Community assembly of tropical Fagaceae-dominated forests in Thailand dates back at least to the Late Palaeogene

The Late Oligocene to Early Miocene flora of the Ban Pa Kha Subbasin (Li Basin, northern Thailand) provides a record of montane dry tropical oak-pine forests. The rich ensemble of Fagaceae typical of these forests might have existed in the wider region of Southeast Asia since Eocene times and various fossil plant assemblages represented both lowland (Fagaceae, Dipterocarpaceae) and upland (Fagaceae, Pinaceae) tropical forests. These findings are in conflict with previous interpretations of vegetation development in northern Thailand, which stressed that stratigraphically older (possibly Late Oligocene) spore and pollen assemblages in northern Thailand were markedly different from the modern tropical flora and had a distinct northern temperate character. A major change in climate would have caused a dramatic shift to tropical conditions since the Mid-Miocene. Considering palaeobotanical data from adjacent regions in Southeast Asia, we suggest that differences in spore and pollen assemblages in intermontane basins in northern Thailand are more likely to represent different facies and lowland/upland settings. Assembly of these forest ecosystems, typically comprising Quercus sections Cyclobalanopsis , Ilex and Quercus , pollen of Castaneoideae with affinities to Castanopsis and Lithocarpus , and extinct fagaceous genera such as Eotrigonobalanus , most probably dates back to the Eocene. The absence of oaks of Quercus section Cerris in the spore and pollen assemblage of the Ban Pa Kha Subbasin, despite this group being part of the modern vegetation, might reflect the late arrival (secondary radiation) of this chiefly temperate group in tropical Southeast Asia.

In the present study, we investigated fossil pollen grains of Fagaceae from the Ban Pa Kha Subbasin, Li Basin, Lamphun Province, using a combined light microscopy (LM) and scanning electron microscopy (SEM) analysis to obtain the highest possible taxonomic resolution.The great diversity and systematic affinities of pollen of Fagaceae identified in our investigation along with a preliminary assessment of the entire spore and pollen assemblage from the Ban Pa Kha Subbasin formed the basis to infer the palaeoenvironment(s) for this subbasin during Late Oligocene to Early Miocene times.In addition, we discussed the timing of the assembly of modern forest types in Thailand.

GeoloGical settinG of the Ban Pa Kha suBBasin
The Li Basin is a Cenozoic rift basin in the northern part of Thailand.Cenozoic sediments are exposed in the centre of the basin, whereas Palaeozoic rocks surround Downloaded from https://academic.oup.com/botlinnean/article/202/1/1/7010368 by Faculty of Life Sciences Library user on 11 December 2023 the basin (Morley & Racey, 2011).The depositional history of the basin was described by Nichols & Uttamo (2005).The Cenozoic sequence can be divided into two formations (Fm), the Oligocene to Early Miocene Li Fm comprising sandstone, mudstone and oil shale, and the Early to Mid-Miocene Mae Long Fm consisting of calcareous mudstone and lignite intercalations.The Li Basin comprises four main subbasins, Ban Pu and Ban Pa Kha subbasins in the east, Mae Long Subbasin in the south-west and Ban Na Sai Subbasin in the south (Fig. 1A) (Ratanasthien, 1990).
The sedimentary succession of the Ban Pa Kha Subbasin comprises five sedimentary rock layers c. 250 m thickness.According to Nichols & Uttamo (2005), the sediments accumulated in fluvial to lacustrine environments.The underburden layer, > 30 m thick, is composed of coarse sandstone with gravels.The lower coal seam, 15-20 m thick, consists mainly of coal, claystone and shale.The interburden layer, 10-15 m thick, is mostly composed of different types of shale.The upper unit of this layer is made up of coaly shale partly with claystone, whereas the lower unit is composed of oily shale with claystone and sandstone.
Due to the lack of datable rocks or vertebrate remains, the small subbasins in the Li Basin are difficult to date.On the basis of the geological structure and tectonic evolution, the age of the Li rift basin was determined as the Late Oligocene to Early Miocene (Morley, 1998;Morley, Charusiri & Watkinson, 2011).Watanasak (1990) and Songtham et al. (2003Songtham et al. ( , 2005) ) also suggested that within the Li Basin, basin filling of the Ban Pa Kha Subbasin took place from the Late Oligocene to the Early Miocene based on the presence of fossil pollen assigned to temperate elements.However, as  Sawangchote et al., 2009Sawangchote et al., , 2010)).B, Stratigraphic chart illustrating sedimentary rocks of the Ban Pa Kha Subbasin (modified from Morley et al., 2001) and rock samples processed for the present study.
pointed out by Morley et al. (2011), dating solely based on palynological data is difficult and a number of plant macrofossils from the Ban Pa Kha Subbasin could be affiliated with subtropical/tropical taxa (Grote & Srisuk, 2021).Nevertheless, for the present study we accept a Late Oligocene to Early Miocene age for the Ban Pa Kha Subbasin.

