resources to a low level and/or being able to survive and grow at lower resource levels (Goldberg, 1990). The relative importance of the two processes for the outcome of competition between plant individuals strongly depends on the level of resource availability. In high resource environments, species with high relative growth rates are effective competitors because rapid growth enables them to dominate available space and to acquire the most resources (Grime, 2001). In low resource environments, traits that lead to resource retention are far more important for plant performance than traits that lead to a high ability for resource uptake (Aerts, 1999). Because there is a fundamental trade-off between the traits allowing maximization of resource acquisition through rapid growth and the traits allowing resource conservation through reductions in tissue turnover, there is no superior competitor that out competes all other species in all environments, but rather a series of competitive strategies adapted to specific environments (Lambers et al., 1998).

Traits associated with competition for nutrients and water

Numerous studies have shown that in fertile environments, species maximizing nutrient acquisition compete most effectively. The rate of nutrient acquisition by a plant is a function of the rate of uptake per gram of biomass, its total biomass, and the proportion of biomass allocated to the nutrient-absorbing roots. Enzymatic kinetics of nutrient uptake by roots are an important determinant of the uptake of mobile elements such as nitrogen, whereas the uptake of immobile ions such as phosphate primarily depends on morphological traits (Casper and Jackson, 1997; Aerts and Chapin, 2000) and the less understood mycorrhizae associations (Allen and Allen, 1990; Sands and Mulligan, 1990).

Plant performance is determined not only by the amount of nutrients absorbed, but also by the amount of nutrients lost. Nutrient losses may occur through tissue turnover, herbivory, leaf leaching and root exudation. Most nutrients that are lost cannot simply be taken up again by the same plant, but in order to maintain biomass, a plant needs to replace the same quantity of nutrients that it has lost (Berendse and Elberse, 1990). In infertile habitats where nutrient acquisition is a costly process, successful competitors are characterized by numerous features that reduce nutrient losses, such as long tissue lifespan and low nutrient concentration in senesced tissues (Aerts, 1999) due to an efficient retranslocation prior to abscission (Vitousek, 1982; Nambiar and Fife, 1991).

On sites with high water availability, successful competitors are characterized by a high growth rate, associated with a maximization of stomatal conductance and water loss. On water-limited sites, plant performance and competitive ability are related to morphological and physiological traits designed to reduce water loss and increase water use efficiency (low stomatal conductance, low stem hydraulic conductivity, and low transpiring surface area through adapted leaf morphology). Some features such as early leaf abscission during drought conditions characterize tree strategies to reduce moisture loss that also influences photosynthetic rates, while mycorrhiza symbionts may also assist in water uptake on dry sites and require shared photosynthates (Sands and Mulligan, 1990).

In forests, nutrient and water availability exhibits high spatial and temporal heterogeneity, to which plants must adjust (Nobel, 1994; Stark, 1994). Important vertical gradients in both nutrient and water availability exist, with nutrients generally largely concentrated in the upper soil layer, whereas water availability generally increases with depth (Sands and Mulligan, 1990). Significant variations in nutrient concentration occur at scales relevant to individual plants, with the existence of highly localized zones of high nutrient availability, resulting from the activity of plants, animals and microorganisms. Furthermore, nutrient concentration and water content usually exhibit large seasonal fluctuations. In spatially or temporally variable environments, rapid adjustments in morphology and physiology in response to highly localized and temporally transient zones of high nutrient or water availability allow a species to maintain dominance over other competing species, by facilitating its resource acquisition from nutrient-rich or moist patches (Caldwell, 1994; Fitter, 1994).

Traits associated with competition for light

Young trees may grow under contrasted light conditions, from full sunlight for seedlings planted after a clearcut to very low light levels for naturally regenerated seedlings occurring under a dense shelterwood. In high light environments, competitive ability is related to a suite of morphological and physiological traits, allowing the plant to expand rapidly its leaf area above its neighbours through high leaf nitrogen content, high photosynthetic and dark respiration rates, high growth rate, high leaf and root turnover, high allocation to stem growth, and long shoots and petioles (Bazzaz, 1996).

In low light environments, fast-growing species usually have low survival rates and do not compete effectively. Successful competitors are characterized by their ability to survive in shaded understorey and to respond effectively to canopy openings (Messier et al., 1999). Ability to survive in understorey environments is linked to the maintenance of a positive carbon budget, allowed by the optimization of light capture in shade (thin leaves, high efficiency to respond to sunflecks, crown architecture designed to reduce self-shading and to maximize the capture of diffuse light) and by the maintenance of low respiration rates, low leaf photosynthetic capacity, long tissue life-span and low total biomass (Kozlowski et al., 1991).

Spatial and temporal segregation

Competition occurs when plant individuals prospect the same volume of above- or below-ground space at the same time. Spatial and temporal segregation of roots and shoots by neighbouring plants may be a means of reducing competition and hence species that forage different zones, or species that take up resource at different periods, may coexist without competing. This is the reason why in agroforestry systems (Schroth, 1995), and to a much lesser extent in forestry, plants having different spatial and temporal development patterns are chosen as a priority to avoid or at least decrease competition between plants and hence optimize their growth.

Initially roots of competitors and tree seedlings equally occupy the same soil horizons (Zutter et al., 1999; Balandier et al., 2002). Vertical stratification of root systems is an eventual pattern observed in many habitats, with shallow-rooted herbaceous species utilizing shallower resources and deep-rooted woody plants acquiring separate resource from deeper soil horizons (Nambiar and Sands, 1993; Casper and Jackson, 1997). When grown in isolation, both herbaceous and woody species have their roots concentrated in the topsoil. For vertical stratification to happen when grown in competition, a shift in the vertical root distribution of at least one species has to occur, and only species with plastic root systems are able to reduce root competition from neighbouring species by adjusting their root distribution (Zutter et al., 1999).

However, even with a vertical stratification of root systems, the plant that has roots in the deepest horizon can suffer from the plant colonizing the upper horizon as a consequence of an almost complete withdrawal of soil water coming from rainfall by the roots in the upper soil layer and the gradual drying of the deeper soil layers. Hence, the filling of all the soil layers with water in spring is fundamental for both plant types (Balandier, unpublished data).

Similarly, temporal differences in resource use facilitate species coexistence, and species with different seasonal phenologies may reduce the intensity of competition (Kimberley and Richardson, 2004). Differences may range from a complete separation when one species grows after the other has died, to differences in the timing of maximal resource uptake (Schroth, 1999).

Competitive effects of different plant species groups on tree seedlings

The different species commonly found in forest stands at different stages can be separated into five large groups: graminoids (including Poaceae, Cyperaceae and Juncaceae), forbs (herbaceous dicots), small shrubs (including semi-woody plants and ferns), large shrubs and mid-storey trees, and main-storey trees. We hypothesize that plant species within a group can be considered to follow similar temporal patterns of competitiveness against crop trees. The competitive pattern describes the main resource for which the plants compete with the crop trees, the survival and growth response of the crop trees, and the time-course of the competitive effects in the years following stand establishment. For each of the five groups, the competitive pattern and the main plant traits associated with the pattern will be described. These features are summarized in Table 1 (Frochot et al., 2002). This does not mean that a plant within a certain growth-form group is always a predictable competitor, but rather that it could be a competitor in some contexts and must be considered with care as far as crop tree growth and vigour are concerned.

Table 1: 

Main traits conferring competitive ability and potential effects on the crop tree of different groups of plants according to their growth form (modified from Frochot et al., 2002 by kind permission of the author)


Growth form
 

Example of genus
 

Main traits
 

Main possible effects
 

Main references
 
Graminoids (and mainly social perennial grasses) Agropyron Dense root system with high physiological ability to uptake water and nutrients Competition for water, nutrients and soil space Adams et al., 2003 
 Agrostis   Balandier et al., 2005 
 Andropogon   Coll et al., 2003, 2004 
 Brachypodium   Frochot et al., 2002 
 Calamagrostis   Hangs et al., 2002, 2003 
 Deschampsia   Ludovici and Morris, 1997 
 Digitaria High growth rate  Mitchell et al., 1999 
 Holcus   Mohammed et al., 1998 
 Molinia   Morris et al., 1993 
 Panicum   Picon-Cochard et al., 2001 
    Reynolds et al., 1998, 2002a, b 
    Schütz, 2004 
Forbs with a dense cover Chenopodium High aerial growth rate Competition for light, in some situations also for water and nutrients Davies, 1987 
 Conyza   Frochot et al., 2002 
 Epilobium Dense cover  Hangs et al., 2002, 2003 
 Rumex   Hänninen et al., 1999 
 Trifolium   Löf, 2000 
    Sands and Nambiar, 1984 
    Schütz, 2004 
Small shrubs with a dense cover Cytisus High aerial growth rate Competition for light, in some situations also for water and nutrients Carrère et al., 2003 
 Genista   Fotelli et al., 2001, 2002 
 Rubus   Frochot et al., 2002 
 Sesbania Dense cover  Morris et al., 1993 
    Schütz, 2004 
    Watt et al., 2003 
Tall shrubs and mid-storey trees Alnus Can overtop crop tree Competition for light, in some situations also for water and nutrients Balandier et al., 2004 
 Betula   Cain, 1997 
 Carpinus   Frochot et al., 2002 
 Populus   Jobidon, 2000 
 Salix   Reynolds et al., 1998, 2002a, b 
    Rose et al., 1999 
    Schütz, 2004 
Main storey trees Acer Can overtop crop tree and be codominant in final stand Competition for light, water, nutrients, and space Albaugh et al., 2003 
 Carya   Fredericksen et al., 1993 
 Liquidambar   Glover and Zutter, 1993 
 Pinus   Glover and Quicke, 1999 
 Prunus   Mitchell et al., 1999 

 
Quercus
 

 

 

 

Growth form
 

Example of genus
 

Main traits
 

Main possible effects
 

Main references
 
Graminoids (and mainly social perennial grasses) Agropyron Dense root system with high physiological ability to uptake water and nutrients Competition for water, nutrients and soil space Adams et al., 2003 
 Agrostis   Balandier et al., 2005 
 Andropogon   Coll et al., 2003, 2004 
 Brachypodium   Frochot et al., 2002 
 Calamagrostis   Hangs et al., 2002, 2003 
 Deschampsia   Ludovici and Morris, 1997 
 Digitaria High growth rate  Mitchell et al., 1999 
 Holcus   Mohammed et al., 1998 
 Molinia   Morris et al., 1993 
 Panicum   Picon-Cochard et al., 2001 
    Reynolds et al., 1998, 2002a, b 
    Schütz, 2004 
Forbs with a dense cover Chenopodium High aerial growth rate Competition for light, in some situations also for water and nutrients Davies, 1987 
 Conyza   Frochot et al., 2002 
 Epilobium Dense cover  Hangs et al., 2002, 2003 
 Rumex   Hänninen et al., 1999 
 Trifolium   Löf, 2000 
    Sands and Nambiar, 1984 
    Schütz, 2004 
Small shrubs with a dense cover Cytisus High aerial growth rate Competition for light, in some situations also for water and nutrients Carrère et al., 2003 
 Genista   Fotelli et al., 2001, 2002 
 Rubus   Frochot et al., 2002 
 Sesbania Dense cover  Morris et al., 1993 
    Schütz, 2004 
    Watt et al., 2003 
Tall shrubs and mid-storey trees Alnus Can overtop crop tree Competition for light, in some situations also for water and nutrients Balandier et al., 2004 
 Betula   Cain, 1997 
 Carpinus   Frochot et al., 2002 
 Populus   Jobidon, 2000 
 Salix   Reynolds et al., 1998, 2002a, b 
    Rose et al., 1999 
    Schütz, 2004 
Main storey trees Acer Can overtop crop tree and be codominant in final stand Competition for light, water, nutrients, and space Albaugh et al., 2003 
 Carya   Fredericksen et al., 1993 
 Liquidambar   Glover and Zutter, 1993 
 Pinus   Glover and Quicke, 1999 
 Prunus   Mitchell et al., 1999 

 
Quercus
 

 

 

 

Graminoid species vary tremendously in their growth rates, ability to form continuous canopies, below-ground root density, and stature. Sparse, low-growing grasses may have little influence on crop tree growth, while dense fields of tall grass can completely exclude crop tree survival (Kozlowski, 2002). The detrimental effects of perennial grasses such as Andropogon sp., Calamagrostis sp., Dechampsia sp., Imperata sp. and Molinia sp. on tree seedling establishment has been widely reported and they are perhaps the most widely recognized competitors of tree seedlings (Davies, 1987; Otsamo et al., 1997; Mitchell et al., 1999). They generally have a very dense root system, made up of a very high number of thin roots (Ludovici and Morris, 1997; Zutter et al., 1999; Balandier, unpublished data). Their specific root length (i.e. the root length by unit of root weight, which can be considered as the possibility of exploring soil volume for a given investment in carbon) can reach 200–700 m g DW−1 (Zutter et al., 1999; Atkinson, 2000) while that of some tree species such as Fagus sylvatica or Pinus sylvestris is often limited to 10–35 m g−1 (Curt et al., 2005). This feature allows graminoids to colonize efficiently new soil volume and to take up rapidly available resources. Grasses have a shallow root system with maximum root depth being much less than 1 m for most grass species (Kutschera and Lichtenegger, 1982). The root system of the newly established tree seedlings is localized in the same soil horizons as the grass roots. The grass root system constitutes a physical barrier that strongly restricts the growth and resource uptake of seedling roots. The competitive effect of the grasses may be extremely high in the first years after seedling establishment but, if the seedling survives, it progressively develops its root system in deeper soil horizons and escapes grass root competition. Grass root competition usually causes high seedling mortality following planting, but its effects on seedling growth are often restricted to a few years after planting (Miller et al., 2003b; Figure 1).

