NAT. NAT. HIST. BUL L. SIAM So c. 49: 295-303 ， 2001
PHOTOSYNTHETIC CAPACITY IN THAI CONIFERS
Philip Philip W. Rundel 1，Mark Patterson 2， Kansri Boonpragob 3 and Santi Watthana 4
ABSTRACT
Ecophysiological Ecophysiological studies were carried out to detennine photosynthetic capacity and as-
sociated sociated gas exch 飢 .ge characteristics of seven species of conifers growing under cornrnon g訂 den conditions in the Queen Sirikit Botanic Garden in the Mae Sa ne Valley 訂 Chi 釦 g Mai ， northem northem Th ailand. Rates of net photosynthesis under conditions of non ・limiting light and water availability availability ranged from a high of 7.9-8.0μmol m- 2・S-I in Pinus kesiya and P. merkusii to a
low low of 2.0 in Podocarpus wallichianus. C 訂 bon isotope ratios (o) of -24 .1 %0 in this latter species species indicated a high degree of water use efficiency (WUE) ，while 出巴 two pines ，C ephalot a.x us griffithii griffithii and Dacrydium elatum ， showed low WUE with o values of -29.3 to -30 .4 %0. ・Thai conifers conifers appear to have ecophysiological traits of photosynthetic capacity ，stomatal conduct-
如 ce ，組d water use efficiency comp 訂 'able to those of in North American temperate conifers Our data suggest that inherent limitations in the structural characteristics of the photosynthetic and and water 甘ansport systems in conifers are equally applicable to tropical as well as temperate conifers conifers in mainland Southeast Asia.
INTRODUCTION
Wh ile there has been a rapidly increasing interest in recent years in the physiological ecology of conifers (SMITH & HINCKLEY ，1995) ， this work has focused almost exclusively on temperate zone conifers ，particularly those in th 巴 genera Pinus ，Abies 叩 d Picea. Relatively little little is known about tropical conifers. Even within a well studied group like the pines in which there are many studies of photosynthetic capacity ， there have been almost no ecophysiological studies of tropical species growing in areas where frost is not an environmental limiting factor (RUNOEL & YOOER ， 1998). Th e conifer flora of 百四land is a small one ，with only 11 species. Six of these 紅 e 加 the the genus Podocarpus sensu latu ，with other genera being Pinus (2 species) ，and Calocedrus ， Da C1 ッdium ，and Cephalot a.x us with a single species each. While all of these species are Southeast Asian endemics in the broad sense ，and occur naturally in lower montane forests of northern and northeastern Thailand ，they represent distinctly different patterns of biogeographic dis 凶 bution. The genus Cephalot ω， us ， for example ， is largely centered in Japan and China ，and reaches its southern limit of occurrence with C. gr 伊 thii in northern
IOepartmen 直of Organismic Biology ，E ∞ logy and Evolutio 旧， University of Califomia ， Los An geles ，Califo rnI a USA 90096 (rundelial biolo l! v .ucla.edy)
20epartment 20epartment of Botany ， Oregon State Univ 引 sity ，Corvallis ，01 官gon USA 97331
30 epartm 叩 t of Biology ，Ramkhamhaeng University ， Bangkok 10242 ，Thailand (kansriialr 百m 1. ru.ac 出) 4Queen 4Queen Sirikit Bot 四 ic Gar 台 n， P.O. Box 7，Mae Rim ， Chiang Mai 50180 ，Thailand
Received Received 16 J飢 uary 2001; accepted 6 November 200 1.