RESULTS
Fifteen fossil pollen taxa of Fagaceae from subfamilies Castaneoideae and Quercoideae were observed from the Ban Pa Kha Subbasin, Li Basin, with high percentages (Table 1).
Still, none of these is comparable with the Quercus section Cyclobalanopsis sp. 1 pollen type described here.
QuErcus section cyclobalanopsis sP. 2 (fiG.3a-c)  (Hofmann, 2010), the late Early Eocene (Ypresian) Princeton Chert, British Columbia, Canada (Grímsson et al., 2016a), the Early Oligocene of Cospuden, Saxony, Germany (Denk et al., 2012), the Mid-Miocene of Lavanttal, Austria (Grímsson et al., 2016b), the Mid-Miocene of Anatolia, Turkey (Bouchal et al., 2016a(Bouchal et al., , 2017;;Bouchal, 2019), the Late Miocene of north-western Greece (Bouchal et al., 2020), the Pleistocene of south-eastern Albania (Denk et al., 2021b)     (2011).The ultrastructure (TEM) of section Quercus has been studied to a lesser extent, but see Wang & Pu (2004) and Denk & Tekleva (2014).The areolate sculpture elements, observed in the fossil Quercus section Quercus pollen type, that are apparently composed of smaller agglomerated elements but masked/covered by sporopollenin is a typical feature for this section (see fig. 3 in Denk & Grimm, 2009).The palynological record of Quercus section Quercus type pollen, documented using combined LM and SEM, includes dispersed grains from the Mid-Eocene (Late Lutetian to Early Bartonian) of Hareøen, western Greenland (Grímsson et al., 2015), the Late Eocene (Priabonian) of Florissant, Colorado, USA (Bouchal et al., 2014), the Late Oligocene of southwestern Siberia (Denk et al., 2021a), the Miocene of eastern China (Liu et al., 2007), the Mid-Miocene of Anatolia, Turkey (Bouchal et al., 2016a(Bouchal et al., , 2017;;Bouchal, 2019), the Late Miocene of Iceland (Denk et al., 2010) and north-western Greece (Bouchal et al., 2020) and the Pleistocene of south-eastern Albania (Denk et al., 2021b).Some of these records bear similarities to the fossil Quercus section Quercus sp.pollen type presented here.We recognized 15 fossil species of Fagaceae belonging to two subfamilies in the pollen assemblage of the Li Basin (Table 2).Four species belong to extinct fagaceous lineages (Eotrigonobalanus, two species; Fagaceae gen.& spec.indet., two species).The remaining species can be assigned to three sections in two subgenera of Quercus (sections Cyclobalanopsis, Ilex and Quercus), and to unresolved taxa of subfamily Castaneoideae, of which one pollen morphotype shows morphological affinities with the extant genera Castanopsis and Lithocarpus.Overall, this suggests that except for the extinct taxa, most of the fagaceous diversity in the Late Palaeogene/Early Neogene of northern Thailand is also found in the modern vegetation of Thailand.

