Evaluation of the growth response of six invasive species to past, present and future atmospheric carbon dioxide

Abstract

The response of plant species to future atmospheric carbon dioxide concentrations [CO₂] has been determined for hundreds of crop and tree species. However, no data are currently available regarding the response of invasive weedy species to past or future atmospheric [CO₂]. In the current study, the growth of six species which are widely recognized as among the most invasive weeds in the continental United States, Canada thistle (Cirsium arvense (L.) Scop.), field bindweed (Convolvulus arvensis L.), leafy spurge (Euphorbia esula L.), perennial sowthistle (Sonchus arvensis L.), spotted knapweed (Centaurea maculosa Lam.), and yellow star thistle (Centaurea solstitialis L.) were grown from seed at either 284, 380 or 719 µmol mol^–1 [CO₂] until the onset of sexual reproduction (i.e. the vegetative period). The CO₂ concentrations corresponded roughly to the CO₂ concentrations which existed at the beginning of the 20th century, the current [CO₂], and the future [CO₂] projected for the end of the 21st century, respectively. The average stimulation of plant biomass among invasive species from current to future [CO₂] averaged 46%, with the largest response (+72%) observed for Canada thistle. However, the growth response among these species to the recent [CO₂] increase during the 20th century was significantly higher, averaging 110%, with Canada thistle again (+180%) showing the largest response. Overall, the CO₂‐induced stimulation of growth for these species during the 20th century (285–382 µmol mol^–1) was about 3× greater than for any species examined previously. Although additional data are needed, the current study suggests the possibility that recent increases in atmospheric CO₂ during the 20th century may have been a factor in the selection of these species.

Key words: Carbon dioxide, invasive weeds, leaf area ratio, net assimilation rate, relative growth rate.

Received 21 May 2002; Accepted 10 September 2002

Introduction

Invasive plants are generally recognized as those species, usually non‐native for a given system, whose introduction, commonly by human transport, results in economic or environmental harm. Although the impact of such species on native plant communities is well documented (Rejmanek and Randall, 1994), introduced plant species also present a threat to agronomic productivity. In agriculture, invasive plants out‐compete crops for soil and water resources, reduce crop quality, interfere with harvesting operations, and reduce land values. The US Department of Agriculture estimates the annual productivity loss of 64 crops due to invasive or ‘noxious’ species at $7.4 billion (USDA‐NRCS, 1999).

Of course, not all introduced plant species become invasive. What then are the characteristics associated with successful invasive species? Understanding these characteristics is crucial since, if they can be identified, future invasions could be identified and controlled. While attention has been given to the lack of native predators or parasites (Vitousek et al., 1996), less work has focused on the role of the abiotic environment per se in invasions (D’Antonio and Vitousek, 1992)

One aspect of the environment which is of obvious interest is global climate change. The concentration of atmospheric CO₂ has already risen 30% during the 20th century from ∼285 µmol mol^–1 to a current estimate of 370 µmol mol^–1 with most of the increase occurring since the 1950s (Keeling and Whorf, 2001). The United Nations Intergovernmental Panel on Climate Change (UN‐IPCC) predicts that [CO₂] could exceed 700 µmol mol^–1 by the end of the current century (Houghton et al., 1996). Recent IPCC reports provide strong evidence that rising CO₂ and other trace gases could lead to a 3–12 °C increase in global surface temperatures with subsequent effects on climate, although the degree of temperature/climatic change remains uncertain (Kaiser, 2001).

While the extent of temperature increases remains speculative, there is acknowledged consensus on the direct physiological impact of increasing [CO₂] on plant photosynthesis and metabolism (see reviews by Stitt, 1991; Bowes, 1996). Increasing [CO₂] has been shown to stimulate growth and development significantly in hundreds of plant species (see Kimball, 1983; Kimball et al., 1993; Poorter, 1993, for reviews examining the response to future CO₂ concentrations; Sage, 1995, for a review of the response to pre‐industrial CO₂ concentrations).