295 296 PHILIP W. RUNDEL ，MARK PATIERSON ，KANSRI BOONPRAGOB ， AND SANTI WATIHANA
Th ailand. In contrast to this pattern ，出 e genus Dac η Idium is largely a southern hemisphere group ， with its primary occuη'ence in southern Chile ，New Ze aland ，Tasmania ，and New Caledonia. Caledonia. It reaches its northern limit of distribution with D. elatum in northernτ 'h ailand. Two tropical pine species in Th ailand ， P. kesiya and P. merkusii ， are widespread species in in Southeast Asia and form 白e most southern occuηence of Pinus 泊 the world ， with P. P. merkusii reaching south of the equator in Sumatra. We 釘 e not aw 釘 'e of any previous studies of photosynthetic capacity in conifer species 合om mainland Southeast Asia. Nor 仕lern Th ailand presents an appropriate 紅 ea to study the ecophysiology ecophysiology of such conifers because all five genera and nine of the 11 Th ai conifers occur occur naturally in this region. Two questions interested us in carry 泊g out 出is research.τ 'h e frrst was: Do tropical conifers conifers growing 泊 a mild ， seasonal monsoon climate have higher photosyn 出etic rates than than temperate conifers? If w 釘 m growing season conditions with abundant water ，low vapor vapor pressure gradients with humid air ， and the absence of a cold season present ideal conditions conditions for conifer growth ， one might expect photosyn 出etic rates higher th 叩 those of temperate temperate conifers. However ， if the structural characteristics of conifer foliage and their associated associated hydraulic systems with fiber tracheids constrain the potential for high rates of carbon carbon fixation ， then comparable rates would be expected. Direct comp 紅 isons within a single single genus can be achieved here with Pinus. τ'h e second question is a special case of the frrs t. Th e genus Podocarpus sensu latu includes a number of species whose foliage is flattened flattened and broad ， with “needles" reaching 5ι70 mm in width in P. wallichian 札Th ere have have been very few reports of photosynthetic rates measured under field conditions for members of the Podocarpaceae. Recent studies with Podocarpus oleifolius in the high
An des of Venezuela (CA VIE 阻 S ET AL. ，2000) 佃 d P. coriaceus in the West In dies DUCREY ， 1994) 1994) have reported relatively low rates of maximum net photosynthesis. Do broad-leaved conifers conifers such as these have higher rates of phoωsynthesis than those species of Podocarpus and other co 凶fers with n紅 TOW needles? If such higher rates of photosynthesis exist in broad-leaved broad-leaved Podocarpus ， are they comparable to those of temperate hardwood trees?
MATERIALS AND METHODS
Study Study Species
Trees Trees of seven conifer species were investigated 泊 common g紅 den pl 釦 tings at the Queen Sirikit Botanic Garden in the Mae Sa Valley (ca. 700 m elevation; la t. 18 0 53' N ， long. long. 78 0 56' E) ne 紅 Chiang Mai ， close to the habitat natural of these species. Wi 白白e exception exception of Pinus kes か'a ，which averaged approximately 10m tall ，experimental trees were young individuals 3ー7m in height. Two individuals of each 住民 and multiple leaves on each individual were studied ， with the exception of Podocarpus wallichianus which was represented represented by on1 ya single individual. All study trees were healthy individuals with good growth growth characteristics.τ 'h e general characteristics of the species conifer included for study are are described below. For more information see PHENG 乱 AI (1972) ，KRUSSMAN (1985) ， HIEP & VIDAL (1 996). Cephalotaxus Cephalotaxus griffithii Hook. is a small-to-medium size trl 田. It has stiff needles 2-5 cm 泊 length ， glossy green above but white below. It is distributed from Assam in East PHOTOSYNTHETIC CAPACITY IN THAI CONIFERS 297
India India across North Burma to North and Northeast Thailand where it occurs at 100 0- 1800 m elevation in the lower montane zone of Chiang Mai ， Chiang Rai ， and Loei Provinces ， as as a subcanopy tree in moist evergreen fores t. Dacrydium elatum (Roxb.) Wal l. ex Hook. Typically reaches 35 m in height and 7ι120 cm in diameter. The leaves are small (8-16 mm) and scale-like ， in densely overlapping overlapping a町 angement on the fertile branches. It is reported to occur from East India across across North Burma to Thailand ， and south to Cambodia and Malaysia ， and eastward to the Philippines Philippines and F討i. It occurs at 100 0- 3000 m elevation in North ，Northeast ，East ， and Central Central Th ailand and is typically found along streams on soils derived from sandstone. In Indochina ，this species has an elevational range of 70 0- 2000 m. In the Cardamom and 日ephant Mountains of South Cambodia it occurs in dwarf conifer stands on boggy sites ， frequently frequently with Podocarpus imbricatus. Pinus Pinus kesiya Royale ex Gard. has a spreading or rounded crown ， and needles occur in in bundles of 3 and 100-250 mm in length. Th is species is distributed from Burma across mainland mainland Southeast Asia to the Philippines. In Th ailand is common in the North and Northeast Northeast at 100 0- 1600 m elevation. P. kes 切 may occur mixed with montane hardwood trees trees or in relatively pure stands. It is relatively tolerant of frequent buming ， and thus has expanded its dominance in many montane habitats under human intervention. The biogeographical biogeographical and ecological pattems of distribution of this species in Th ailand have been been described in detail (WERNER ， 1993; SANTISUK ， 1997). Pinus Pinus merkusii Jungh. & De Vriese likewise has a spreading to rounded crown; needles occur occur with 2 in a fascicle ， and reach 150 ー250 mm in length. Th is pine occurs from East In dia across Southeast Asia to the Philippines to the east ， and south to Sumatra where it becomes the only pine species that naturally occurs south of the equator. In Th ailand it is a common species in the North ，Northeast and East where it occurs in dry deciduous dipteroca 叩 forests or lower montane forests at 60 0- 1300 m elevation (SANTISUK ， 1997). Rarely ，this species may occur together with the higher elevation P. kesiya. Podocarpus Podocarpus imbricatus B l. [=D αcη carpus imbricatus (B l.) De Laubenf.] is a large forest forest tree reaching 30 m in height and 120 cm in diamete r. Two forms of leaves 紅 e commonly present in this species ， with both occasionally found on the same individua l.