Subfamily
A strikingly similar taxonomic composition and diversity of Fagaceae has previously been reported from the Mid-Eocene strata of southern China (Changchang Fm, Hainan Island;Hofmann, 2010;Spicer et al., 2014), including the extinct Eotrigonobalanus and sections Cyclobalanopsis, Ilex and Quercus among oaks.In addition, several Castaneoideae pollen morphotypes were recorded from the Changchang Fm.The leaf fossil record from the Changchang Fm (Spicer et al., 2014;Liu et al., 2020) included Castanopsis, Lithocarpus, extinct Castaneoideae and Quercus section Cyclobalanopsis.Palaeoclimate estimates derived from leaf physiognomy suggested a fully humid cool tropical climate for the Changchang leaf assemblage, whereas the palynological record with diverse oaks of section Ilex points to precipitation seasonality and a possible monsoon influence.Pollen assemblages similar to the one from the Li Basin are further known from the Late Eocene/Early Oligocene Maoming Basin, South China (Herman et al., 2017).The pollen flora from the Maoming Basin records Fagaceae, Dipterocarpaceae Downloaded from https://academic.oup.com/botlinnean/article/202/1/1/7010368 by Faculty of Life Sciences Library user on 11 December 2023 and Pinaceae as dominant elements.Among Fagaceae, Eotrigonobalanus, unspecified Quercus, Quercus spp.(verrucate; possibly corresponding to Quercus section Quercus), Quercus section Cyclobalanopsis and Castanopsis were identified.The Early Oligocene leaf assemblage from the Maoming Basin suggested a clear monsoon signal for this flora (Herman et al., 2017).From roughly coeval strata in the Wenshan Basin, south-western China, Li et al. (2015b;see Tian et al., 2021 for a corrected age) reported an Early Oligocene pollen assemblage with Castanopsis, Castaneoideae (as Castanea), Quercus unspecified and Quercus section Cyclobalanopsis.Overall, the spore and pollen assemblage from Wenshan suggested a seasonal climate with weak monsoon intensity.
Well-preserved wood remains from the Oligocene Yongning Fm, South China, comprised the Fagaceae Lithocarpus (one species) and Castanopsis (two species; Huang et al., 2018).The preserved wood characteristics, weak to absent growth ring boundaries and prominent ring-porous wood, suggested a tropical monsoon climate for this part of China during the Oligocene.Finally, Mu et al. (2015) described fruits of Lithocarpus from Miocene strata of the Nanlin Fm (Yunnan, southwestern China).The age for this formation is not well constrained, but on the basis of palynology, an Early to Mid-Miocene age has been suggested.