Based on these reviews, it is clear that plant species respond differently to the ongoing increase in atmospheric [CO₂]. In fact, differential response of C₃ and C₄ plant species to increasing CO₂ during the 20th century has been suggested as one potential explanation for recent invasions of native C₄ grasslands in North America by woody C₃ species (Johnson et al., 1993). However, a specific evaluation of growth responses among recognized invasive plant species to past, present and future CO₂ concentrations is not available.

To determine the relative extent of potential growth stimulation systematically, six widely acknowledged North American invasive weeds, Canada thistle (Cirsium arvense (L.) Scop.), field bindweed (Convolvulus arvensis L.), leafy spurge (Euphorbia esula L.), perennial sowthistle (Sonchus arvensis L.), spotted knapweed (Centaurea maculosa Lam.), and yellow star thistle (Centaurea solstitialis L.) were grown from seed at either 284, 380 or 719 µmol mol^–1 [CO₂] until the onset of sexual reproduction (i.e. the vegetative period). These CO₂ values correspond approximately to those which existed at the beginning of the 20th century, those which exist today, and those which are predicted to occur by the end of the 21st century. Vegetative growth was considered critical since early development is a key factor in determining crop/weed competition (Kropff and Spitters, 1991).

Materials and methods

Seeds from six weedy species, Canada thistle, field bindweed, leafy spurge, perennial sowthistle, spotted knapweed, and yellow star thistle, were grown using three controlled environmental chambers (EGC Corp., Chagrin Falls, OH, USA) with each chamber kept at one of three 24 h carbon dioxide concentrations set‐points, 285, 380 or 720 µmol mol^–1 (actual [CO₂] values averaged on a 24 h basis through the experiment were 284±18, 380±10 or 719±6 µmol mol^–1 CO₂). Five to ten seeds of each species were sown in 0.6 l pots filled with vermiculite and were thinned to one seedling 4–6 d after emergence. For each [CO₂] treatment 24–26 plants of a given species were used. Pots were rotated biweekly inside the chambers and arranged to avoid shading from other plants. All pots were watered to the drip point daily with a complete nutrient solution containing 14.5 mMol m^–3 nitrogen (Robinson, 1984). All seed was obtained from Herbiseed (Herbiseed Corp., Berkshire, UK).

For all environmental chambers, temperature was altered in a diurnal fashion from an overnight low of 20 °C to a maximum afternoon value of 30 °C, with an average daily (24 h) value of 23.1 °C. Similarly, light (photosynthetically active radiation, PAR) was also altered diurnally in conjunction with temperature, with the highest PAR value (∼1200 μmol m^–2 s^–1) occurring during the afternoon. Daily PAR was 15 h, supplied by a mixture of high pressure sodium and metal halide lamps and averaged 38 mol m^–2 d^–1 for all chambers. The CO₂ concentration of the air was controlled by adding either CO₂ or CO₂‐free air to maintain the desired CO₂ concentration. Injection of CO₂ and CO₂‐free air was controlled by a TC‐2 controller using input from an absolute infrared gas analyser (WMA‐2, PP Systems, Haverhill, MA, USA). Temperature, humidity and [CO₂] were recorded every 15 min, and daily averages determined for a given experimental run. Additional details concerning the operating system can be found in Ziska et al. (2001).

Plants were grown until 54–60 d after sowing (DAS) at which time, depending on species, initiation of floral spikes or reproductive development was evident. Reproductive development occurred 2–3 d earlier at 284 µmol mol^–1 CO₂ for spotted knapweed and yellow star thistle relative to ambient CO₂ conditions, but was approximately equivalent for all other species among CO₂ treatments. Because of different growth rates and timing of reproductive development, Canada thistle, field bindweed and yellow star thistle were harvested at 17, 23, 35, and 54 DAS, while leafy spurge, spotted knapweed and perennial sowthistle were harvested at 23, 35, 54, and 60 DAS. To minimize root binding, all species were transplanted to 21.1 l pots by 36 DAS. With the exception of leafy spurge, leaf area was determined photometrically on all leaves at 17, 23 and 35 DAS using a leaf area meter (model 3100, Li‐Cor Corp., Lincoln NE, USA). Because of the large leaf area at 54 and 60 DAS, subsamples of 10 leaves per CO₂ treatment and species were placed in moistened paper towels to prevent leaf rolling and leaf area was determined as described previously. These leaves were then dried at 65 °C and total leaf area per plant and specific leaf weight estimated by the linear regression of leaf area to leaf dry weight (r² >0.91 for all species) obtained from these leaves. Because of the milky latex and the subsequent difficulty in running leaves of leafy spurge through the leaf area meter, this technique was also used to determine total leaf area per plant for this species at 54 and 60 DAS. In addition to leaf area, dry mass was determined separately for all leaves, stems and roots at each harvest for all species and [CO₂] treatments after drying at 65 °C for a minimum of 48 h or until dry mass was constant. Relative growth rate (RGR), net assimilation rate (NAR) and leaf area ratio (LAR) were calculated according to Jones (1983). Because NAR is determined on a per leaf area basis, NAR of whole plants (NARp) was also determined by multiplying NAR by existing leaf area at a given harvest.