Leaves Leaves on young or rapidly extending branches are linear ，6- 12 mm in length ， and arr 佃 ged in in two flattened ranks that collectively resemble a compound leaf. Older trees typically have have small ，awl-like leaves ，resembling a small Cryptomeria ， but only 2-5 mm long. Th is species species is widely distributed across Southeast Asia from East India across Burma to Laos and and Cambodia ， and on to the west and south to the Philippines ，Malaysia ，Indonesia and New Guinea. It occurs in North ，Northeast and Southeast Thailand scattered in evergreen forests forests at 700 ー1200 m elevation. It is a common associate of Dac η diun elatum in dwarf conifer conifer forests growing in boggy sites in the Cardamom and Elephant Mountains of South Cambodia. Cambodia. Podocarpus neriifolius o. Oon. is a large tree reaching 30 m or more in height and 200 cm diameter. It s coriaceous leaves 訂 e linear-lanceolate in shape but highly variable in in size ， with a typic a1 1ength ranging from 50 to 200 mm and a width of 5-25 mm.τbis species species is widely distributed from eastem India and southem China through Th ailand ， Laos ，Cambodia ， and Vietnam to Indonesia and F訪i. It occurs throughout Th ailand where it it is frequent in moist evergreen forests at 600 ー1300 m elevation. 298 PHILIP W. RUNDEL ，MARK PA TfER SON ，KANSRI B∞NPRAGOB ，AND SANTl WATIHANA
Podocarpus wallichianus Pr es l. [=Nageia wallichiana (Pr es l.) De Laubenf.] can reach 48 m in height ， although only 10 ー20 m 泊Thai1 and. Its coriaceous leaves are typically 10 0- 180 mm 泊 leng 出， but remarkably 3ι70mm 泊 width. It is dis 凶buted from East In dia across mainland Southeast Asia to In donesia and New Guinea. is It found 也roughout Th ailand 泊 wet evergreen forests from sea level up to 2000 m elevation.
Field Field Measurements
All All field measurements were carrled out on December 5. and 6，1996 ，泊 the Queen S廿訟it Botanic Garden. 百lis season corresponded to 白 e frrst month of the dry season when cle 訂 skies ，good soil moisture availability and moderate temperature extremes provide ideal ideal conditions for tr ，閃 growth. Study trees were growing under comm 'O n garden c'O nditi 'O ns with with c'O mp 釘 'able s 'O ils and water availability. Midday water p 'O tential measurements were made between 11:20 and 13:00 h'O urs with a Schollander-type pressure chamber f'O ll 'O w 泊g standard standard procedures.τ 'h e water potential 'O f3 -4 samples 'O f each species were summed and averaged averaged t 'O calculate mean midday water potentials in mega Pascals. G 槌 exch 佃 ge me 儲 urements were made with a LICOR ・6200 g出 exchange system using using a cl 'O sed fl 'O w architecture. An artificiallight source was used t 'O bring ambient light levels levels ab 'O ve 1200 tJ. m 'O l m- 2.s- t， a level 企om 'O ur experience 白紙 is 副知rating f'O rc 'O nifers. A c 'O ld mirr 'O r prevented heating 'O f the c 'O nifer f'O liage. F 'O rm 'O st species studied ，6- 10 cm 2 of of the y'O ungest fully mature f 'O liage was placed int 'O血 e cuvette. F 'O r the fme leaves 'O f Podocarpus imbricatus ，13 ー18 cm 2 of f 'O liage was used ， while the dense wh 'O rls 'O f small awl-shaped awl-shaped needles of Dacrydium elatum put 29-36 cm 2 of foliage int 'O the cuve 悦 .τ 'h e actual actual pr ，吋 ected leaf 紅 ea 'O ff 'O liage used 恒 each measurement was determined after measurements using a LICOR leaf area meter. Two foliar measurements 'O f net ph 'O tosynthesis ，st 'O matal c 'Onduc剛院姐d 凶ern~ CO 2 c'O ncen 回 ti 'O n were made f'O r each individual individual studied. Leaf temperatures averaged 28" C (range 26 ー30" C) during gas exchange measurements ，cl 'O sely 仕'acking ambient air tempera 佃 res.