Figure 1.

Duration and intensity of herbaceous vs woody competition in Pinus taeda plantations in the south-eastern United States. The annual (years 1–11) and periodic (years 11–15 and 15–20) loss in loblolly pine (Pinus taeda) growth (percentage loss relative to total vegetation control per year or period) from woody competition only (hardwoods mainly and shrubs) and herbaceous competition only at three study locations differing by their hardwood density (Miller et al., 2003b, and Miller, unpublished 20-year data). Values for the hardwood basal area at year 20 are provided on the figure. During the first years following pine plantation, herbaceous competition is predominant, particularly when the standing volume of hardwood is low. Conversely, some years after, competition by woody species become higher than that of herbaceous species. Both decrease with time and the competition effect is close to zero at about age 15–20 years.

Figure 1.

Duration and intensity of herbaceous vs woody competition in Pinus taeda plantations in the south-eastern United States. The annual (years 1–11) and periodic (years 11–15 and 15–20) loss in loblolly pine (Pinus taeda) growth (percentage loss relative to total vegetation control per year or period) from woody competition only (hardwoods mainly and shrubs) and herbaceous competition only at three study locations differing by their hardwood density (Miller et al., 2003b, and Miller, unpublished 20-year data). Values for the hardwood basal area at year 20 are provided on the figure. During the first years following pine plantation, herbaceous competition is predominant, particularly when the standing volume of hardwood is low. Conversely, some years after, competition by woody species become higher than that of herbaceous species. Both decrease with time and the competition effect is close to zero at about age 15–20 years.

The dense and shallow root system of grasses strongly reduces the penetration of rainfall into the soil but usually does not withdraw any water from horizons deeper than 1 m (Balandier, unpublished data). On soils where water is available in deeper horizons that can be reached by the tree roots, the competitive effects of the grass are limited. By contrast, on shallow soils or on soil with no deep water-table, rainwater is the major source of water and the presence of a dense grass layer that pre-empts most of this water has a highly negative impact on tree seedlings.

Grasses differ considerably in their ecological strategies: some species which are specific to fertile and well-drained sites, such as Agrostis sp. or Calamagrostis sp., show high growth rate, high nutrient uptake capacity and high physiological and morphological plasticity (Grime, 1994; Collet et al., 1996). These species are able to very rapidly colonize the stand following partial or clearcutting and, because of their ability to deplete rapidly available soil resources, are very competitive towards tree seedlings (Picon-Cochard et al., 2001; Hangs et al., 2003; Coll et al., 2004). Other grass species that are more specific to infertile, dry or waterlogged sites, such as Deschampsia cespitosa or Molinia sp. show low growth rate and slow tissue turnover (Aerts and Chapin, 2000). These species are generally not as efficient in colonizing newly opened stands even if, in certain circumstances, Deschampsia can also rapidly respond to increased light after cutting. However, once they are installed, they are able to prevent the establishment of almost any new species, because of their ability to grow effectively under resource-limited conditions (Coll et al., 2003; Figure 2).

Figure 2.

Root growth ability according to soil water content for two plant groups, graminoids and forbs. Root length relative growth rates (RGR) were computed from weekly measurements of root growth through rhizotrons during the growing season in 2001. Soil water content in the 0–20 cm soil layer was measured at the same time with a TDR probe in a close vicinity of each rhizotron. (Data from central France, adapted from Coll et al., 2003 by kind permission of the author) The lines give the maximum growth for the two groups of plants. Obviously graminoid roots are able to grow better at low soil water content than forb roots.

Figure 2.

Root growth ability according to soil water content for two plant groups, graminoids and forbs. Root length relative growth rates (RGR) were computed from weekly measurements of root growth through rhizotrons during the growing season in 2001. Soil water content in the 0–20 cm soil layer was measured at the same time with a TDR probe in a close vicinity of each rhizotron. (Data from central France, adapted from Coll et al., 2003 by kind permission of the author) The lines give the maximum growth for the two groups of plants. Obviously graminoid roots are able to grow better at low soil water content than forb roots.

As erectophiles (leaf angle ≥65°) with clumped foliage, grasses generally do not intercept a great amount of light in comparison with other species groups (Reynolds et al., 2002a; Sonohat et al., 2002; Figure 3). In the southern US, dense stands of two common early competitive grasses of Pinus taeda L., Panicum dichotomiflorum and Andropogon virginicus, both significantly decreased soil moisture but not light in the first 2 years after planting (Morris et al., 1993). Mitchell et al. (1999) studied Pinus taeda growth responses to gradients in the density of the tree Liquidambar styraciflua and the grass Andropogon virginicus. Liquidambar affected Pinus growth mainly by reducing light availability while Andropogon reduced pine growth mainly by decreasing soil water content. At the end of the first growing season, four Andropogon plants per square metre reduced Pinus stem volume index by 60 per cent, while increasing Andropogon density to 16 plants per square metre only reduced pine stem volume index by an additional 22 per cent. Even low densities of bunch grasses in droughty conditions present severe competition to tree seedlings.

Figure 3.

Effects of seven important boreal weed species on Picea mariana mid-crown photosynthetically active radiation (PAR) seasonal availability. Data are given as mean (± standard error) of three soil types (clay, loam and sand soils) six growing seasons after planting in Ontario, Canada (Reynolds et al., 2002a). Weed species were cultivated as monospecific culture in small plots with different controlled densities. Tree competitors (Salix, Alnus, Betula and to a lesser extent Populus) generally overtopped spruce 6 years after plantation and, hence, decreased more PAR availability for spruce than herbaceous species, resulting in reduced spruce seedling growth.

Figure 3.

Effects of seven important boreal weed species on Picea mariana mid-crown photosynthetically active radiation (PAR) seasonal availability. Data are given as mean (± standard error) of three soil types (clay, loam and sand soils) six growing seasons after planting in Ontario, Canada (Reynolds et al., 2002a). Weed species were cultivated as monospecific culture in small plots with different controlled densities. Tree competitors (Salix, Alnus, Betula and to a lesser extent Populus) generally overtopped spruce 6 years after plantation and, hence, decreased more PAR availability for spruce than herbaceous species, resulting in reduced spruce seedling growth.

Herbaceous forbs can be less competitive than graminoids, at least for soil resources (Coll et al., 2003; Figure 2). Their root growth is slower than grass species, especially during increasing soil drought. This limits the forb's capacity to explore new soil volumes and reach patches with high water or nutrient availability, in comparison with grasses. Therefore forbs generally present a sparse cover in a resource-limited environment (Miller et al., 1995; Lautenschlager, 1999). In fertile sites, they can compete efficiently for nutrients as shown by Hangs et al. (2003) for Epilobium angustifolium and Morris et al. (1993) for forb mixtures. However, light is the resource for which forbs compete the most efficiently (Frochot et al., 2002). The leaf area index (LAI) of species such as Epilobium angustifolium can reach 3.4 (Kuessner et al., 1998; Table 2). Morris et al. (1993) found that Pinus taeda seedlings experienced less competition for soil moisture and nutrients from a dense mixture of forbs compared with Panicum dichotomiflorum and Andropogon virginicus grasses, on one hand, or Sesbania exaltata subshrub, on the other hand. By contrast, forbs decreased light more than these in the second year. All in all, seedling growth was higher with forbs than with grasses or small shrubs.

Table 2: 

Mean LAIs (m2 m−2) of seven important boreal competing species, fourth growing season (1997) in Ontario, Canada (adapted from Kuessner et al., 1998 by kind permission of the author)


 

Clay site
 
  
Loam site
 
  
Competitor
 
Mean
 
SD
 
n
 
Mean
 
SD
 
n
 
Alnus crispa 1.98 ± 0.89 27 0.79 ± 0.38 27 
Betula papyrifera 1.96 ± 0.57 27 0.78 ± 0.52 27 
Populus tremuloides 0.97 ± 0.57 37 0.53 ± 0.28 26 
Salix humilis 1.57 ± 0.60 31 0.72 ± 0.28 27 
Calamagrostis canadensis 1.06 ± 0.71 32 1.04 ± 0.45 27 
Epilobium angustifolium 3.44 ± 0.82 27 1.01 ± 0.41 27 
Rubus idaeus
 
2.81
 
± 1.25
 
27
 
0.58
 
± 0.49
 
27
 

 

Clay site
 
  
Loam site
 
  
Competitor
 
Mean
 
SD
 
n
 
Mean
 
SD
 
n
 
Alnus crispa 1.98 ± 0.89 27 0.79 ± 0.38 27 
Betula papyrifera 1.96 ± 0.57 27 0.78 ± 0.52 27 
Populus tremuloides 0.97 ± 0.57 37 0.53 ± 0.28 26 
Salix humilis 1.57 ± 0.60 31 0.72 ± 0.28 27 
Calamagrostis canadensis 1.06 ± 0.71 32 1.04 ± 0.45 27 
Epilobium angustifolium 3.44 ± 0.82 27 1.01 ± 0.41 27 
Rubus idaeus
 
2.81
 
± 1.25
 
27
 
0.58
 
± 0.49
 
27
 

Small shrubs such as those of the genera Rubus, Genista and Sesbania are often much more problematic. These species usually develop a root system not as dense as the grass root systems, but much deeper (Kutschera and Lichtenegger, 1992). They can take up significant amounts of water and nutrients from soils (Morris et al., 1993; Thevathasan et al., 2000; Fotelli et al., 2001, 2002; Reynolds, 2002b), and thus reducing seedling growth. Their most dramatic effect on tree seedlings is the reduction of available light. They develop a medium-sized stature (up to 2 m in some cases) that may rapidly overtop the seedlings. The LAI of species such as Rubus idaeus may be as high as 2.8 (Kuessner et al., 1998; Table 2), and the relative light intensity under the shrub canopy may be as low as 5 per cent of full light intensity (Kuessner et al., 2000). The LAI for the legume Sesbania exaltata can reach 4.2 in 1 year and presented strong competition for water and nutrients as well as light, comparable to grasses (Morris et al., 1993).

In the early post-harvest years when small shrubs can completely overtop seedlings, they usually induce high seedling mortality. For instance, in natural regeneration of Quercus petraea and Q. robur stands, the presence of a 50-cm-high continuous layer of Rubus fruticosus may totally suppress the new Quercus seedlings (Frochot et al., 2002). In two sites in northern Europe, reduction of shading from Pteridium aquilinum resulted in increased growth of Fagus sylvatica and Pinus sylvestris seedlings while the role of below-ground competition was negligible (Den Ouden, 2000). This can be due to the low amount of roots attached to bracken rhizomes. The detrimental effects of small shrubs and tall ferns (such as Pteridium aquilinum) are limited to a few years after seedling establishment and, if the seedlings are able to survive the strong competitive effects of the deciduous small shrubs, they will eventually reach the top of the shrub canopy and progressively escape their competitive effect while others will not, due to light requirements (Horsley, 1993; Reynolds et al., 2002a).