Because only three chambers were available, a randomized complete block design was used with runs over time as replications (blocks). Each chamber was assigned one of the three CO₂ treatments. At the end of a given run (i.e. 60 DAS), CO₂ treatments were randomly assigned to a given chamber and the entire experiment was repeated. The entire experiment was repeated three times, and the mean value from each run used as a single replicate. Growth characteristics, biomass and vegetative characteristics were analysed using a one‐way ANOVA with CO₂ as the independent variable. To determine differences as a function of [CO₂] for a given species and harvest date, a Student’s t‐test (assuming unequal variances) was used. Unless otherwise stated, significant differences for any measured parameter were determined as significant at the P ≤0.05 level.

Results

Increasing [CO₂] above levels present at the beginning of the 20th century (i.e. 285 µmol mol^–1) stimulated NAR at either 23 and/or 35 DAS for all invasive species except for spotted knapweed (Table 1). By 54–60 DAS, the additional stimulation of NAR by [CO₂] was no longer consistent among species (and had actually declined for perennial sowthistle) (Table 1). Although this would suggest increasing insensitivity to assimilation with increasing CO₂, NAR is expressed per unit leaf area. Consequently, if total leaf area per plant is taken into consideration, increasing [CO₂] from either 284 to 380 or from 380 to 719 µmol mol^–1 resulted in significant stimulation of NAR per plant (NARp) through 54 or 60 DAS for all species except leafy spurge with no evidence of a decline in the relative stimulation through this period as a function of [CO₂] (Table 2).

Leaf area was also significantly stimulated with increasing [CO₂] with a greater relative increase from early 20th century to current CO₂ than from current to projected [CO₂] levels (Fig. 1). LAR in contrast, increased with decreased [CO₂], especially during the early part of the growing season for five out of the six species (Fig. 2). That is, at 284 µmol mol^–1 CO₂, LAR significantly increased for Canada thistle, field bindweed, perennial sowthistle, spotted knapweed, and yellow star thistle between 17–35 DAS. Increased LAR for the lowest [CO₂] was also observed for leafy spurge at 54 and 60 DAS. The increase in LAR indicates a greater allocation to leaf production during early development for the lowest [CO₂] (Fig. 2).

The CO₂‐induced stimulation of plant biomass was consistent with that observed for leaf area, with a greater relative increase from 284 to 380 than from 380 to 719 µmol mol^–1 CO₂ (Table 3). Yellow star thistle and perennial sowthistle showed the largest absolute increase in plant biomass at 54 and 60 DAS, respectively. The smallest absolute response was observed for leafy spurge, with no significant increase in biomass observed at 60 DAS between current and future [CO₂]. By the final harvest date (i.e. either 54 or 60 DAS), significant increases in leaf, stem and root weight were also observed in all species with increasing [CO₂], with the exception of root weight in leafy spurge (Table 3). The largest relative increase, averaged across all species was observed for root biomass (∼120% from 284 to 389 µmol mol^–1; 55% from 380 to 719 µmol mol^–1). While there is a significant effect of [CO₂] on R:S ratio averaged for all species, for individual species, no consistent effect of [CO₂] was observed for root:shoot ratio (Table 3).