τ 'h e me 組 relative humidity was 75% (range 63 -8 0%) ， giving a mean vap 'O r press 町 'e gradient 'O f 0.9 kP a. Natural 13C rati 'O s we 陀 measured at the Duke University Phytotr 'O n (D 町 ham ，N 'O rth C 釘 olina) 'O na SIRA Series Series 11 is 'O t'O pe rati 'O mass spectr 'O meter (VG Isotech ，Middlewich ，UK) 'O perated in aut 'O matic 回 pping m 'O de after c'O mbusti 'O n 'O f samples 泊佃elemental analyzer (N AI500 ， Carlo Carlo Erba In strumentazi 'O n，Milan ，It aly).τ 'h e reference CO 2，calibrated aga 泊st standard Pee Dee belemnite (PDB) ，was obtained 合'O m Oztech (Dallas ，Texas). Carb 'O n isot 'O pe rati 'O was expressed as o13 C 泊 units 'O f negative parts per th 'O usand (0/ 00). Values 'O f O 13 Cc 'O rrelate directly directly with integrated water use efficiency over 白.e period 泊 which carbon tissues were formed 泊 developing leaves (FARQUHAR Ef AL. ， 1982). T 'O talleaf nitrogen values were obtained obtained during these analyses by 出.e Carla Erba elemen ta1 analyzer.
RESULTS Mi dday water potentials were similar in all seven species conifer studied and their relatively relatively high values suggested 白紙 these were all well-watered individuals (Table 1). 百le highest highest mean midday water potential (ー 0.70 MPa) was found in Podocarpus neriifolius ， while while the lowest w 部加 Cephalotaxus griffithii ←1. 24 MPa). Th ere was little variation among replicate s創 nples of water potential wi 血加 any single species. PHOTOSYNTHETIC CAPAC 汀 Y IN THAI CONlFERS 299
Table Table 1. Ecophysiologic a1 characteristics of water relations ， gas exchange ，foliar nitrogen level level and carbon isotope ratio in seven species of Thai conifers growing in a common garden plantation at the Queen Sirikit Botanic Garden in the Mae Sa Valley Valley ne 訂 Chiang Mai ，Thailand.
Midday Midday Net Stomatal Species Species water potential photosynthesis conductance Leaf nitrogen oJ3 c (MPa) (MPa) (件 mol m-2.cl) (μmol m- 2ぷ 1) (mg ・g-I) (%0)
Cepla10t 似 us griffi 血11 -1.2 4 5.5 2 59 21. 0 -30 .4 Dacrydium Dacrydium elatum ND 4.05 50 14 .4 一29 .3 Pinus Pinus kesiya -0 .99 7.92 156 14 .5 29.4 Pinus Pinus merkusii -0 .90 7.98 143 14.0 -29.6 Podocarpus Podocarpus imbricatus -1.1 0 6.5 3 85 13. 0 -28.0 Podocarpus Podocarpus neriifolius -0 .70 3. 06 80 13. 0 -27.6 Podocarpus Podocarpus wallichianus -0 .95 1. 99 46 13.9 -24 .1 」ー
Net photosynthetic capacity under conditions with saturating light intensity was low ， with with values comparable to those seen in many temperate region conifers. Expressed on a projected projected leaf area basis ， as is recommended for conifers (RUNDEL & YODER ，1998) ， the highest highest rates were found in Pinus merkusii and P. kesiya with 7.98 and 7.92μmol m- 2.s- l ，respectively (Table 1). Rates of maximum photosynthesis in Podocarpus speci 回 varied varied from a high of 6.53 f! mol m- 2.s- 1 in P. imhricatus to a low of 1.99 f! mol m- 2.s- 1 in P. P. wallichianus. Intermediate values were found for Cephalotaxus griffithii (5.52 μmol m- 2.s- 1 and Dacrydium elatum (4.05 μmol m- 2.s 一 1). Maximum rates of stomatal conductance measured in these seven conifers closely followed followed the pattem seen in the photosynthetic data (Table 1). There is a significant positive positive linear relationship between net photosynthetic rate and stomatal conductance in our our data (Fig. 1). 百le highest rates of stomatal conductance were found in Pinus merkusii and P. kesiya with 143 and 156 mmol m- 2.s 一 1，respectively. The lowest rate of stomat a1 conductance conductance was 46 mmol m- 2.s- 1 measured in Podocarpus wallichianus. Leaf nitrogen contents were very similar in six of the seven species studies ， with concentrations concentrations of 13. 0- 14 .4 mg ・g-I. A significantly higher nitrogen concentration was found in Cephalotaxus griffi 的ii with 2 1. 