The competitive effects of tall shrubs and mid-storey trees vary strongly according to their origin (seedlings or sprouts), abundance and phenology (evergreen or deciduous). Severe competition by tall evergreen shrubs is common in the coastal plain forests of the southern US and boreal forests of Canada. In the north-western forests of the US, shrub competition is highly variable from slight to severe (Harrington et al., 1995; Rose et al., 1999). Their main competitive ability is via light interception (Kuessner et al., 1998, 2000; Jobidon, 2000; Reynolds et al., 2002a, b; Figure 3 and Table 2). The competitiveness of these species depends primarily on the dynamics of their development and the rate at which they may overtop the seedlings. Many of the broadleaved species are capable of sprouting vigorously from stumps or established rootstocks producing rapidly growing sprouts that quickly overtop the slower growing seedlings. Other species, such as Betula papyrifera, Populus tremuloides, Ilex glabra, Cytisus scoparius and Buddleja davidii, produce an abundant seed crop that can quickly invade open sites and are capable of rapid juvenile growth. Whatever the origin of the competing broadleaves (sprout or seed), they are capable of dominating the site for many years and can greatly reduce crop tree stocking and growth (Zutter and Miller, 1998; Richardson et al., 1999).

Tree species of main-storey stature are severe competitors of young trees in many forest regions, especially those from sprout origin (Glover and Zutter, 1993; Wagner et al., 2005). Within a region, main-storey tree abundance can vary widely, unlike the more ubiquitous grass and forb competitor components. The competitive effects can occur early in stand establishment following abundant resprouting, and are comparable to herbaceous competitors in magnitude (Miller et al., 2003a; Figure 1). With main-storey tree species, the competitive effects persist the longest of all competitive groups, especially when tree competitors are dominant or co-dominant with the desired trees (Shainsky and Radosevitch, 1991; Glover and Zutter, 1993; Glover and Quicke, 1999, Reynolds et al., 2000a, b). In some world forests, non-sprouting, fast-growing conifers also can form dense early stands to compete with regeneration, which can even be the same species as crop trees, although not genetically improved. Volunteer Pinus taeda commonly compete with planted improved seedlings in the southern US along with other pines species, Pinus echinata and P. virginiana (Miller et al., 2003a).

Main-storey tree competitors have been shown in both boreal and temperate forests to exert severe competition even at low densities. In the boreal forests of Canada studies showed that Rubus idaeus or Calamagrostis canadensis exerted a strong competition, particularly for nutrients in the first years after plantation, but by year 6, available light, soil temperatures and soil moisture were reduced most by arborescent competitors (Alnus crispa, Betula papyrifera) that generally overtopped Picea mariana seedlings (Reynolds et al., 2002a; Figure 3).

These findings for major boreal weed competitors are consistent with those reported by other researchers carrying out similar density studies with major temperate weed competitors in the US. Glover and Quicke (1999), working with two arborescent competitors, Liquidambar styraciflua and Quercus nigra, planted in a gradient of densities in Pinus taeda plantation, reported that both competitors greatly reduced pine growth at the lowest weed densities, but had progressively less additional effect as weed density increased. Using a designed experiment established by Mitchell et al. (1993), Perry et al. (1993) studied Pinus taeda growth responses to gradients in Liquidambar styraciflua and Andropogon virginicus density during the first year of Pinus growth. Liquidambar densities as low as one plant per square metre signficantly reduced Pinus growth as early as May during the first growing season. A density of one sprout per square metre of Liquidambar resulted in a 75 per cent reduction in Pinus stem volume index. Adding an additional one or three sprouts per square metre only reduced Pinus growth by an additional 4 or 16 per cent, respectively. Liquidambar sprouts resulted in more growth loss than established Andropogon. Shainsky and Radosevich (1991) worked with mixtures of Pseudotsuga menziessi and Alnus rubra planted at differing densities and at the same time. During the third year, the lowest density of Alnus (1 plant m−2) inflicted a substantial reduction in Pseudotsuga stem volume, with progressively less additional growth reduction as Alnus density increased.

In some circumstances crop trees can tolerate this competition and become dominant and the harmful effect of competitors can lessen (Miller et al., 2003a; Figure 1). Depending on the objectives of the forester, this situation can be a good compromise, non-crop trees being not too deleterious for crop tree growth, while adding diversity to the stand and preventing competition by shading the plants of the other groups of competitors like grasses, forbs and small shrubs (Wagner et al., 1999). The magnitude of these competitive reductions, especially by mid-storey and overstorey hardwoods, depends on the shade tolerance of the crop tree. Some crop trees such as Fagus sylvatica, Acer pseudoplatanus or Abies alba in Europe are tolerant enough to survive and grow in the understorey of the competing trees for decades and will eventually reach the overstorey and achieve dominance. Other crop trees such as Quercus petraea, Q. robur, Q. rubrum, Prunus avium and P. serotina, which are shade intolerant, are not able to survive in the understorey and are rapidly suppressed by the competing hardwoods (Marquis, 1990).

Like the species from the other groups, the tall shrubs and trees may have both detrimental and beneficial effects on seedling development (e.g. Frochot et al., 2002; Schütz, 2004). However, the beneficial effects of tall shrubs and trees (mainly obtaining a straight stem free of knots by natural pruning) are of primary importance for some crop trees such as Quercus petraea and Fagus sylvatica. The sale price of wood of high quality intended for furniture in Europe can be more than fivefold higher than wood for carpentry for species like Prunus avium, Fraxinus excelsior, Acer pseudoplatanus and Quercus rubra (Ancel, 1995), justifying a crop tree growth reduction for a better stem quality provided by the shelter created by other trees and tall shrubs. Positive (better stem quality) and negative (growth reduction) effects are intrinsically correlated (Balandier et al., 2004), being both the consequence of the light availability reduction caused by the neighbouring species, and it is therefore difficult to disentangle them (Collet et al., 1998). The relative importance of detrimental and beneficial effects depends on site conditions, vegetation composition, and the stage reached by the crop trees (Cain, 1997; Jobidon, 2000; Frochot et al., 2002).

Short-term effects of silvicultural treatments on vegetation dynamics

Determinants of vegetation dynamic

Whatever the vegetation management treatment or the silvicultural operation, vegetation is never at a complete equilibrium. The technical operations can be seen as more or less intense disturbances of the ecosystem that induce secondary plant succession with changes, both in plant cover and species composition. Therefore in order to design management strategies, it is necessary to predict for the various vegetation management options the changes in both plant cover and species composition, which usually leads to changes in the overall competitive effect of the vegetation. Unfortunately, successions and particularly vegetation composition and species dominance are not easy to predict. Succession is determined by initial floristic composition, the arrival of new species from adjacent areas (Haeussler and Bergeron, 2004), and the changes in the relative dominance and interactions among the different species. It is recognized that stochasticity plays a central role in all these processes (Egler, 1954) and it seems unrealistic to predict with accuracy vegetation composition and cover, species by species, resulting from a silvicultural intervention.

However, succession science indicates that it is possible to anticipate the vegetation changes in wide plant groups, as those defined in the previous section. Following a clearcut, pioneering plants with widely dispersed seeds such as grasses and some forbs that often reside in the soil seed bank characterize the first stage of succession, if resprouting woody plants are not present (Miller et al., 1995). In some situations, a dense cover of monopolistic plants, such as a grass or invasive plant, can stop succession (Otsama et al., 1997). The following stages are often characterized by the occurrence of small shrubs and later by pioneering small trees before the establishment of true forest trees (Connell and Slatyer, 1977; Perry, 1995; Rameau, 1999). These stages do not necessarily all occur in all situations. For example, after a clearcut, re-sprouts of shrubs or trees can directly lead to a shrubby stage, and pioneering trees (such as Pinus sylvestris and Betula pendula) can directly establish in meadows characterized by grass and annual forbs (Curt et al., 2003).

Taking into account this general plant succession pattern, we will describe in this section the short-term effects of various silvicultural operations on the vegetation dynamics, and analyse their consequences on the competitiveness of the vegetation against the crop trees.

Herbicide application and species replacement

Herbicides are commonly used to control vegetation competition in some forest regions (Reynolds, 1988; Campbell, 1990, 1991; Reynolds et al., 1993; Richardson, 1993; Wagner, 1993, 1994; Lautenschlager et al., 1998; Willoughby, 1999; Thompson and Pitt, 2003), with the objective being to eliminate or reduce growth of one, several or all herbaceous or woody plants competing with crop trees, in order to improve their establishment and growth. Herbicides are used for afforestation of former agricultural or grass lands as well as for reforestation of forest stands by planting or natural regeneration. At first, the herbicide leads to the suppression of the target species or at least to a strong reduction of their development. But this effect is usually only temporary and vegetation redevelops. The vegetation cover can reach between 0 per cent and 80 per cent at the end of the first year after the treatment, depending on site richness (Dreyfus, 1984; Zutter et al., 1986; Cain, 1999; Miller et al., 2003a) and residual activity of the herbicide. Vegetation composition can be partially or completely different from the initial one (Dreyfus, 1984; Pitt et al., 1988; Reynolds and Obarymskij, 1994; Willoughby and McDonalds, 1999) which may be less detrimental for crop trees (Pitt et al., 1988), but alternatively can also be more competitive than the initial vegetation (Horsley, 1988; Groninger et al., 2004). Several examples of such vegetation replacement are found in the literature (Dreyfus, 1984; Pitt et al., 1988; Dumas et al., 2000; Table 3). In practice, it is often difficult to predict the changes in vegetation following a herbicide treatment in different environments, even for a vegetation characterized by the same dominant species (or a group of the same dominant species) as it depends also on the ecological features of the site, phenological plant stage, application technique, eventual soil tillage before treatment and soil seedbank composition.

Table 3: 

Some examples of the evolution of the dominant vegetation after a herbicide treatment in different site conditions (from Dreyfus, 1984; Pitt et al., 1988; Dumas et al., 2000)


Soil
 

Chemical composition of herbicides
 

Initial dominant species
 

Dominant species after treatment
 
Waterlogged soil Atrazine Agrostis spp. Deschampsia cespitosa 
  Juncus spp. Holcus lanatus 
   Ranunculus repens 
   Molinia caerulea 
Well-drained soil Atrazine Agrostis spp. Rubus fruticosus 
Rendzina Hexazinone Brachypodium pinnatum Herbaceous dicotyledons 
 Dalapon + Dichlobenil Carex flacca Carex flacca 
   Brachypodium pinnatum 
   Clematis vitalba 
Granitic soil Dalapon + Dichlobenil Deschampsia flexuosa Deschampsia flexuosa 
  Agrostis capillaris  
Granitic soil Atrazine Cytisus scoparius Agrostis capillaris 
   Cytisus scoparius 
Acid and waterlogged soil Asulame Pteridium aquilinum Molinia caerulea 
   Calluna vulgaris 
Acid soil Asulame Pteridium aquilinum Holcus lanatus 
   Cytisus scoparius 
Clay
 
Hexazinone
 
Rubus ideaus
 
Epilobium angustifolium
 

Soil
 

Chemical composition of herbicides
 

Initial dominant species
 

Dominant species after treatment
 
Waterlogged soil Atrazine Agrostis spp. Deschampsia cespitosa 
  Juncus spp. Holcus lanatus 
   Ranunculus repens 
   Molinia caerulea 
Well-drained soil Atrazine Agrostis spp. Rubus fruticosus 
Rendzina Hexazinone Brachypodium pinnatum Herbaceous dicotyledons 
 Dalapon + Dichlobenil Carex flacca Carex flacca 
   Brachypodium pinnatum 
   Clematis vitalba 
Granitic soil Dalapon + Dichlobenil Deschampsia flexuosa Deschampsia flexuosa 
  Agrostis capillaris  
Granitic soil Atrazine Cytisus scoparius Agrostis capillaris 
   Cytisus scoparius 
Acid and waterlogged soil Asulame Pteridium aquilinum Molinia caerulea 
   Calluna vulgaris 
Acid soil Asulame Pteridium aquilinum Holcus lanatus 
   Cytisus scoparius 
Clay
 
Hexazinone
 
Rubus ideaus
 
Epilobium angustifolium
 

In some particular situations, a herbicide application may eventually lead to a more diversified flora by suppressing the dominant species (e.g. shrubs and trees) and freeing space and resources for the other species (e.g. diverse forbs) (Miller et al., 2003a). However, in most cases, a new colonizing species becomes dominant in the following years leading again to heavy competitive effects on crop trees (Dumas et al., 2000). Therefore in some cases, it can be necessary to continue herbicide applications for several years and often until canopy closure occurs and crop trees start to dominate and suppress weed growth (Willoughby and McDonald, 1999). However, it has been well substantiated by multiple studies that vegetation control in the initial 1–2 years after establishment yields the most crop tree response, a ‘critical period’ for control (Bacon and Zedaker, 1987; Newton and Preest, 1988; Lauer et al., 1993; Glover and Lauer, 1996; Wagner et al., 1999).