Present veGetation in thailand and climatic
and Biotic enveloPes of forest trees in faGaceae Forest vegetation in the western and north-western parts of Thailand comprises remnants of closed monsoon forests in the lowlands, dry dipterocarp savannah forests at lower elevations and montane pine-oak forests at mid to high elevations (Menitsky, 2005).Closed monsoon forests contain a rare Quercus section Cyclobalanopsis, whereas higher up, dipterocarp forests occur on dry soils and mixed dipterocarp-Fagaceae forests on mesic soils of gentle slopes.The latter forests are rich in Fagaceae (Quercus, Lithocarpus and Castanopsis).On dry soils, Quercus section Ilex thrives in the upper tropical belt.
Above c. 900 m, dipterocarp forest is replaced by subtropical dry or wet pine-oak or oak (dominated by Fagaceae) forest.The relatively drier pine-oak forest is dominated by diverse oaks of all sections of subgenus Cerris and by Lithocarpus spp.and Castanopsis spp.In some places, species of Quercus section Ilex grow up to 2000 m a.s.l.
A c c o r d i n g t o t h e K ö p p e n -G e i g e r c l i m a t e classification, Thailand is characterized by a 'tropical savannah' climate (Aw, Kottek et al., 2006;Peel, Finlayson & McMahon, 2007).This is in accordance with the grassy ground cover found in many pine-oak forests and corresponds to the Tropical and Subtropical Dry Broadleaf Forests biome of Olson et al. (2001) and oriGin of seasonal troPical faGaceaedominated forests in northern thailand Fossil assemblages with a particularly high diversity of Fagaceae have been reported from several mid-to high-latitude localities in the Northern Hemisphere (e.g.Denk et al., 2012, Central Europe;Bouchal et al., 2014, western North America;Pavlyutkin, Chekryzhov & Petrenko, 2014, Russian Far East;Grímsson et al., 2016a, western Greenland;Grímsson et al., 2016b, Central Europe;Prader et al., 2020, eastern North America;Sadowski et al., 2020, Baltic amber region of North Europe).In these areas, hyper-diverse fagaceous assemblages were of ephemeral nature coinciding with warm phases in Earth history (western North America, Greenland, Baltic amber, Far East) and their diversity decreased dramatically during the Neogene when the modern vegetation in these areas developed (e.g.Mai, 1995;Graham, 1999).
In contrast, characteristic forest communities of tropical lowlands and mid-elevations (Tropical and Subtropical Moist Broadleaf Forests, Tropical and Subtropical Dry Broadleaf Forests) in Southeast Asia might have existed since the Eocene, reflected by fossil plant assemblages (pollen and leaf assemblages), with the dominance of Fagaceae and variably co-dominating Dipterocarpaceae or Pinaceae (e.g.Herman et al., 2017).The presence of species of Quercus sections Cyclobalanopsis and Ilex is important in distinguishing wet forests from monsoonal, seasonal forest vegetation.For example, Huang et al. (2022) reported forest vegetation with Fagaceae and Dipterocarpaceae from the Oligocene Ha Long flora of Vietnam.Extinct Castaneoideae (Berryophyllum Jones & Dilcher and Castaneophyllum Jones & Dilcher), Lithocarpus and Quercus section Cyclobalanopsis represent a lowland tropical evergreen forest corresponding to a modern vegetation in southern Thailand and less so to the palaeovegetation of the Ban Pa Kha Subbasin of the Li Basin.However, extant members of Quercus sections Cyclobalanopsis and Ilex can also be found in northern Thailand (Phengklai, 2006;Santisuk, 2012).
Our recognition of tropical montane pine-oak and mesic 'oak' (Fagaceae-dominated) forest in the Late Oligocene to Early Miocene Ban Pa Kha Subbasin of the Li Basin, based on the high diversity and proportion of fossil Fagaceae pollen (Tables 1 and 2), conflicts with previous interpretations of the palynological record from Cenozoic deposits of Thailand (Morley et al., 2001;Songtham et al., 2005), which suggested a north temperate (Late Oligocene) flora that would have been replaced by a tropical flora after the Early Miocene.According to these authors, sediment fill of the Ban Pa Kha Subbasin would have coincided with temperate conditions during the Late Oligocene/Early Miocene.We interpret the varying contributions of 'temperate' and 'tropical' taxa in the dispersed spore and pollen record of Thailand as reflecting different facies and lowland/upland settings.This is in agreement with previous findings of macrofossils in the Ban Pa Kha Subbasin that could be affiliated with subtropical/ tropical taxa (Grote & Srisuk, 2021).
A typical temperate element in the Late Oligocene/ Early Miocene flora of Thailand would be Fagus (Table 3).However, previous leaf records (Endo, 1967) and pollen records (Songtham et al., 2003;Sattraburut et al., 2021) clearly indicate alliances outside Fagaceae.Endo (1967) assigned leaf remains from the Ban Pa Kha Subbasin as Fagus feroniae Unger, a taxon that has subsequently been transferred to Alnus Mill.(Kvaček & Holý, 1974).Similar leaf morphotypes are not only encountered in modern Alnus, but also in Dipterocarpaceae (see Khan et al., 2020).Pollen described as Faguspollenites by Songtham et al. (2003) only superficially resembles Fagus but differs by its wide colpi and sculpture elements of the colpus membrane that are much too large for Fagus.Likewise, a pollen grain figured by Sattraburut et al. (2021) from Miocene strata of Laos close to the Li Basin and referred to as Fagus, clearly differs from this genus by the structure of its pori and the conspicuous thickening in the colpusporus region (suggesting Cornaceae/Nyssa Gronov.ex L.).Therefore, records of Fagus from the Late Oligocene and Miocene of Thailand and adjacent regions would need to be re-investigated.In combination with a highly diverse assemblage of Quercus pollen types representing the evergreen sections Cyclobalanopsis and Ilex, along with diverse Castaneoideae and tropical elements among leaf macrofossils (cf.Sawangchote, 2003;Sawangchote et al., 2009Sawangchote et al., , 2010) ) and fossil pollen (Table 3), we note the following: (1) The clear distinction between temperate and tropical forest associations is not given whereby the Late Oligocene/Early Miocene assemblages have a distinct tropical component; (2) The presence of evergreen oak lineages indicative of seasonal climates would suggest that the Tropical and Subtropical Moist/Dry Forests biomes were in place by the Late Palaeogene; and (3) The change to true tropical forest assemblages with increased numbers of tropical taxa, such as Dipterocarpaceae, might as well reflect changes from lowland to upland vegetation, slope aspect and soil moisture.Hence, in addition to reflecting a profound climatic change across the Palaeogene/Neogene boundary, these different plant associations may also reflect different facies/  (Santisuk, 2012).In summary, our new data suggest that seasonal (monsoon) tropical climates, which were probably cooler than they are today, and their corresponding forest vegetation existed in northern Thailand at least since the Late Palaeogene and persisted into the modern vegetation of Thailand.Apparently, no sharp change from (northern) temperate to tropical environments occurred between the Late Oligocene and the Mid-Miocene.
Although Quercus sections Cyclobalanopsis and Ilex are abundant in the Late Oligocene/Early Miocene flora of the Li Basin, deciduous or semi-deciduous elements of Quercus section Cerris are absent from the fossil pollen assemblage.In contrast, the latter is part of contemporaneous dry oak-pine forests (Menitsky, 2005).Likewise, Quercus section Cerris is absent from the Palaeogene forest in Southeast Asia and the southern Himalayas (Hofmann, 2010;Denk & Bouchal, 2021).Members of Quercus section Cerris are essentially temperate species (Naryshkina & Evstigneeva, 2020;Denk & Bouchal 2021) and may have reached tropical areas during a secondary radiation in the later part of the Miocene.