Although there are clear differences in plant biomass and its components as a consequence of [CO₂], little actual change in RGR as a function of [CO₂] was observed for a given sampling date (Table 4). Although RGR did decline over time, overall RGR did not differ as a function of CO₂ treatment during the measured growth interval. Since there are obvious biomass differences as a result of [CO₂] by the end of the experiment, the actual change in RGR resulting from [CO₂] must have occurred prior to 17 DAS for Canada thistle, field bindweed and yellow star thistle and prior to 23 DAS for leafy spurge, perennial sowthistle and spotted knapweed.

Discussion

Because of the recent increase in atmospheric [CO₂], and its importance in plant physiology, the response of plant species to future levels of CO₂ has been the object of numerous studies (for reviews see Poorter, 1993; Kimball et al., 1993; Curtis and Wang, 1998; inter alia). In examining the species listed in these reviews, it is clear that a principal focus of many studies has been on commercially important species (i.e. crops and trees). Although weeds have been investigated (see Patterson, 1995, for a review), specific evaluations of the growth response of recognized invasive or noxious weeds to past, present or future CO₂ concentrations are not available.

The invasive species chosen for this study were taken from Skinner et al. (2000) who ranked noxious weeds from surveys of the United States and Southern Canada. (‘Noxious’ refers to the fact that these plants have been deemed harmful in a legal sense and not that the plants are poisonous.) Their inclusion in such a list recognizes the detrimental nature of such species and their wide‐spread occurrence. Obviously a plant known to be noxious and invasive is likely to be regulated by law in order to prevent its introduction or spread into new areas. Six of the top 15 weeds determined by Skinner et al. (2000) (all C₃ species) were used in the current study. Among these, Canada thistle, is recognized as the most noxious weed, occurring on 33 noxious lists (the next closest weed, musk thistle, occurs on 24 lists) (Skinner et al., 2000).

Relative to other species, do invasive species show a stronger or lesser response to increasing atmospheric [CO₂]? What is the expected response? One of the earliest attempts to integrate plant response to elevated [CO₂] was published by Kimball (1983) who examined 430 previous studies. Bases on his analysis he determined that the average response of plants (±SE) to future elevated CO₂ conditions was 34±6% (330–360 µmol mol^–1 for ambient versus 600–1000 µmol mol^–1 for future elevated; Kimball, 1983). Other studies which have quantified the variation in the response of plants to future [CO₂] show similar results (e.g. 37% for 156 plant species; Poorter, 1993). In the current experiment with invasive weeds, the average response after 54 DAS was ∼45%, similar to previously published values (Fig. 3A). For individual weedy species, only Canada thistle (+72%) and spotted knapweed (+60%) indicate a substantially stronger than expected response to future elevated [CO₂] relative to the baseline devised by Kimball (1983). The strong response to future elevated [CO₂] relative to current ambient levels observed for these two species is consistent with the observed stimulation of NARp (+63% and +50% for Canada thistle and spotted knapweed, respectively, from ambient to future elevated [CO₂]), but could not be predicted based on relative biomass allocation (e.g. R/S, alterations in source:sink). Overall, the response of the invasive weeds examined here to projected future increases in atmospheric [CO₂], does not differ considerably from that of other species.

But the ‘current’ [CO₂] is in flux. Atmospheric carbon dioxide has already risen from ∼285 to 378 µmol mol^–1 during the 20th century, with most of the observed increase coming since the late 1950s (c. 312 µmol mol^–1 in 1959) (Keeling and Whorf, 2001). Prior to 1900, [CO₂] fluctuated between 180–290 µmol mol^–1 for at least 220 000 years (Barnola et al., 1987; Jouzel et al., 1993). With respect to the recent and rapid increase in atmospheric [CO₂] during the 20th century, do invasive species show a stronger than expected response?

In contrast to future elevated CO₂ levels, less work has integrated the response of plants to the recent increase in atmospheric [CO₂], even though it is recognized that leaf and plant photosynthesis can be particularly sensitive to the low [CO₂] concentrations of the past (Polley et al., 1993). This is due in part to technical considerations since it is more difficult to scrub CO₂ than it is to add it. Nevertheless, Sage (1995) summarized 12 studies over the range of preindustrial relative to current levels. Including only C₃ species, the estimated average relative biomass response between 270 and 380 µmol mol^–1 is approximately 29% (see Fig. 3b in Sage, 1995). If this response is updated to include more recent reports, (i.e. wheat varieties Seri M82 and Yaqui; Mayeux et al., 1997; Albutilon theophrasti; Dippery et al., 1995; Ward et al., 1999), the average relative growth response (±SD) is 33±11% for [CO₂] values between 250–270 µmol mol^–1 and 360–380 µmol mol^–1.