It is important to note that herbicide application has more impact on relative species dominance than species composition or species diversity (i.e. the species composition after a treatment is basically the same as in an untreated control, but it is not the same species that are dominant; Dreyfus, 1984). Such results were also reported by Boyd et al. (1995) and Miller et al. (1999) in central Georgia (USA) who observed in Pinus taeda plantations that 10 and 11 years after different single herbicide applications, species diversity was similar among the plots while relative species dominance differed strongly. Similarly, Biring et al. (2003) reported that 10 years after repeated manual cutting and a single application of glyphosate in a mixed-shrub community in British Columbia, total vegetation cover and species diversity remained similar, but community structure was altered with a reduction in the percentage cover of Populus balsamifera, an increase in the percentage cover of Picea engelmannii and differences in the percentage cover of other conifers, shrubs and herbs. It has been reported that multiple herbicide applications for woody plant control over several years during establishment has the potential to decrease species richness of Pinus plantation communities for up to 15 years (Miller et al., 2003a). Further, Schabengerger and Zedaker (1999) in Virginia (USA) found that canopy diversity was inversely related to pine yield 12–14 years after a range of competition control treatments was tested. Herbaceous control had less longer-term influence than woody plant control in both investigations, while woody control treatments often initially increase richness of herbaceous species but can permanently decrease long-term woody plant diversity (Miller et al., 2003a). It is also noteworthy that low rates of herbicide can suppress certain species, hence changing dominance, even if species are not actually killed (Boyd et al., 1995; Miller et al., 1999).

Mechanical treatment

Mechanical treatments can be effective in controlling woody competitors and particularly tall shrubs and understorey trees (Gjerstad and Barber, 1987; Walstad et al., 1987; Bovey, 2001). When the goal is only to reduce the competitive pressure of hardwoods, mechanical treatments can completely suppress hardwood stems. When the goal is to reduce the competitive pressure of hardwoods while maintaining their beneficial effects on seedling morphological quality, the treatment will release only the top of the crop trees from light competition (Collet et al., 1998). In most cases, the treatment only temporarily (Fiddler and McDonald, 1991; McDonald and Fiddler, 1993) removes the hardwood and shrub above-ground parts unless root dislodging implements are used. Moderate mechanical methods (e.g. cutting) yield moderate increases in growth of crop trees but less drastic changes to richness and structure than more intensive methods such as burning or herbicides (Locasio et al., 1991; Lautenschlager et al., 1998; Haeussler et al., 1999). McDonald and Fiddler (1993) have stated, ‘In most instances, forests cannot be managed economically without herbicides if the goal is to grow seedlings at the potential of the site and the plant community includes sprouting hardwoods and shrubs or rhizomatous forbs and ferns’.

Other mechanical treatments such as mowing may induce significant changes in species composition. Some herbaceous species, being capable of vegetative spread, are tolerant of mowing and will be favoured by mowing in comparison with other species. These species (often grasses) are very competitive for below-ground resources and have a very harmful effect on trees (Davies, 1987). Willoughby and McDonald (1999) reported such an effect in a 5-year experiment of afforestation of previous agricultural land in England, with the increasing development of Trifolium repens and Lolium perenne in the mown treatment in comparison with the unmanaged control treatment dominated by tall perennial species such as Senecio jacobea.

Soil cultivation has various effects on vegetation depending on the site characteristics and the tool used. For example, in young stands characterized by a high light availability at the ground level, destroying the vegetation by a disc plough is often reported to have no effect (on Populus sp. stands; Berthelot et al., 2002) or to increase species richness (on Populus sp. stands; Laquerbe, 1999). Plant communities are generally different from the untreated control with a higher proportion of annual and biennials species (Berthelot et al., 2002). After annual cultivation treatments end, species richness generally decreases and perennial species progressively establish.

Finally many different types of organic or inorganic mulches can be used to prevent vegetation development around the crop tree, the main aim often being to improve soil water or nutrient availability (Davies, 1987; McDonald and Helgerson, 1990; Fiddler and McDonald, 1991; McDonald and Fiddler, 1993; McDonald et al., 1994; Van Lerberghes, 2004). The quickness of vegetation come back and its specific composition depends on the duration of the material used as mulch, the soil seed bank and the shade resistance of the plant species covered by the mulch.

Alternative silvicultural systems, thinning intensity and understorey vegetation composition

Partial cutting systems (e.g. shelterwoods, group selection and strip cutting) are practices increasingly used to regenerate forest stands in some forest regions, in association with planting or natural regeneration (Martin and Hornbeck, 1990; Ruark, 1990; Hannah, 1991; Tubbs and Lamson, 1991; McDonald and Fiddler, 1993; Lieffers and Stadt, 1994; Riegel et al., 1995; McDonald and Reynolds, 1999; Holgen and Hanell, 2000; Wetzel and Burgess, 2001). Clearcutting has a greater potential for soil disturbance than other uneven-aged silvicultural systems, inclusive of shelterwood or group selection cutting (Turcotte et al., 1991; Reynolds, 1992). Disturbance exposes bare mineral soil, producing a fertile seedbed for potential competitors (Roberts and Dong, 1991, 1993). Coupled with maximal light availability following clearcutting, optimal conditions prevail for regeneration of potential plant competitors. Regeneration systems other than even-aged clearcutting offer less reliance on herbicides for controlling these competitors, since they create less favourable light and soil disturbance conditions that enable the early establishment of severe plant competitors. In many cases after a clearcut, vegetation is composed of very competitive species (e.g. Populus tremuloides, Calamagrostis canadensis, Epilobium angustifolium, etc. in Alberta (Lieffers and Stadt, 1994); Carex geyeri and Calamagrostis rubescens in the Pacific Northwest (Riegel et al., 1995)), making seedling establishment difficult or even impossible without vegetation control. The general idea supporting the practice of shelterwood, group selection, and strip cutting is that it is possible to avoid very competitive light-demanding species such as grasses, brambles, ferns and other invaders by limiting light availability to a threshold that still allows tree regeneration, even if seedling growth is not at its maximum (Martin and Hornbeck, 1990; Hannah, 1991; Reynolds, 1992; McDonald and Fiddler, 1993; McDonald and Reynolds, 1999; Schütz, 2004).

An example of this is the study conducted in Larix sp. stands in France and Belgium (Balandier and Pauwels, 2002; Figure 4). Two or three years after different thinning intensities, species diversity changed with relative light intensity (RLI) following a bell-shape curve from bare soil at very low RLI (<2 per cent) to a maximum for higher light availability (10–20 per cent) and to lower values for highest RLI (>20 per cent). The RLI value allowing maximum species diversity differs with stand type, site conditions and initial vegetation composition. The decrease in species diversity above the optimal RLI value can be explained by the rapid colonization of the total stand by a limited number of opportunistic and monopolistic species such as Pteridium aquilinum, Rubus fruticosus, Calluna vulgaris, and several graminoids. Many studies have reported such a colonization of forest stand by competitive species with high light availability (Alaback and Herman, 1988; McDonald and Reynolds, 1999) and often this fact is explained by the competition intensification among species with the increase in resources (e.g. Grime, 1994; Thomas et al., 1999). Of course the vegetation found in the highest light percentages (close to clearcut conditions) can lead to a complete standstill or at least to a severe slowdown of the succession (Frochot et al., 2002). It is noteworthy that in addition to the appearance of these competing species, their biomass and cover are promoted by the high light availability (Balandier and Pauwels, 2002; Figure 4). It is clear from this study that RLI values greater than 10–20 per cent should be avoided in order to promote seedling establishment and growth.

Figure 4.

Evolution of the understorey plant species diversity and total biomass according to the light transmitted by the overstorey in larch (Larix sp.) stands in France and Belgium. Light availability at the ground level is given in relative value (transmittance), i.e. the value measured under the overstorey divided by the value measured above the overstorey; the relative scale of species diversity corresponds to the number of species found at a given transmittance value divided by the total number of species found on a site whatever the transmittance value is; and the relative scale of plant biomass corresponds to the biomass recorded for a given transmittance value divided by the maximum biomass recorded whatever the transmittance value is (adapted from Balandier and Pauwels, 2002). Plant species diversity peaks at about 10–20 per cent transmittance; light limits species number under this value and promotes the plot invasion by a small number of light-demanding species above it. Plant biomass is at maximum with the occurrence of the small number of light-demanding species and does not coincide with the maximum of biodiversity. Of course, transmittance values above 20–30 per cent are not favourable to tree regeneration because they promote the occurrence of a high density of very competitive species.

Figure 4.

Evolution of the understorey plant species diversity and total biomass according to the light transmitted by the overstorey in larch (Larix sp.) stands in France and Belgium. Light availability at the ground level is given in relative value (transmittance), i.e. the value measured under the overstorey divided by the value measured above the overstorey; the relative scale of species diversity corresponds to the number of species found at a given transmittance value divided by the total number of species found on a site whatever the transmittance value is; and the relative scale of plant biomass corresponds to the biomass recorded for a given transmittance value divided by the maximum biomass recorded whatever the transmittance value is (adapted from Balandier and Pauwels, 2002). Plant species diversity peaks at about 10–20 per cent transmittance; light limits species number under this value and promotes the plot invasion by a small number of light-demanding species above it. Plant biomass is at maximum with the occurrence of the small number of light-demanding species and does not coincide with the maximum of biodiversity. Of course, transmittance values above 20–30 per cent are not favourable to tree regeneration because they promote the occurrence of a high density of very competitive species.

Another example of such a bell-shaped curve of species diversity is given by Jobidon et al. (2004). Stands of Picea mariana were established in the Québec region with different intensities of mechanical vegetation control and pre-commercial thinning. As the proportion of Picea mariana increased in the stand to the detriment of non-crop trees, total species richness and plant diversity first increased to a maximum and then decreased. By contrast to the previously cited study on larch, these variations of species richness and diversity are more due to the composition of the upper stratum and particularly non-crop tree diversity rather than to the composition of the intermediate and lower strata.

In another study, Klinka et al. (1996) in West Vancouver Island forests (Canada) observed an increase in the shrub layer (mainly Gaultheria shallon and Rubus spectabilis) with increasing light in the 6 years following canopy opening, while the herb and moss layers were not affected, probably because of the shade created by the shrub layer. Similar results were found in eastern Canada by Wetzel and Burgess (2001) with an increase in shrub diversity 2 years after a thinning but no change in the herb layer.

Light is not the single resource that affects understorey vegetation composition and cover and drives competition between vegetation and crop tree. Below-ground resource availability may lead to different responses of the understorey vegetation to light availability and to contrasted effects on crop trees (Lindh et al., 2003). This was clearly demonstrated by Riegel et al. (1995) in north-eastern Oregon where a reduction in tree canopy cover (Pinus ponderosa) was conducted in a split-plot study with or without root trenching to remove below-ground competition. Some species responded positively to light, others positively to an increase in soil resources, and some to both factors. In a literature review, Ricard et al. (2003) observed that contradictory results were reported on the subject. They noticed that, since the relative importance of competition for below- or above-ground resources depends on resource availability, the competitive effects of understorey vegetation against the seedlings and its changes with RLI vary with the site water and nutrient availability. Therefore optimal shelterwood density to control the vegetation and improve crop trees is site dependent.

Consequences regarding FVM

General considerations

As shown in the previous sections, the size, habit, density and growth rate of competing plants influence crop trees in different ways. Crop trees rarely compete with a single plant species, and the combined effects of competitors are difficult to analyse. Furthermore, the possible competitors for the crop trees on a given site are numerous, and the effective species composition of the competing vegetation is very difficult to predict because of the intrinsically stochastic nature of species establishment. As stated in the previous sections, there are, however, general patterns in the competitive effects of the various species summarized in Table 1 (Frochot et al., 2002) by growth forms into five groups. Generally, graminoids, and to a lesser extent forbs with a dense cover, are more detrimental in the first years after stand establishment and mainly compete for below-ground resources, while shrubs and trees compete for light later in the rotation. A conceptual model, resulting from and summarizing, to a certain extent, the knowledge gathered in the previous sections, is proposed in Figure 5a and b.

Figure 5.