Figure 1 .
Figure 1.The geographical and geological setting of the Li Basin, northern Thailand.A, Map showing the geographical location of the Li Basin and its subbasins (BP = Ban Pu; BPK = Ban Pa Kha; BNS = Ban Na Sai; ML = Mae Long) (modified from Sawangchote et al., 2009, 2010).B, Stratigraphic chart illustrating sedimentary rocks of the Ban Pa Kha Subbasin (modified from Morley et al., 2001) and rock samples processed for the present study.

Table 1 .
Percentages of fagaceous and pinaceous fossil pollen recovered from the Ban Pa Kha Subbasin Sadowski et al. (2020)rímsson et al., , 2016a) )o Eotrigonobalanus pollen from the Cenozoic of North America and Europe (see Remarks for Eotrigonobalanus sp.1).Although we recognize two distinct (pollen) fossil species, it is possible that they reflect the morphological range of a single biological taxon.Previous studies on both fossil pollen (e.g.Grímsson et al., 2015Grímsson et al., , 2016a) )and extant pollen (e.g.Makino et al., 2009)of Fagaceae have shown that several fagaceous taxa exhibit highly variable pollen grains considering their pollen size and particularly sculpture arrangement.In contrast,Sadowski et al. (2020)established different fossil species of Eotrigonobalanus from Eocene Baltic amber deposits based on distinct male flowers with in situ pollen.In this case, different fossil species produced nearly identical pollen morphologies.
Stuchlik et al. (2014)//academic.oup.com/botlinnean/article/202/1/1/7010368 by Faculty of Life Sciences Library user on 11 December 2023 Castaneoideae pollen grains are typical elements of numerous palynofloras from the Upper Cretaceous and Cenozoic of the Northern Hemisphere.According to summaries inMuller (1981)andStuchlik et al. (2014), the earliest Castaneoideae-type pollen records are from the Santonian/Campanian of North America and Campanian of Europe.The indet. 1 type has conspicuous endopori and the nexine is thickened around the pori.This is not observed in the indet. 2 pollen type.Also, the size of the microrugulae is smaller in the indet. 1 pollen and pollen grains have a granulate suprasculpture not noted for the indet. 2 type.
(De Langhe, 2022)9)tQuercus pollen demonstrated that by using LM features (pollen size, wall thickness and aperture arrangement/ peculiarities) and sculpture elements overserved with SEM, it is possible to discriminate major subgenera and sections of genus Quercus (e.g.Denk & Grimm, 2009).The only exception is that sections Quercus and Lobatae Loudon (red and white oaks) have several taxa that produce comparable pollen, and the small sections Ponticae Stefanoff., Protobalanus (Trelease) Schwarz and Virentes Loudon correspond largely with the pollen morphology of white oaks.In Quercus subgenus Cerris Oerst.(sectionsCerrisDumort., Cyclobalanopsis and Ilex Loudon), pollen sculpture is highly useful for recognition of sections.Quercus championii currently occurs at low-to mid-elevations (100-1700 m) in Taiwan and south-western to southern China(De Langhe, 2022).The fossil pollen record of Quercus section Cyclobalanopsis is meagre and only a few isolated grains investigated with combined LM and SEM have been assigned to this section.These include records from the Eocene of Hainan Crepet & Daghlian, 1980) are aware of only a single species, Quercus championii Benth.(seefig.26inCrepet&Daghlian, 1980), producing pollen with comparable sculpture elements.
This is the first detailed study into Fagaceae diversity during the Late Palaeogene/Early Neogene of Thailand.
: not determined

Table 2 .
Taxonomy of dispersed Fagaceous pollen from the Ban Pa Kha Subbasin Downloaded from https://academic.oup.com/botlinnean/article/202/1/1/7010368 by Faculty of Life Sciences Library user on 11 December 2023 its wetter variant, the Tropical and Subtropical Moist Broadleaf Forests biome.Overall, the fossil record of Fagaceae from the Li Basin suggests that similar forests existed in northern Thailand since the start of the Early Neogene, if not since the Palaeogene (see the previous section).

Table 3 .
Continued Downloaded from https://academic.oup.com/botlinnean/article/202/1/1/7010368 by Faculty of Life Sciences Library user on 11 December 2023 environmental conditions.Sepulchre et al. (2010) reported high amounts of Poaceae pollen in the Mid-Miocene deposits of Thailand.Poaceae are co-dominant with Castanopsis, Quercus and Pinus, reflecting a dry tropical pine-oak forest with grass ground cover