In the current study, the response of the six invasive weeds to recent increases in [CO₂] (i.e. since the beginning of the 20th century) averaged 110%. Although only a limited number of preindustrial studies are available, the responses from the current experiment are considerably higher than previously reported for any plant species over this [CO₂] range (Fig. 3b). It could be argued that limitations of nutrients or water could limit the response of these species to atmospheric CO₂ in the field; however, many of these species are associated with managed agronomic environments where water and nutrients would be optimal (Patterson, 1995).

For individual species, Canada thistle again showed the largest relative increase (+180%), but no species showed a relative growth increase less than 80%. There was a larger relative increase in leaf area and NARp between 284 and 380 than from 380 to 719 when averaged for all species, but the relative response of NARp, or other growth characteristics from 284 to 380 µmol mol^–1 was independent of the CO₂‐induced stimulation of growth observed at 54 DAS for individual species.

Why such a strong response to recent increases in atmospheric [CO₂]? It has been shown in numerous studies that below‐ground growth can show dramatic increases with increased [CO₂] (Rogers et al., 1986; Bernsten and Woodward, 1992; Prior et al., 1994). Interestingly, of the most noxious weedy species listed by Skinner et al. (2000), many have a strong below‐ground root or rhizome system (e.g. Canada thistle, purple loosestrife, field bindweed, leafy spurge, Russian knapweed, whitetop, perennial sowthistle, quackgrass, dalmation toadflax) which can generate new stems from below‐ground structures. Averaged among all species in the current experiment, below‐ground biomass showed the strongest relative increase to the recent [CO₂] increase (+121% versus 54% for CO₂ concentrations from 284–380 versus 380–719 µmol mol^–1, respectively, see Table 3).

It is possible that substantial below‐ground sinks contributed to the large growth stimulation from 284 to 380 µmol mol^–1 providing a link between invasiveness and CO₂ responsiveness. However, at this time, the selection of propagation by vegetative means over floral reproduction remains only an intriguing possibility. The evolutionary role of increasing [CO₂] in the recent past cannot be fully elucidated by a single study done under growth chamber conditions for individual species. Clearly, the responses of the species considered here will vary in situ as a function of competition and environment with other factors antagonistic to the direction of selection (Etterson and Shaw, 2001). Additional field experiments to clarify the response to recent increases in [CO₂] are difficult due to technical considerations (cf. Mayeux et al., 1993). Nevertheless, understanding the role of climate, particularly the sudden and dramatic rise in atmospheric carbon dioxide within recent decades, as a possible factor in the invasiveness of the these species deserves additional consideration. Such an assessment may be crucial in quantifying the response and potential agronomic threat posed by invasive weeds as atmospheric [CO₂] continues to increase.

Acknowledgements

I would like to thank Mr Shaun Faulkner, and Mr Ernie Goins for expert technical assistance and to thank Dr Jim Bunce, Dr Joe Sullivan and Dr John Lydon for their comments and suggestions.

Fig. 1. Leaf area for six invasive weedy species, Canada thistle (Cirsium arvense), field bindweed (Convolvulus arvensis), leafy spurge (Euphorbia esula), perennial sowthistle (Sonchus arvensis), spotted knapweed (Centaurea maculosa), and yellow star thistle (Centaurea solstitialis) as a function of 284, 380 and 719 µmol mol^–1 [CO₂]. Each point is the average of 15–18 plants averaged over three runs. Bars are ±SE. Different letters indicate significant differences on the last day of harvest (54–60 DAS) as a function of [CO₂].