Proposition of conceptual models of (a) the influence of weed density independently of time and (b) the influence of time after stand establishment independently of weed density, on crop yield for different groups of competing vegetation. Crop yield is expressed as a percentage of maximum growth (supposed to be in the absence of competitive vegetation). (a) Small densities of forbs and graminoids do not affect yield of crop trees but depreciate it strongly at high density while a few individuals of mid-storey trees, large shrubs and main-storey trees lead to a significant loss of crop tree yield. (b) Graminoids and forbs compete early in the stand rotation but the depressive effect stops as soon as they are overtopped by crop trees, while the depressive effect of main-storey non-crop trees can last until the end of the rotation if they stay co-dominant with crop trees.

Figure 5.

Proposition of conceptual models of (a) the influence of weed density independently of time and (b) the influence of time after stand establishment independently of weed density, on crop yield for different groups of competing vegetation. Crop yield is expressed as a percentage of maximum growth (supposed to be in the absence of competitive vegetation). (a) Small densities of forbs and graminoids do not affect yield of crop trees but depreciate it strongly at high density while a few individuals of mid-storey trees, large shrubs and main-storey trees lead to a significant loss of crop tree yield. (b) Graminoids and forbs compete early in the stand rotation but the depressive effect stops as soon as they are overtopped by crop trees, while the depressive effect of main-storey non-crop trees can last until the end of the rotation if they stay co-dominant with crop trees.

At later stages in the rotation, a considerable number, cover, or density of herbaceous plants can exist in forest stands without appreciable (detectable) declines in crop tree yield. This is especially true if a weed-free zone, immediately adjacent to crop trees, is established early in the rotation (Carter et al., 1984; Dougherty and Lowery, 1991; Richardson et al., 1996). In many stands, herbaceous plants exist in the understorey throughout the rotation and their control leads to little or no yield gains. However, beyond a certain point of weed density, crop tree growth decreases rapidly and dramatically (Figure 5a). Conversely, even one weed tree in a forest canopy decreases the yield of crop species and an increasing number of weed trees in the canopy causes an exponential decline in crop yield (Figure 5a). Understorey shrubs and sub-canopy trees have intermediate effects. The reduction of seedling growth, even with the low densities of woody competitors, has been widely reported (e.g. Shainsky and Radosevich, 1991; Mitchell et al., 1993; Perry et al., 1993; Zutter et al., 1998; Glover and Quicke, 1999; Bell et al., 2000; Reynolds et al., 2002a, b).

In many plantation settings, crop tree growth and yield are modelled as a function of stand age, the number of crop trees per unit area, and a site index curve. Differences in competition by different growth forms lead managers to use different models to assess the effects of weed control (Richardson et al., 2005). As herbaceous weeds often delay crop tree establishment and growth in the first years after plantation establishment, but do not affect subsequent growth (i.e. treated and untreated stands take parallel paths with no further increase in time gain after the initial period), crop trees are often modelled as if they were older than their actual age (South et al., 2005). Functionally, the herbaceous weeds retard growth until overtopped, but once that is achieved, the stand will grow at a rate determined largely by the number of trees and the site quality (site index). However, there are some cases where colonizing species such as perennial grasses can totally prevent crop tree establishment and growth (inhibition model of Connell and Slatyer, 1977; Kozlowski, 2002). Conversely, tree competition is modelled as a change in site quality; as if each weed tree present was reducing light or water and nutrient availability to the crop trees to a level that would be characteristic of poorer sites. The implication for shrub and sub-canopy tree impacts is that they could have an intermediate effect of retarding both stand development and also reducing ultimate long-term yield. In fact, a considerable body of evidence from yield studies in US southern yellow pine plantations points to this very effect. D’Anieri et al. (1986), Oppenheimer et al. (1989), Shiver and Daniel (1994), Fortson et al. (1996), Zedaker et al. (2002) and Albaugh et al. (2003) have demonstrated that the control of dense understorey shrubs and trees can lead to increased growth and yield in stands ranging from 15 to over 60 years old.

Principles of sustainable FVM

Competition control treatments can enhance crop trees while maintaining a diverse flora when the most severe competitors are specifically targeted and non-competitive components are permitted to remain as cohorts (Hartley, 2002).

Grasses and forbs are often early severe competitors of desired tree seedlings planted in open stands, due to their initial abundance. Grasses in general are more prevalent and can be competitive even on poorer sites with lower soil moisture and drier forest regions. Early control in the first and sometimes second years of both herbaceous components predictably yields greater crop seedling growth according to species, and sometimes improves survival on droughty sites (Wagner et al., 1999). Suppression of grasses and forbs invariably will increase growth of cohort trees and increase their competitiveness along with crop trees but does not predictably enhance shrub growth (Miller et al., 2003a). The initial presence of shrub and tree sprouts and crop tree seedlings suppresses grass and forb abundance and decreases competition for moisture. Therefore a planned balance of components (e.g. as sought by the shelterwood practice or spot control around seedling) can optimize crop tree growth, reduce suppression expenses, and provide habitat attributes.

Competition components of all types immediately surrounding the seedlings, have been found the most competitive for water, nutrients and light, especially for dense grasses in the first years. Control strategies that target competitors within 1–2 m of newly established seedlings are most efficient with minimizing herbicide use while conserving diverse plants outside these weed-free zones (Richardson et al., 1996; Hartley, 2002). Graminoids and forbs contribute greatly to diversity in flora as well as providing habitat and food plants for vertebrates and invertebrates, and hence warrant conservation where possible. In a similar manner, small shrubs, while being less competitive than grasses and comparable to forbs with respect to density, often represent valuable food and habitat plants for forest fauna. McDonald (1986) has reported that ‘Grasses are not desirable in conifer plantations less than five years old, but after five years, they can aid conifer seedling growth by physically and chemically excluding more competitive vegetation. In plantations over 5 years old on good sites with deep soils, grasses can be beneficial by excluding deeper rooted shrubs.’

Abundant colony-forming shrub communities occur most commonly in boreal forests, montane forests, and in the sub-tropical coastal plain forests. These forest types require aggressive shrub control treatments to assure crop tree establishment and survival, either around seedlings or area-wide treatments. In both situations, shrub re-establishment from established rootstocks presents the most severe competition and can be the result of prior mechanical treatments. Other shrubs that become established from seed present less immediate competition but will influence seedling regeneration for a sustained period relative to tree and shrub growth rates. Those fruit- and heavy-seed-bearing shrubs are a principal wildlife food source and warrant consideration of partial control with moderate growth gains for habitat features.

Large shrubs and mid-storey trees have been less researched compared with the other growth-form components, while their values as fruit producers for fauna are widely recognized. Dense resprouting components exert competition immediately surrounding seedlings but their competitive influence decreases as plantations or stands develop with dominating crop canopies. Suppression treatments that facilitate later stand participation warrant consideration and further research and development.

Main-storey trees present the most formidable long-term component capable of completely superceding crop tree growth depending upon the relative dynamic canopy position. Low densities are capable of exacting significant growth losses to crop seedlings, depending upon the seedling's shade tolerance. Control treatments should be adjusted relative to anticipated or actual early competitor densities. Conversely, main-storey trees grown with hardwood crop seedlings have been shown to be valuable for improving form and knot reduction in the wood as well as having value for fruit and seed production. In shelterwood regeneration, main-storey trees must be managed to create conditions to favour regeneration over competitive understorey plants.

These principles are generalizations that will need modification relative to various forest regions, sub-regions, topography and site, while providing summaries of numerous research endeavours to guide management and future research.

Future research needs

It is noteworthy that now we have a large database showing that early control of the herbaceous or woody vegetation has long-term effects on crop trees, most noted are increases in wood yield (Wagner et al., 2005). However, whilst there are instances where the necessity to control the vegetation is obvious, for instance in the situation of a dense layer of perennial grasses preventing any tree establishment, this requirement is not universal. Moreover Haeussler and Bergeron (2004) emphasized that small changes in the variability of plant communities today may have dramatic implications for forest composition, biodiversity and ecosystem functioning later. Miller and Miller (2004) amplify these concerns for forest regions that see intensification of FVM during this period of increasing forest fragmentation. As a consequence, designing vegetation management treatments or silvicultural operations for seedling establishment or crop tree growth requires the consideration of the evolution of the vegetation over several years, and across landscapes and regions, before it is possible to conclude on its potential interest or adverse effect. In addition, with the observation of early growth responses of planted trees to vegetation control, patterns and processes of plant succession on a long-term basis should be taken into consideration as this can have late repercussions such as a delay in canopy closure (Groninger et al., 2004). Multiple-scale investigations are needed to examine the repercussions to ecological services and biodiversity exacted by various FVM strategies and especially comparing those anticipated by economic and policy developments.

It is also clear that we need an international common framework and requirements in studies aimed at characterizing the traits that confer competitive ability to a plant in the forest environment. Research using common protocols is more apt for providing the insights required to adjust our groups of competitive plants in different environments in order to adjust treatments and determine when treatments are justified. However, we also emphasized the necessity to include longer-term vegetation dynamics in studies dealing with influences of forest vegetation on crop trees because the intensity and orientation of competition change with floral composition in time and in space. The complexity of the flora in any region warrants more detailed taxonomic guides and keys such that current and future researchers are capable of accurately documenting the diversity.

Finally the role of biotic agents such as animals (and especially grazing animals, see Fiddler and McDonald, 1991; McDonald and Fiddler, 1993), insects, microorganisms and other forest inhabitants have not been examined in any detail in this paper. Following Connell and Slatyer (1977), we recognize that they play a fundamental role in interactions between plants and trees. It is a challenge to include them in future studies. It challenges us to understand the full array of interactions and life webs that connect all components within forest stands in order to develop realistic, efficient and sustainable forest practices crucially needed to supply goods, services and habitat for the swelling world population.

The authors thank Ian Willoughby and Robert Jobidon very much for their valuable comments on the manuscript.

References

Adams, P.R., Beadle, C.L., Mendham, N.J. and Smethurst, P.J.
2003
The impact of timing and duration of grass control on growth of a young Eucalyptus globules Labill. plantation.
New Forests
 
26
,
147
–165.
Aerts, R.
1999
Interspecific competition in natural plant communities: mechanisms, trade-offs and plant–soil feedbacks.
J. Exp. Bot.
 
50
,
29
–37.
Aerts, R. and Chapin III, F.S.
2000
The mineral nutrition of wild plants revisited: a re-evaluation of processes and patterns. In Advances in Ecological Research, Vol. 30. A. Fitter and D. Raffaeli (eds). Academic Press, San Diego, CA, pp. 1–67.
Alaback, P.B. and Herman, F.R.
1988
Long-term response of understorey vegetation to stand density in Picea-Tsuga forests.
Can. J. For. Res.
 
18
,
1522
–1530.
Albaugh, T.J., Allen, H.L., Zutter, B.R. and Quicke, H.E.
2003
Vegetation control and fertilization in midrotation Pinus taeda stands in the southeastern United States.
Ann. For. Sci.
 
60
,
619
–624.
Allen, E.B. and Allen, M.F.
1990
The mediation of competition by mycorrhizae in successional and patchy environments. In Perspectives on Plant Competition. J.B. Grace and D. Tilman (eds). Academic Press, San Diego, CA, pp. 367–389.
Ancel, P.
1995
Approche économique. In Feuillus précieux. Conduite des plantations en ambiance forestière. G. Armand (compiler), Institut pour le Developement Forestier, Paris, France, pp. 100–105.
Anderson, L.S. and Sinclair, F.L.
1993
Ecological interactions in agroforestry systems.
Agroforestry Abstracts
 
6
(2),
57
–90.
Atkinson, D.
2000
Root characteristics: why and what to measure. In Root Methods. A.L. Smit, A.G. Bengough, M. Van Noordwijk, S. Pellerin and S.C. Van De Geijn (eds). Springer-Verlag, Berlin, pp. 1–32.
Bacon, C.G. and Zedaker, S.M.
1987
Third-year response of loblolly pine growth to eight levels of competition control.
South. J. Appl. For.
 