0 mg.g-I ， despite its modest photosynthetic rate. Previous Previous studies with conifers have not shown a consistent positive relationship leaf nitrogen content content and photosynthetic capacity (see review in RUNDEL & YODER ， 1998). The range of of values in our study is within the range of those reported for temperate conifer species (REICH ET A L.， 1995). Analyses Analyses of carbon isotope discrimination in the conifers studied showed three pattems of of water use efficiency. Four species (Cephalotaxus griffithii ， Pinus merkusii ， P. kes 加， and Dacrydium elatum) had low δ13C values of -29.3 to -30 .4 %0 (Table 1) ，signifying a relatively relatively low water use efficiency. At the other extreme ， Podocarpus wallichianus had a O 13 C value of -24.1%0 ，indicating a relatively high level of water use efficiency. Th is is su 中 rising given the wet forest habitats where P. wallichianus characteristically occurs. The remaining two species ， Podocarpus imbricatus and P. neriifolius ，had intermediate 3QQ PH ILIP W . R UNDEL, M ARK PATIERSON, i 9 8 ~ (/) 7 <: 0 0 50 100 150 200 2 1 Stomatal conductance (m mol m· s' ) 2 1 2 1 Figure I. Relationshi p between net photosynthesis (!-!mol m- -s- ) and stomatal conductance (!-! mol m- -s- ) in 7 species of Thai conifers. See text for discussion. 16 14 ~ ~ 12 0 E ...=, 10 ·c;;(/) 8 Q) .r::: E 6 (/)>- 0 0 .r::: 4 Q. Qi z 2 0 Thai conifers TemP.erate Temperate conifers hardwoods 2 1 Figure 2. Comparative values of net photosynthetic (!-!mol m- -s- ) under optimal conditions in Thai conifers and in temperate coni fers and hard wood trees from North America. Data on North American trees are from REICH ET AL. ( 1995). PHOTOSYNTHETIC CAPACITY IN THAI CONIFERS 301 va1 ues of O13 C with -28.0 and -27.6%0 ，respectively. 官lU S， these species had intermediate levels levels of water use efficiency. DISCUSSION Our objectives in this paper have been to describe the photosynthetic capacity of a diverse diverse group of these 住opical conifers growing toge 出er under common garden conditions ， 組 d to comp 紅 'e va1 ues of maximum rates of photosyn 血， etic assimilation with those reported 泊 the literature for temperate conifers. If the tropics provided ideal conditions for conifer grow 血， one might expect photosyn 出etic rates higher than those of temperate conifers. However ，if 血 e structural characteristics of conifer override the potenti a1 for high rates of carbon carbon fixation ， then comparable rates would be expected. Photosynthetic Photosynthetic rates under field conditions for members of the Podoc 釘 paceae have rarely rarely been measured. Recent studies with Podocarpus oleifolius in 出， e high An des of Venezuela Venezuela (CA VIE 阻 S ET AL. ， 2000) and P. coria 印刷 in the West In dies (Duc 阻 Y， 1994) have have reported relatively low rates of maximum net photosynthesis. τ'h e range of relatively modest photosynthetic rates reported here 釘 'e comparable or or slightly lower 由佃 those that have been widely reported for temperate conifer species (see (see reviews 泊 SMπH&H 別 CKLEY ， 1995; RUNDEL & YODER ，1998). In P泊es for example ， reported reported rates of photos 戸曲目is in temperate species are commonly 6ー14μmol m- 2.s- 1 (RUNDEL &YODER ，1998) ，bracketing the rate of 8μmol m- 2.s- 1 reported here. Figure 2 shows data on photosyn 曲目ic capacity for 官lai conifers comp 釘 ed to ranges of va1 ues reported reported for temperate North American conifers and North American hardwoods. 