Fig. 2. Leaf area ratio (LAR) determined as the ratio of existing leaf area to above‐ground plant biomass at different harvest dates for six invasive weedy species, Canada thistle (Cirsium arvense), field bindweed (Convolvulus arvensis), leafy spurge (Euphorbia esula), perennial sowthistle (Sonchus arvensis), spotted knapweed (Centaurea maculosa), and yellow star thistle (Centaurea solstitialis) as a function of 284, 380 and 719 µmol mol^–1 [CO₂]. * Indicates a significant difference relative to the 284 µmol mol^–1 value for the 719 µmol mol^–1 treatment.

Fig. 3. (A) The average relative increase (%) determined from the ratio of dry weight at elevated (719 µmol mol^–1), relative to ambient (380 µmol mol^–1) CO₂ for three experimental runs for each of six invasive weeds, Canada thistle (Cirsium arvense), field bindweed (Convolvulus arvensis), leafy spurge (Euphorbia esula), perennial sowthistle (Sonchus arvensis), spotted knapweed (Centaurea maculosa), and yellow star thistle (Centaurea solstitialis) at a common harvest date of 54 DAS. Dotted line is the average response of plants as reported by Kimball (1983). (B) Same as (A), but for [CO₂] of 284 and 380 µmol mol^–1 CO₂. Dotted line is the published response of plants from 270–380 µmol mol^–1 (Sage, 1995). See text for additional details.

Table 1.

Change in net assimilation rate (NAR, g cm^–2 d^–1×10³) for six noxious weed species grown at either 284, 380 or 719 µmol mol^–1 CO₂ Different letters indicate significant differences between CO₂ treatments for a given sampling date and species.

Species	CO2	DAS
		23	35	54	60
Canada thistle	284	1.16 b	1.74 c	1.45 b
(Cirsium arvense)	380	1.85 a	2.10 b	1.68 ab
	719	1.08 b	2.55 a	1.74 a
Field bindweed	284	1.32 b	1.55	1.03 b
(Convolvulus arvensis)	380	1.28 b	1.78	1.07 b
	719	2.54 a	1.95	1.49 a
Leafy spurge	284		NA	0.92 b	1.01
(Euphorbia esula)	380		NA	1.21 ab	0.90
	719		NA	1.48 a	1.10
Perennial sowthistle	284		1.31 b	1.61 a	0.70 c
(Sonchus arvensis)	380		1.76 a	1.31 ab	1.16 b
	719		2.00 a	1.18 b	1.50 a
Spotted knapweed	284		2.23	1.27 b	0.65 b
(Centaurea maculosa)	380		2.13	1.41 ab	1.29 a
	719		2.55	1.55 a	1.15 ab
Yellow star thistle	284	2.73 a	1.74	1.29
(Centaurea solstitialis)	380	2.17 b	2.02	1.26
	719	2.53 ab	2.10	1.28
Average	284	1.74 b	1.71 b	1.26	0.79 b
	380	1.77 b	1.96 b	1.32	1.12 ba
	719	2.05 a	2.23 a	1.45	1.25 a

Table 2.

Change in net assimilation rate on a per plant basis (NARp, g plant^–1d^–1) for six noxious weed species grown at either 284, 380 or 719 µmol mol^–1CO₂ Different letters indicate significant differences between CO₂ treatments for a given sampling date and species.

Species	CO2	DAS
		23	35	54	60
Canada thistle	284	0.014 b	0.249 c	1.532 c
(Cirsium arvense)	380	0.056 a	0.543 b	3.332 b
	719	0.052 a	1.016 a	5.424 a
Field bindweed	284	0.057 b	0.488 b	2.177 c
(Convolvulus arvensis)	380	0.078 b	0.866 a	3.383 b
	719	0.245 a	1.164 a	4.293 a
Leafy spurge	284		NA	0.270 c	0.650 ab
(Euphorbia esula)	380		NA	0.608 a	0.450 b
	719		NA	0.721 a	0.928 a
Perennial sowthistle	284		0.165 c	1.491 c	0.895 c
(Sonchus arvensis)	380		0.526 b	2.537 b	2.966 b
	719		0.891 a	3.592 a	6.279 a
Spotted knapweed	284		0.265 c	1.672 c	0.742 b
(Centaurea maculosa)	380		0.453 b	2.775 b	3.627 a
	719		0.837 a	4.162 a	4.418 a
Yellow star thistle	284	0.106 b	0.473 c	2.108 b
(Centaurea solstitialis)	380	0.111 b	0.804 b	4.279 a
	719	0.211 a	1.356 a	4.642 a
Average	284	0.059 b	0.328 c	1.542 c	0.762 c
	380	0.082 b	0.638 b	2.819 b	2.348 b
	719	0.169 a	1.053 a	3.806 a	3.875 a

Table 3.