11
,
91
–95.
Balandier, P. and Pauwels, D.
2002
La lumière, outil sylvicole pour favoriser la diversité végétale ou la gestion cynégétique des peuplements de mélèze (Larix sp.).
Forêt Wallone
 
61
,
9
–13.
Balandier, P., Rapey, H., Ruchaud, F. and De Montard, F.X.
2002
Agroforesterie en Europe de l’Ouest: pratiques et expérimentations sylvopastorales des montagnes de la zone tempérée.
Cahiers Agricultures
 
11
,
103
–113.
Balandier, P., Guitton, J.L. and Prevosto, B.
2004
Forest restoration in the French Massif Central mountains. In Restoration of Boreal and Temperate Forests. J. Stanturf and P. Madsen (eds). CRC Lewis Press, Boca Raton, Florida, pp. 355–369.
Bazzaz, F.A.
1990
Plant–plant interactions in successional environments. In Perspectives on Plant Competition. J.B. Grace and D. Tilman (eds). Academic Press, San Diego, CA, pp. 239–263.
Bazzaz, F.A.
1996
Plants in Changing Environments. Linking Physiological, Population, and Community Ecology. Cambridge University Press, Cambridge, UK.
Bell, F.W., Ter-Mikaelian, M.T. and Wagner, R.G.
2000
Relative competitiveness of nine early-successional boreal forest species associated with planted jack pine and black spruce seedlings.
Can. J. For. Res.
 
30
,
790
–800.
Begon, M., Harper, J.L. and Townsend, C.R.
1990
Ecology: Individuals, Populations and Communities, 2nd edn, Blackwell Scientific Publications, Cambridge, MA.
Berendse, F. and Elberse, W.T.
1990
Competition and nutrient availability in heathland and grassland ecosystems. In Perspectives on plant competition. J.B. Grace and D. Tilman (eds). Academic Press, San Diego, CA, pp. 93–116.
Berthelot, A., Landeau, S. and Roguier, S.
2002
Tending method and vegetation diversity under poplar. In Popular Summaries from the Fourth International Conference on Forest Vegetation Management. H. Frochot, C. Collet and P. Balandier (eds). 17–21 June 2002, Nancy, France, INRA – Cemagref, pp. 133–135.
Biring, B.S., Comeau, P.G. and Fielder, P.
2003
Long-term effect of vegetation control treatments for release of Engelmann spruce from a mixed-shrub community in Southern British Columbia.
Ann. For. Sci.
 
60
,
681
–690.
Bovey, R.W.
2001
Woody Plants and Woody Plant Management: Ecology, Safety, and Environmental Impact. Marcel Dekker Press, New York.
Boyd, R.S., Freeman, J.D., Miller, J.H. and Edwards, M.B.
1995
Forest herbicide influences on floristic diversity seven years after broadcast pine release treatments in central Georgia, USA.
New Forests
 
10
,
17
–37.
Cain, M.D.
1997
Woody and herbaceous competition effects on the growth of naturally regenerated loblolly and shortleaf pines through 11 years.
New Forests
 
14
,
107
–125.
Cain, M.D.
1999
Woody and herbaceous competition effects on stand dynamics and growth of 13-year-old natural, precommercially thinned loblolly and shortleaf pines.
Can. J. For. Res.
 
29
,
947
–959.
Caldwell, M.M.
1994
Exploiting nutrients in fertile soil microsites. In Exploitation of Environmental Heterogeneity by Plants. M.M. Caldwell and R.W. Pearcy (eds). Academic Press, San Diego, CA, pp. 325–348.
Campbell, R.A.
1990
Herbicide use for forest management in Canada: where we are and where we are going.
For. Chron.
 
66
,
355
–360.
Campbell, R.A.
1991
Silvicultural herbicides in Canada: registration status and research trends.
For. Chron.
 
67
,
520
–527.
Carrère, P., Loiseau, P., Orth, D., Picon-Cochard, C., Prévosto, B. and Soussana, J.F.
2003
Dynamique d’invasion par des petits ligneux de prairies sous exploitées de moyenne montagne du Massif Central: le cas du genêt à balais (Cytisus scoparius). Cemagref edn, Antony, France.
Carter, G.A., Miller, J.H., Davis, D.E. and Patterson, R.M.
1984
. Effect of vegetative competition on the moisture and nutrient status of loblolly pine.
Can. J. For. Res.
 
14
,
1
–9.
Casper, B.B. and Jackson, R.B.
1997
Plant competition underground.
Annu. Rev. Ecol. Syst.
 
28
,
545
–570.
Coll, Ll., Balandier, P., Picon-Cochard, C., Prevosto, B. and Curt, T.
2003
Competition for water between beech seedlings and surrounding vegetation in different light and vegetation composition conditions.
Ann. For. Sci.
 
60
,
593
–600.
Coll, Ll., Balandier, P. and Picon-Cochard, C.
2004
Morphological and physiological responses of beech seedlings to grass-induced belowground competition.
Tree Physiol.
 
24
,
45
–54.
Collet, C., Frochot, H. and Guehl, J.M.
1996
Growth dynamics and water uptake of two forest grasses differing in their growth strategy and potentially competing with forest seedlings.
Can. J. Bot.
 
74
,
1555
–1561.
Collet, C., Ningre, F. and Frochot, H.
1998
Modifying the microclimate around young oaks through vegetation manipulation: effects on seedling growth and branching.
For. Ecol. Manage.
 
110
,
249
–262.
Connell, J.H. and Slatyer, O.
1977
Mechanisms of succession in natural communities and their role in community stability and organization.
Am. Nat.
 
111
, 982,
1119
–1144.
Curt, T., Prévosto, B., Klesczewski, M. and Lepart, J.
2003
Post-grazing Scots pine colonization of mid-elevation heathlands: population structure, impact on vegetation composition and diversity.
Ann. For. Sci.
 
60
,
711
–724.
Curt, T., Coll, L., Prévosto, B., Balandier, P. and Kunsler, G.
2005
Plasticity in growth, biomass allocation and root morphology in beech seedlings as induced by irradiance and herbaceous competition.
Ann. For. Sci.
 
62
,
51
–60.
D’Anieri, P.A., Zedaker, S.M. and Hairston, A.B.
1986
Understory hardwood control in Virginia Coastal Plain loblolly pine stands. In Proceedings of the 39th Southern Weed Science Society 20–22 January, Nashville, TN. Southern Weed Science Published by Editor 39, 253.
Davies, R.J.
1987
Trees and weeds. Forestry Commission Handbook No. 2. HMSO, London.
Den Ouden, J.
2000
The role of bracken (Pteridium aquilinum) in forest dynamics. Thesis, Wageningen University, The Netherlands.
Dietz, H., Steinlein, T. and Ullmann, I.
1998
The role of growth form and correlated traits in competitive ranking of six perennial ruderal plant species grown in unbalanced mixtures.
Acta Oecologia
 
1
,
25
–36.
Dougherty, P.M. and Lowery, R.F.
1991
Spot-size of herbaceous control impacts loblolly pine seedling survival and growth.
South. J. Appl. For.
 
15
,
193
–199.
Dreyfus, P.
1984
Substitution de flore après entretien chimique des plantations forestières.
Rev. For. Fr.
 
36
,
385
–396.
Dumas, Y., Gama, A. and Berges, L.
2000
Substitution de flores après traitements herbicides. Cemagref edn, Antony, France.
Egler, F.E.
1954
Vegetation science concepts. I. Initial floristic composition, a factor in oldfield development.
Vegetatio
 
4
,
412
–417.
Fiddler, G.O. and McDonald, P.M.
1991
Integrated vegetation management in California – advances and future.
For. Chron.
 
67
,
528
–531.
Fitter, A.H.
1994
Architecture and biomass allocation as components of the plastic response of root systems to soil heterogeneity. In Exploitation of Environmental Heterogeneity by Plants. M.M. Caldwell and R.W. Pearcy (eds). Academic Press, San Diego, CA, pp. 304–324.
Fortson, J.C., Shiver, B.D. and Shackelford, L.
1996
Removal of competing vegetation from established pine plantations increases growth on Piedmont and upper Coastal Plain sites.
South. J. Appl. For.
 
20
,
188
–193.
Fotelli, M.N., Gessler, A., Peuke, A.D. and Rennenberg, H.
2001
Drought affects the competitive interactions between Fagus sylvatica seedlings and an early successional species, Rubus fruticosus: responses of growth, water status and δ13C composition.
New Phytol.
 
151
,
427
–435.
Fotelli, M.N., Rennenberg, H. and Gessler, A.
2002
Effects of drought on the competitive interference of an early successional species (Rubus fruticosus) on Fagus sylvatica L. seedlings: 15N uptake and partitioning, response of amino acids and other N compounds.
Plant Biol.
 
4
,
311
–320.
Fredericksen, T.S., Zedaker, S.M., Smith, D.W., Seiler, J.R. and Kreh, R.E.
1993
Interference-interactions in experimental pine-hardwood stands.
Can. J. For. Res.
 
23
,
2032
–2043.
Frochot, H., Armand, G., Gama, A., Nouveau, M. and Wehrlen, L.
2002
La gestion de la vegetation accompagnatrice: état et perspective.
Rev. For. Fr.
 
54
, 6,
505
–520.
Gemmel, P., Nilsson, U. and Welander, T.
1996
Development of oak and beech seedlings planted under varying shelterwood densities and with different site preparation methods in southern Sweden.
New Forests
 
12
,
141
–161.
Gitay, H. and Noble, I.R.
1997
What are functional types and how should we seek them? In Plant Functional Types. T.M. Smith, H.H. Shugart and F.I. Woodward (eds). Cambridge University Press, Cambridge, pp. 3–19.
Gjerstad, D.H. and Barber, B.L.
1987
Forest vegetation problems in the South. In Forest Vegetation Management for Conifer Production. J.D. Walstad and P.J. Kuch (eds). Wiley Interscience, New York, NY, pp. 55–75.
Glover, G.R. and Lauer, D.K.
1996
Growth response of Pinus taeda to varying levels of hardwood control.
N. Z. J. For. Sci.
 
26
,
74
–83.
Glover, G. and Quicke, H.
1999
Growth response of loblolly pine, sweetgum, and water oak in a pine-hardwood density study.
Can. J. For. Res.
 
29
,
968
–978.
Glover, R.G. and Zutter, B.R.
1993
Loblolly pine and mixed hardwood stand dynamics for 27 years following chemical, mechanical and manual site preparation.
Can. J. For. Res.
 
23
,
2126
–2132.
Goldberg, D.E.
1990
Components of resource competition in plant communities. In Perspectives on Plant Competition. J.B. Grace and D. Tilman (eds). Academic Press, San Diego, CA, pp. 27–49.
Goldberg, D.H.
1996
Competitive ability: definitions, contingency and correlated traits.
Philosophical Transaction of the Royal Society of London
 , B,
351
,
1377
–1385.
Grace, J.B. and Tilman, D.
1990
Perspective on Plant Competition. Academic Press, San Diego, CA.
Grime, J.P.
1979
Plant Strategies and Vegetation Processes. John Wiley & Sons, New York, NY.
Grime, J.P.
1994
The role of plasticity in exploiting environmental heterogeneity. In Exploitation of Environmental Heterogeneity by Plants. M.M. Caldwell and R.W. Pearcy (eds). Academic Press San Diego, CA, pp. 1–19.
Grime, J.P.
2001
Plant Strategies, Vegetation Processes, and Ecosystem Properties, 2nd edn. John Wiley & Sons, New York, NY.
Groninger, J.W., Baer, S.G., Babassana, D.A. and Allen, D.H.
2004
Planted green ash (Fraxinus pennsylvanica Marsh.) and herbaceous vegetation responses to initial competition control during the first 3 years of afforestation.
For. Ecol. Manage.
 
189
,
161
–170.
Haeussler, S. and Bergeron, Y.
2004
Range of variability in boreal aspen plant communities after wildfire and clear-cutting.
Can. J. For. Res.
 
34
,
274
–288.
Haeussler, S., Bedfor, L., Boateng, J.O. and MacKinnon, A.
1999
Plant community responses to mechanical site preparation in northern interior British Columbia.
Can. J. For. Res.
 
29
,
1084
–1100.
Hangs, R.D., Knight, J.D. and Van Rees, C.J.
2002
Interspecific competition for nitrogen between early successional species and planted white spruce and jack pine seedlings.
Can. J. For. Res.
 
32
,
1813
–1821.
Hangs, R.D., Knight, J.D. and Van Rees, C.J.
2003
Nitrogen uptake characteristics for roots of conifer seedlings and common boreal forest competitor species.
Can. J. For. Res.
 
33
,
156
–163.
Hannah, P.R.
1991
Regeneration of northern hardwoods in the northeast with the shelterwood method.
North. J. Appl. For.
 