百le range range of va1 ues for the North Am erican hardwoods is wide ， but within 出is range most species species have higher rates than those present 加 either Thai or North Am erican conifers. Rates Rates of stomat a1 conductance in 百団 conifers are also comparable to those seen in temperate temperate co 凶fers ，and generally lower than those measured 泊 temperate hardwood leaves 侭EICH ET AL. ，1995). Our data suggest that inherent limitations 加血es 官邸側 ral characteristics of of the photosynthetic and water transport systems in conifers 釘 'e equ a1 1y applicable to 住opic a1 部 well 出 temperate conifers in mainland Southeast Asia. Podocarpus wallichianus with with leaves up to 70 mm in width ，likely the broadest of any species conifer in the world today ，had the lowest photosynthetic rate of 佃 y species studied. Although Although 百lailand has a strongly season a1 monsoon climate ，conifers with the exception of of Pinus merkusii ， are largely absent from lowland habitats. It is not clear whether low rainf a1 1， strongly season a1 climates ， high temperatures ， or ecological competition with angiosperm 甘'ees limit 血位 surviv a1.百 le me 佃 annu a1 rainf a1 1 levels of 110 0- 1500 mm 白紙 characterize most of the lowlands of 官lailand are well within the r叩 ge of seasonal rainf a1 1 regimes where temperate pines thrive. Moreover ， the low 即位ient conditions present over over the sh a1 10w lateritic soils 白紙 characteristica I1 y underlie deciduous dipterocarp forest might might be expected to provide a competitive advantage to pines over hardwood species. Pinus Pinus merkusii is unique among Southeast Asian conifers in being adapted for surviv a1泊 血ese low elevation deciduous dipterocarp forests as on 出e Kh orat Plateau (WE 悶 ER ， 1993; 1993; SANTISUK ， 1997). Cooler Cooler temperatures and higher rainfall in montane habitats ， with no more 出an3 -4 months of functional dry season (compared to 5-6 months in most lowland habitats) ， 302 PHILIP W. RUNDEL ，MARK PA 1T ERSON ，KANSRI BOONPRAGOB ， AND SANTI WA 1T HANA provide provide conditions for the highest diversity of conifers. Pines in Th ailand show their greatest greatest dominance in lower mont 加巴 habitats with low nutrient substrates. Relatively pure savannas savannas of Pinus kesiya and/or P. merkusii with a grass or sedge understory 紅 e well developed developed on 白e tops of sandstone plateaus at 100ι1800 m elevation in Northeast Th ailand ， as as at Phu Soidao and Phu Kr adueng National Parks. Th e great majority of 官1a i conifer species species are found in lower montane evergreen forests above the zone of dipteroc 創? dominated dominated forest communities where 血ey may occur as tall canopy trees (e.g. Podocarpus species and Dacrydium elatum) ， or subcanopy shade-tolerant species (e.g. Cephalotaxus griffi 的ii). This is similarly true throughout Southeast Asia. The Dalat Plateau in the Central Highlands Highlands of southem Vietnam supports 16 species of native conifers (Loc & HIEP ， 1997; RUNDEL ， unpublished data) ， and there are 12 conifer species present in the Fan Si Pan massif massif in northem Vietnam near the Chinese border (THIN & THOI ， 1998). Wet evergreen lowland forests such as those in the Malay Peninsula of southem Th ailand have lower diversities of conifers 血an montane forest habitats. Only six conifer species species are found in pe 凶nsul 訂 and southeastem 百1a iland in areas with high precipitation and and relatively little rainfall seasonality. In addition to their tall forest growth forms ， dwarf forms forms of Dac ηdium elatum and Podocarpus imbricatus form major canopy dominants in boggy 訂 'eas of the Cardamom and Elephant Mountains in Cambodia ， and in Kh ao Yai National National Park ，Central Th ailand ，suggesting a high tolerance of water-logged soil conditions. ACKNOWLEDGMENTS We thank the staff of the Queen Sirikit Botanical Garden ，especially Director Weerachi Nanakom ， for their support 泊 carrying out this projec t. Field expenses were made possible with with a grant from the University of Califomia Pacific Rim Research Program. REFERENCES CAVIERES ， L. A. ，F. RADA ， A. Az OCAR ，C. GARCIA-NuNEZ ， AND H. 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