Final biomass characteristics for six noxious weeds Canada thistle, field bindweed and yellow star thistle were harvested at 54 DAS; leafy spurge, perennial sowthistle and spotted knapweed were harvested at 60 DAS. Weeds were grown from sowing at either 284, 380 or 719 µmol mol^–1 CO₂. Values are expressed as g plant^–1. R:S is the ratio of root to shoot biomass. Different letters indicate significant differences as a function of CO₂ concentration for a given parameter and species.

Species	[CO2]	Leaf wt.	Stem wt.	Root wt.	R:S
Canada thistle	284	6.79 c	1.27 c	3.71 c	0.50
(Cirsium arvense)	380	15.02 b	3.42 b	7.82 b	0.42
	719	24.71 a	6.06 a	13.58 a	0.44
Field bindweed	284	8.38 c	4.55 c	7.01 c	0.51 c
(Convolvulus arvensis)	380	12.31 b	6.55 b	12.99 b	0.71 a
	719	17.52 a	8.73 a	17.01 a	0.64 b
Leafy spurge	284	2.75 c	1.02 c	1.84 b	0.49 a
(Euphorbia esula)	380	4.21 b	1.27 b	2.37 a	0.43 ab
	719	6.13 a	2.17 a	2.93 a	0.35 b
Perennial sowthistle	284	7.33 c	1.91 b	5.30 c	0.58 b
(Sonchus arvensis)	380	16.17 b	2.27 b	16.16 b	0.88 a
	719	25.89 a	4.79 a	26.72 a	0.86 a
Spotted knapweed	284	10.81 c	2.49 c	3.64 c	0.26 b
(Centaurea maculosa)	380	21.72 b	4.76 b	9.94 b	0.38 a
	719	30.03 a	6.30 a	16.84 a	0.46 a
Yellow star thistle	284	11.43 b	2.61 b	5.25 c	0.42 ab
(Centaurea solstitialis)	380	21.01 a	5.35 a	9.74 b	0.36 b
	719	24.63 a	6.75 a	14.25 a	0.46 a
Average	284	7.92 c	2.31 c	4.46 c	0.49 b
	380	15.07 b	3.94 b	9.81 b	0.52 b
	719	21.49 a	5.79 a	15.22 a	0.56 a

Table 4.

Change in relative growth rate (RGR, g g^–1d^–1) for six noxious weed species grown at either 284, 380 or 719 µmol mol^–1CO₂ Different letters indicate significant differences between CO₂ treatments for a given sampling date and species.

Species	CO2	DAS
		23	35	54	60
Canada thistle	284	0.210 b	0.232	0.118
(Cirsium arvense)	380	0.307 a	0.224	0.126
	719	0.169 a	0.232	0.122
Field bindweed	284	0.214 b	0.191	0.103
(Convolvulus arvensis)	380	0.192 b	0.193	0.107
	719	0.283 a	0.177	0.099
Leafy spurge	284		0.160	0.091	0.105
(Euphorbia esula)	380		0.150	0.106	0.065
	719		0.137	0.101	0.077
Perennial sowthistle	284		0.230	0.139	0.059 b
(Sonchus arvensis)	380		0.245	0.120	0.083 a
	719		0.236	0.114	0.098 a
Spotted knapweed	284		0.197	0.121	0.044 b
(Centaurea maculosa)	380		0.200	0.124	0.096 a
	719		0.198	0.116	0.080 a
Yellow star thistle	284	0.395 a	0.173	0.113
(Centaurea solstitialis)	380	0.311 b	0.194	0.119
	719	0.321 b	0.190	0.103
Average	284	0.273	0.197	0.114	0.069
	380	0.270	0.201	0.117	0.081
	719	0.258	0.195	0.109	0.085