8
,
99
–104.
Hänninen, K., Ohtonen, R. and Huttunen, S.
1999
Effects of leguminous ground cover competition on red birch and soil nutrient status in the nursery.
Plant Soil
 
216
,
129
–138.
Harrington, T. B., Wagner, R.G., Radosevich, S.R. and Walstad, J.D.
1995
Interspecific competition and herbicide injury influence 10-year responses of coastal Douglas-fir and associated vegetation to release treatments.
For. Ecol. Manage.
 
76
,
55
–67.
Hartley, J.H.
2002
Rationale and methods for conserving biodiversity in plantation forests.
For. Ecol. Manage.
 
155
,
81
–95.
Haywood, J.D., Tiarks, A.E. and Sword, M.A.
1997
Fertilization, weed control, and pine litter influence loblolly pine stem productivity and root development.
New Forests
 
14
,
233
–249.
Holgen, P. and Hanell, B.
2000
Performance of planted and naturally regenerated seedlings in Picea abies- dominated shelterwood stands and clearcuts in Sweden.
For. Ecol. Manage.
 
127
,
129
–138.
Horsley, S.B.
1988
Control of understory vegetation in Allegheny hardwood stands with Oust.
North. J. Appl. For.
 
5
,
261
–262.
Horsley, S.B.
1993
Mechanisms of interference between hay-scented fern and black cherry.
Can. J. For. Res.
 
23
,
2059
–2069.
Jobidon, R.
2000
Density-dependent effects of northern hardwood competition on selected environmental resources and young white spruce (Picea glauca) plantation growth, mineral nutrition, and stand structural development – a 5-year study.
For. Ecol. Manage.
 
130
,
77
–97.
Jobidon, R., Cyr, G. and Thiffault, N.
2004
Plant species diversity and composition along an experimental gradient of northern hardwood abundance in Picea mariana plantations.
For. Ecol. Manage.
 
198
,
209
–221.
Kimberley, M.O. and Richardson, B.
2004
Importance of seasonal growth patterns in modeling interactions between radiata pine and some common weed species.
Can. J. For. Res.
 
34
,
184
–194.
Klinka, K., Chan, H.Y.H., Wang, Q. and De Montigny, L.
1996
Forest canopies and their influence on understory vegetation in early-seral stands on West Vancouver Island.
Northwest Sci.
 
70
,
193
–200.
Kozlowski, T.T.
2002
Physiological ecology of natural regeneration of harvested and disturbed forest stands: implication for forest management.
For. Ecol. Manage.
 
158
,
195
–221.
Kozlowski, T.T., Kramer, P.J. and Pallardy, S.G.
1991
Radiation. In The Physiological Ecology of Woody Plants. Academic Press, San Diego, CA, pp. 123–167.
Kuessner, R., Reynolds, P.E. and Bell, F.W.
1998
Growth response of black spruce seedlings as affected by competition for radiation. In Third International Conference on Forest Vegetation Management: Popular Summaries. R.G. Wagner and D.G. Thompson (compilers). Ontario Ministry of Natural Resources, Ontario Forest Research Institute, Forest Research Information Paper No. 141, pp. 145–147.
Kuessner, R., Reynolds, P.E. and Bell, F.W.
2000
Growth response of Picea mariana seedlings to competition for radiation.
Scand. J. For. Res.
 
15
,
334
–342.
Kutschera, L. and Lichtenegger, E.
1982
Wurzelatlas mitteleuropäischer Grünlandpflanzen. Band 1. Monocotyledoneae. Gustav Fischer Verlag, Stuttgart.
Kutschera, L. and Lichtenegger, E.
1992
Wurzelatlas mitteleuropäischer Grünlandpflanzen. Band 2. Pteridophyta und Dicotyledoneae (Magnoliopsida). Gustav Fischer Verlag, Stuttgart.
Lambers, H., Chapin III, F.S. and Pons, T.L.
1998
Plant Physiological Ecology. Springer Verlag, New York, NY.
Laquerbe, M.
1999
Communautés de sous-bois des peupleraies artificielles: relation entre phytomasse, richesse spécifique et perturbations.
Ann. For. Sci.
 
56
,
607
–614.
Lauer, D.K., Glover, G.R. and Gjerstad, D.H.
1993
Comparison of duration and method of herbaceous weed control on loblolly pine response through midrotation.
Can. J. For. Res.
 
23
,
2116
–2125.
Lautenschlager, R.A.
1999
Environmental resource interactions affect red raspberry growth and its competition with white spruce.
Can. J. For. Res.
 
29
,
906
–916.
Lautenschlager, R.A., Bell, F.W., Wagner, R.G. and Reynolds, P.E.
1998
The Fallingsnow Ecosystem Project: documenting the consequences of conifer release treatments.
J. For.
 
96
(11),
20
–27.
Lavorel, S. and Garnier, E.
2002
Predicting changes in community composition and ecosystem functioning from plant traits: revisiting the Holy Grail.
Funct. Ecol.
 
16
,
545
–556.
Lieffers, V.J. and Stadt, K.J.
1994
Growth of understory Picea glauca, Calamagrostis canadensis, and Epilobium angustifolium in relation to overstory light transmission.
Can. J. For. Res.
 
24
,
1193
–1198.
Lindh, B.C., Gray, A.N. and Spies, T.A.
2003
Responses of herbs and shrubs to reduced root competition under canopies and in gaps: a trenching experiment in old-growth Douglas-fir forests.
Can. J. For. Res.
 
33
,
2052
–2057.
Locasio, C.G., Lockaby, B.G., Caulfield, J.P., Edwards, M.B. and Causey, M.K.
1991
Mechanical site preparation effects on understory plant diversity in the Piedmont of the southern USA.
New Forests
 
4
,
261
–269.
Löf, M.
2000
Establishment and growth in seedlings of Fagus sylvatica and Quercus robur: influence of interference from herbaceous vegetation.
Can. J. For. Res.
 
30
,
855
–864.
Ludovici, K.H. and Morris, L.A.
1997
Competition-induced reductions in soil water availability reduced pine root extension rates.
Soil Sci. Soc. Am. J.
 
61
,
1196
–1202.
McDonald, P.M. 1986 Grasses in young conifer plantations - hindrance and help. Northwest Science 60 (4), 271–278.
McDonald, P.M. and Fiddler, G.O.
1993
Feasibility of alternatives to herbicides in young conifer plantations in California.
Can. J. For. Res.
 
23
,
2015
–2022.
McDonald, P.M. and Helgerson, O.T.
1990
Mulches aid in regenerating California and Oregon forests: past, present, and future. USDA Forest Service General Technical Report No. PSW-123. Pacific Southwest Research Station, September 1990.
McDonald, P.M. and Reynolds, P.E.
1999
Plant community development after 28 years in small group selection openings. USDA Forest Service Service Paper No. PSW-RP-241. Pacific Southwest Research Station. December 1999.
McDonald, P.M., Fiddler, G.O. and Henry, W.T.
1994
Large mulches and manual release enhance growth of ponderosa pine seedlings.
New Forests
 
8
:
169
–178.
Marquis, D.A.
1990
Black cherry. In Silvics of North America. Vol. 2. Hardwoods. R.M. Burns and B.H. Honkala (eds). USDA Forest Service Agricultural Handbook No. 654, pp. 594–604.
Martin, C.W. and Hornbeck, J.W.
1990
Regeneration after strip cutting and block clearcutting in northern hardwoods.
North. J. Appl. For.
 
7
,
65
–68.
Messier, C., Doucet, R., Ruel, J.C., Claveau, Y., Kelly, C. and Lechowicz, M.J.
1999
Functional ecology of advance regeneration in relation to light in boreal forests.
Can. J. For. Res.
 
29
,
812
–823.
Miller, J.H., Zutter, B.R., Zedaker, S.M., Edwards, M.B. and Newbold, R.A.
1995
Early plant succession in loblolly pine plantations as affected by vegetation management.
South. J. Appl. For.
 
19
,
109
–126.
Miller, J.M., Boyd, R.S. and Edwards, M.B.
1999
Floristic diversity, stand structure, and composition 11 years after herbicide site preparation.
Can. J. For. Res.
 
29
,
1073
–1083.
Miller, J.H., Zutter, B.R., Newbold, R.A., Edwards, M.B. and Zedaker, S.M.
2003
a Stand dynamics and plant associates of loblolly pine plantations to midrotation after early intensive vegetation management – a southeastern United States regional study.
South. J. Appl. For.
 
27
,
221
–236.
Miller, J.H., Zutter, B.R., Zedacker, S.M., Edwards, M.B. and Newbold, R.A.
2003
b Growth and yield relative to competition for Loblolly pine plantations to Midrotation – a southeastern United States regional study.
South. J. Appl. For.
 
27
,
1
–16.
Miller, K.V. and Miller, J.H.
2004
Forestry herbicide influences on biodiversity and wildlife habitat in southern forests.
Wildlife Soc. Bull.
 
32
,
1049
–1060.
Mitchell, R.J., Zutter, B.R., Green, T.H., Perry, M.A., Gjerstad, D.H. and Glover, G.R.
1993
Spatial and temporal variation in competitive effects on soil moisture and pine response.
Ecol. Appl.
 
3
,
167
–174.
Mitchell, R.J., Zutter, B.R., Gjerstad, D.H., Glover, G.R. and Wood, C.W.
1999
Competition among secondary-successional pine communities: a field study of effects and responses.
Ecology
 
80
,
857
–872.
Mohammed, G.H., Noland, T.L. and Wagner, R.G.
1998
Physiological perturbation in jack pine (Pinus banksiana Lamb.) in the presence of competing herbaceous vegetation.
For. Ecol. Manage.
 
103
,
77
–85.
Morris, L.A., Moss, S.A. and Garbett, W.S.
1993
Competitive interference between selected herbaceous and woody plants and Pinus taeda L. during two growing seasons following planting.
For. Sci.
 
39
,
166
–187.
Nambiar, E.K.S. and Fife, D.N.
1991
Nutrient retranslocation in temperate conifers.
Tree Physiol.
 
9
,
185
–207.
Nambiar, E.K.S. and Sands, R.
1993
Competition for water and nutrients in forests.
Can. J. For. Res.
 
23
,
1955
–1968.
Newton, M. and Preest, D.S.
1988
Growth and water relations of Douglas fir (Pseudotsuga menziesii) seedlings under different weed control regimes.
Weed Sci.
 
36
,
653
–662.
Nobel, P.S.
1994
Root–soil responses to water pulses in dry environments. In Exploitation of Environmental Heterogeneity by Plants. M.M. Caldwell and R.W. Pearcy (eds). Academic Press, San Diego, CA, pp. 285–304.
Oppenheimer, M.J., Shiver, B.D. and Rheney, J.W.
1989
Ten-year response of mid-rotation slash pine plantations to control competing vegetation.
Can. J. For. Res.
 
19
,
329
–334.
Otsamo, A., Adjers, G., Hadi, T.S., Kuusipalo, J. and Vuokko, R.
1997
. Evaluation of reforestation potential of 83 tree species planted on Imperata cylindrica dominated grassland.
New Forests
 
14
,
127
–143.
Perry, D.A.
1995
Forests, competition and succession. In Encyclopedia of Environmental Botany, Vol. 2. Academic Press, San Diego, CA, pp. 135–153.
Perry, M.A., Mitchell, R.J., Zutter, B.R., Glover, G.R. and Gjerstad, D.H.
1993
Competitive responses of loblolly pine to gradients in loblolly pine, sweetgum, and broomsedge densities.
Can. J. For. Res.
 
23
,
2049
–2058.
Picon-Cochard, C., Nsourou-Obame, A., Collet, C., Guehl, J.M. and Ferhi, A.
2001
Competition for water between walnut seedlings (Juglans regia) and rye grass (Lolium perenne) assessed by carbon isotope discrimination and δ18O enrichment.
Tree Physiol.
 
21
,
183
–191.
Pitt, D.G., Reynolds, P.E. and Roden, M.J.
1988
Growth and tolerance of white spruce after site preparation with liquid hexazinone.
Proc. Northeast. Weed Sci. Soc.
  Suppl.
42
,
58
–62.
Powers, R.F. and Reynolds, P.E.
1999
Ten-year responses of ponderosa pine plantations to repeated vegetation and nutrient control along an environmental gradient.
Can. J. For. Res.
 
29
,
1027
–1038.
Rameau, J.C.
1999
Accrus, successions végétales et modèles de dynamique linéaire forestière. Ingénierie-EAT Special Issue, pp. 33–48.
Reynolds, P.E.
1988
Prognosis for future herbicide use in Canada.
Can. For. Indus. Mag.
 
108
(2),
35
–42.
Reynolds, P.E.
1992
Silvicultural strategies for managing competing vegetation: future research directions in Canada. In Proceedings of the First International Weed Control Congress. R.G. Richardson (ed.), Monash University, Melbourne, Australia, Vol. 2, pp. 439–441.
Reynolds, P.E. and Obarymskyj, A.M.
1994
Development of raspberry or fireweed competition after site disturbance or treatment with hexazinone.
Proc. Northeast. Weed Sci. Soc.
 
48
,
49
–55.
Reynolds, P.E., Scrivener, J.C., Holtby, L.B. and Kingsbury, P.D.
1993
Carnation Creek herbicide research: a summary of research findings.
For. Chron.
 
69
,
323
–330.
Reynolds, P.E., Kuessner, R. and Bell, F.W.
1998
Effects of controlled weed densities and soil types on seedling microclimate. In Third International Conference on Forest Vegetation Management: Popular Summaries. R.G. Wagner and D.G. Thompson (compilers). Ontario Ministry of Natural Resources, Ontario Forest Research Institute, Forest Research Information Paper No. 141, pp. 278–280.
Reynolds, P.E., Curry, R.D. and Bell, F.W.
2002
a Effects of controlled weed densities, soil types, weed species and duration of weed establishment on seedling microclimate and seedling growth. In Fourth International Conference on Forest Vegetation Management: Popular Summaries. H. Frochot, C. Collet and P. Balandier (compilers) Institut National de la Recherche Agronomique (INRA), Champenoux, France, pp. 258–260.
Reynolds, P.E., Curry, R.D., Tibbels, N.C. and Kuessner, R.
2002
b Physiological responses of black spruce seedlings to predetermined plant competition levels. In Fourth International Conference on Forest Vegetation Management: Popular Summaries. H. Frochot, C. Collet and P. Balandier (compilers). Institut National de la Recherche Agronomique (INRA), Champenoux, France, pp. 261–263.
Ricard, J.P., Messier, C., Delagrange, S. and Beaudet, M.
2003
Do understory saplings respond to both light and below-ground competition? A field experiment in a north-eastern American hardwood forest and a literature review.
Ann. For. Sci.
 
60
,
749
–756.
Richardson, B.
1993
Vegetation management practices in plantation forests of Australia and New Zealand.
Can. J. For. Res.
 
23
,
1989
–2005.
Richardson, B., Davenhill, N., Coker, G., Ray, J., Vanner, A. and Kimberley, M.
1996
Optimising spot weed control: first approximation of the most cost effective spot size.
N. Z. J. For. Sci.
 
26
,
265
–275.
Richardson, B., Kimberley, M.O., Ray, J.W. and Coker, G.W.
1999
Indices of interspecific plant competition for Pinus radiata in the central north island of New Zealand.
Can. J. For. Res.
 
29
,
898
–905.
Richardson, B., Mason, E.G., Watt, M.S. and Kriticos, D.J.
2005
Advances in modeling and decision support systems for forest vegetation management. Forestry 78 (in press).
Riegel, G.M., Miller, R.F. and Krueger, W.C.
1995
The effects of aboveground and belowground competition on understory species composition in a Pinus ponderosa forest.
For. Sci.
 
41
,
864
–889.
Roberts, M.R. and Dong, H.
1991
Effects of clearcutting and forest floor disturbance on establishment and survival of raspberry.
Proc. Northeast. Weed Sci. Soc.
 
45
,
71
.
Roberts, M.R. and Dong, H.
1993
Potential levels of raspberry competition after forest floor disturbance.
Proc. Northeast. Weed Sci. Soc.
 
47
,
206
.
Rösch, H., Van Rooyen, M.W. and Theron, G.K.
1997
Predicting competitive interactions between pioneer plant species by using plant traits.
J. Veg. Sci.
 
8
,
489
–494.
Rose, R. and Ketchum, J.S.
2003
Interaction of initial seedling diameter, fertilization and weed control on Douglas-fir growth over the first four years after planting.
Ann. For. Sci.
 
60
,
625
–635.
Rose, R., Ketchum, J.S. and Hanson, D.E.
1999
Three-year survival and growth of douglas-fir seedlings under various vegetation-free regimes.
For. Sci.
 
45
,
117
–126.
Ruark, G.A.
1990
Evidence for the reserve shelterwood system for managing quaking aspen.
North. J. Appl. For.
 
7
,
58
–62.
Sands, R. and Nambiar, E.K.S.
1984
Water relations of radiata pine in competition with weeds.
Can. J. For. Res.
 
14
,
233
–237.
Sands, R. and Mulligan, D.R.
1990
Water and nutrient dynamics and tree growth.
For. Ecol. Manage.
 
30
,
91
–111.
Schabenberger, L.E. and Zedaker, S.M.
1999
Relationships between loblolly pine yield and woody plant diversity in Virginia Piedmont plantations.
Can. J. For. Res.
 
29
,
1065
–1072.
Schroth, G.
1995
Tree root characteristics as criteria for species selection and system design in agroforestry.
Agrofor. Syst.
 
30
,
125
–143.
Schroth, G.
1999
A review of belowground interations in agroforestry, focussing on mechanisms and management options.
Agrofor. Syst.
 
43
,
5
–34.
Schütz, J.P.
2004
Opportunistic methods of controlling vegetation, inspired by natural plant succession dynamics with spatial reference to natural outmixing tendencies in a gap regeneration.
Ann. For. Sci.
 
61
,
149
–156.
Shainsky, L.J. and Radosevitch, S.R.
1991
Analysis of yield-density relationships in experimental stands of Douglas-fir and red alder seedlings.
For. Sci.
 
37
,
574
–592.
Shiver, B.D. and Daniel, B.
1994
Response and economics of mid-rotation release. In Proceedings of the 47th Southern Weed Science Society, Dallas, TX, Southern Weed Science Published by Editor 47, 85–92.
Sonohat, G., Sinoquet, H., Varlet-Grancher, C., Rakocevic, M., Jacquet, A., Simon, J.C. and Adam, B.
2002
Leaf dispersion and light partitioning in three-dimensionally digitized tall fescue–white clover mixtures.
Plant, Cell Environ.
 
25
,
529
–538.
South, D.B. and Mitchell, R.J.
1999
Determining the ‘optimum’ slash pine seedling size for use with four levels of vegetation management on a flatwoods site in Georgia, U.S.A.
Can. J. For. Sci.
 
29
,
1039
–1046.
South, D.B., Zwolinski, J.B. and Allen, H.L.
1995
Economic returns from enhancing loblolly pine establishment on two upland sites: effects of seedling grade, fertilization, hexazinone, and intensive soil cultivation.
New Forests
 
10
,
239
–256.
South, D.B., Miller, J.H., Kimberley, M.O. and VanderSchaaf, C.L.
2005
Determining productivity gains from forest vegetation management with ‘age-shift’ calculations. Forestry 78 (in press).
Stark, J.M.
1994
Causes of soil nutrient heterogenity at different scales. In Exploitation of Environmental Heterogeneity by Plants. M.M. Caldwell and R.W. Pearcy (eds). Academic Press, San Diego, CA, pp. 255–284.
Thevathasan, N.V., Reynolds, P.E., Kuessner, R. and Bell, F.W.
2000
Effects of controlled weed densities and soil types on soil nitrate accumulation, spruce growth and weed growth.
For. Ecol. Manage.
 
133
,
135
–144.
Thomas, S.C., Halpern, C.B., Falk, D.A., Liguori, D.A. and Austin, K.A.
1999
Plant diversity in managed forests: understorey responses to thinning and fertilization.
Ecol. Appl.
 
9
,
864
–879.
Thompson, D.G. and Pitt, D.G.
2003
A review of Canadian forest vegetation management research and practice.
Ann. For. Sci.
 
60
,
559
–572.
Tilman, D.
1988
Plant Strategies and the Dynamics and Structure of Plant Communities. Princeton University Press, Princeton, NJ.
Tubbs, C.H. and Lamson, N.
1991
Effect of shelterwood canopy density on sugar maple reproduction in Vermont.
North. J. Appl. For.
 
8
,
86
–89.
Turcotte, D.E., Smith, C.T. and Federer, C.A.
1991
Soil disturbance following whole-tree harvesting in north-central Maine.
North. J. Appl. For.
 
8
,
68
–72.
Van Lerberghes, P.
2004
Les paillis biodégradables en plantation ligneuse. Forêt-entreprise 157, 19–46.
Vitousek, P.
1982
Nutrient cycling and nutrient efficiency.
Am. Nat.
 
119
,
553
–572.
Wagner, R.G.
1993
Research directions to advance forest vegetation management in North America.
Can. J. For. Res.
 
23
,
2317
.
Wagner, R.G.
1994
Toward integrated forest vegetation managment.
J. For.
 
92
,
26
–30.
Wagner, R.G., Noland, T.L. and Mohammed, G.H.
1996
Timing and duration of herbaceous vegetation control around four northern coniferous species.
N. Z. J. For. Sci.
 
26
,
39
–52.
Wagner, R.G., Mohammed, G.H. and Noland, T.L.
1999
Critical period of interspecific competition for northern conifers associated with herbaceous vegetation.
Can. J. For. Res.
 
29
,
890
–897.
Wagner, R.G., Keith, M.L., Richardson, B. and McNabb, K.
2005
The role of vegetation management for enhancing productivity of the World's forests. Forestry 78 (in press).
Walstad, J.D., Newton, M. and Boyd Jr, R.J.
1987
Forest vegetation problems in the Northwest. In Forest Vegetation Management for Conifer Production. J.D. Walstad and P.J. Kuch (eds). Wiley Interscience, New York, NY, pp. 15–53.
Wardle, D.A., Barker, G.M., Bonner, K.I. and Nicholson, K.S.
1998
Can comparative approaches based on plant ecophysiological traits predict the nature of biotic interactions and individual plant species effects in ecosystems?
J. Ecol.
 
86
,
405
–420.
Watt, M.S., Whitehead, D., Mason, E.G., Richardson, B. and Kimberley, M.O.
2003
The influence of weed competition for light and water on growth and dry matter partitioning of young Pinus radiata, at dryland site.
For. Ecol. Manage.
 
183
,
363
–376.
Wetzel, S. and Burgess, D.
2001
Understorey environment and vegetation response after partial cutting and site preparation in Pinus strobus L. stands.
For. Ecol. Manage.
 
151
,
43
–59.
Willoughby, I.
1999
Future alternatives to the use of herbicides in British forestry.
Can. J. For. Res.
 
29
,
866
–874.
Willoughby, I. and McDonald, H.G.
1999
Vegetation management in farm forestry: a comparison of alternative methods of inter-row management.
Forestry
 
72
,
109
–121.
Zedaker, S.M., Cheynet, K.I., Stauffer, D.F., Amateis, R.L. and Scrivani, J.A.
2002
Mid-rotation release affects forest structure, wildlife habitat, and pine yield. In Fourth International Conference on Forest Vegetation Management: Popular Summaries. H. Frochot, C. Collet and P. Balandier (compilers) Institut National de la Recherche Agronomique (INRA). Champenoux, France, pp. 249–251.
Zutter, B.R. and Miller, J.H.
1998
Eleventh-year response of loblolly pine and competing vegetation to woody and herbaceous control on a Georgia flatwoods site.
South. J. Appl. For.
 
122
,
88
–95.
Zutter, R.B., Glover, G.R. and Gjerstad, D.H.
1986
Effects of herbaceous weed control using herbicides on a young loblolly pine plantation.
For. Sci.
 
32
,
882
–899.
Zutter, B.R., Glover, G.R., Mitchell, R.J. and Gjerstad, D.H.
1997
Response of loblolly pine and sweetgum to intra- and inter-specific competition and influence of soil organic matter.
Can. J. For. Res.
 
27
,
2079
–2087.
Zutter, B.R., Mitchell, R.J., Gjerstad, D.H. and Glover, G.R.
1998
Intra- and interspecific effects of loblolly pine and sweetgum on resources and responses through three growing seasons. In Third International Conference on Forest Vegetation Management: Popular Summaries, Wagner R.G., Thompson D.G. (compilers), Ontario Ministry of Natural Resources, Ontario Forest Research Institute, Forest Research Information Paper No. 141, pp. 378–380.
Zutter, R.B., Mitchell, R.J., Glover, G.R. and Gjerstad, D.H.
1999
Root length and biomass in mixtures of broomsedge with loblolly pine and sweetgum.
Can. J. For. Res.
 
29
,
926
–933.