NAT. HlST. BULL. SIAM Soc. 52(1): 55-70， 2004

PHOTOSYNTHETIC RESPONSES TO LIGHT AND THE ECOLOGICAL DOMINANCE OF HOPEA FERREA

(DIPTEROCARPACEAE) IN A SEMI-EVERGREEN FOREST OF NORTHEASTERN THAILAND

Philip W. Rundel1， Mark T. Patterson2 and Kansri Boonpragob3

ABSTRACT

Semi-evergreen forests of Thailand have a high diversity of回 :especies， but commonly exhibit a s汀ongecological dominance by single species of Oipterocarpaceae.百lis

situation is evident with Hopea ferrea i目白.esemi-evergreen forest 創出eSakaerat Enviromnental

Research Station of nor廿leastern百lailandwhere this speCI<巴scomprised 32.9% of all trees over

IOcmdbh釦 dmore than 50% of total t間活 basalarea and biomass.百ledominance of H.タrrea

results合'omthe ecological success of light-adapted seedlings and saplings which are able to

utilize the frequent and long-lasting sunflecks which reach the forest floor below an

inhomogeneous upper c飢 opy.Extended seasonal drought in this mons∞n climate of north-

eぉ 旬rnThailand and high ambient air tempera伽resin the lat巴句 seasonmay restrict the use

of a similar growth strat巴gyby other potentially competing species. Saplings of H.タrreawere

able 10 maintain significantly higher rates of net pho1Osynthesis when light saturated (5.4μrnol

m-2 S-I)血another species with similar water use efficiencies. Access to soil water not readily

available to these other species may allow this adaptive growth s住ategyto be successful for

H.ferrea. Photosynthetic production in H 戸rreawas叫soaided by a rapid triggering response

to early morn恒gsunflecks白紙 allowedstomata to open more rapidly than those of other

species. Limited water availability also represented a strong selective pressur草ontree seedlings

in血.efo児 stunderstory， particularly during April and May when ambient air temperatures became very high. Maximum rates of stomatal conductance of H. ferrea canopy leaves in血e

wet season ranged from 220-250 mmol m-2 s-l， comp'卸 dto m依田umrates of only 40-110 mmol m-2 s-1 in出edry season when midday water po旬ntialsdropped below -3.0 MPa. Full irradiance during sunflecks typically brought leaf tempera'加resof saplings of all species to

36ー38"C泊 thelate dry season， reducing maximum photosynthetic capacity through tempera-ture effects as well. Inherent tolerance to high temperaωres in m釦 yOipterocarpaceae may

provide a selective advantage to H.戸rrea.Given the apparent sensitive balance between the

light enviromnent and regional climatic conditions 0目白eecological success of H.戸rrea，small changes凪 forestcanopy coverage or the seasonality and intensity of monsoon conditions could

cause fundamental changes in the structure of semi-evergreen forest泊Th剖land.

Key words: Dip飽rocarpaceae，semi-evergreen forest， Hopea ferrea， photosynthesis， stomatal response， water relations

1Department of Ecology and Evolutionary Biology and Center for Embedded Network Sensing， UnI'l:ersity of

California， Los Angeles， California 90095 USA <rundel@biology.ucla.edu>

20epartment of Botany and Plant Pathology， Oregon State University， Corvallis， Oregon 97331 USA

<pattersm@science.oregonstate.edu>

30epartment of Biology， Ramkhamhaeng University， Bangkok 10240，百lailand<kansri@raml.ru.ac.th>

55

56 PHILIP W. RUNDEL， MARK T. PAITERSON， AND KANSRI.sOONPRAGOB

INTRODUCTION

Tropical forest communities are characterized by high tree diversity with mixed species

dominance (GENTRY， 1982; CONDIT ET AL.， 2002). Typically no single species is able to

maintain strong ecological dominance. Clearly there are exceptions to this pa恥 m(CONNELL & LOWMAN， 1989， HART ET AL~ ， 1989)， but nevertheless there has been little study of the mechanisms that might allow such strong ecological dominance in the face of potentially

strong biological competition. Semi-evergreen forests of northeastem Thailand present an interesting case of single

species dominance in high diversity stands. Here single species of Dipterocarpaceae

frequently are overwhelming ecological dominants in forests where more than 50 tree species are present per hectare (BUNJAV日CHEW町 1986，1999; KANZAKI ET AL.， 1995). Semi-evergreen forests are widespread in the Dangrek Range and areas of foothill regions

around the Khorat Plateau in northeastemτbailand where they occur in a strongly seasonal

monsoon climatic regime with 1200-1400 mm annual precipitation (THOMPSON &

LANSBERG， 1975; RUNDEL & BOONPRAGOB， 1995). These semi-evergreen forests are tall， closed-canopy forests averaging 25-35 m in height with emergent住民sto 40 m. However， environmental selection for drought-adapted species in血esemi-evergre氾nforests of百lailand

and Indochina differs from more mesic Indo-Malaysian elements of tropical rainforest to

the south. As a result， endemism is relatively high at the species level. In this paper we repo目 onfield studies of semi-evergreen forest at the Sakaerat

Environmental Research Station in northeastem Thailand where Hopea ferrea Pierre (Dipterocarpaceae) is the strong ecological dominant in a highly diverse forest community.

We use an ecophysiological approach to ask， What factors and adaptive traits allow this single species to maintain such dominance in the face of competition from so many other

tree species? Our studies were focused on the late dry season of April and May when low soil water availability and high ambient air temperatures provide particularly strong environmental stress for plant growth， and thus strong selective pressures on demographic

success.

STUDY SITE

Field studies were carried out at the Sakaerat Environmental Research Station near

Pak Thong Chai， Nakhon Ratchasima Province， 60 km south of Khorat in northeastem Thailand (lat. 14031'N， long. 10I055'E). This field station encompasses a research area of

81 km2 on the northeastem slopes of the westem Dangrek Range of Thailand， near their transition to the lower Khorat Plateau. The topography of the station is largely gentle slopes and rolling terrain at 300-560 m elevation. The shallow soils紅 esandy clay loams derived from sandstone parent material and紅 epoor in available nutrients and soil water holding capacity. Semi-evergreen forest covers 36% of白eSakaerat reserve， with sharp transitions to deciduous dipterocarp woodland， forest plantations， and disturbed areas with swidden agriculture from local villages (TONGYAI， 1980; BUNJAVEJCHEW別， 1986). Soils in this semi-evergreen forest are reported as acidic with a mean pH of 4.5 for surface mineral soils of 0-15 cm depth (BUNJAVEJCHEWIN， 1986). Cation exchange capacity of these soils was 7.0 meq 100 g-I and organic matter content was 3.2%.

PHOTOSYNTH ETIC RESPONSES AND DOMINANCE OF HOPEA FERREA 57

300

-(.) 0

250 40 -Q.) L-- :J

E ..... E 200 35 0 .....

Q.)

a. E

0 150 30

Q.) ....... ..... c 0 E ,_ :J

c 100 25

E 0 >< Q.)

::::!! 0 E

50 20 c 0 Q.)

~

0 J F M A M J J A s 0 N D

Month

Figure I. Seasonal pattern of mean monthl y rainfall and monthly mean maximum temperature at the Sakaerat Envi ronmental Research Station in northeastern Thailand. Mean rainfall data is for 1969- 1979. Mean­maximum temperatures are 1969 data taken from Sukapanpotharam ( 1979) .

Mean annual prectpttation at Sakae rat from 1969-1982 was 1240 mm (BUNJAVEJCHEW!N, 1986). Peak monthly rainfall is present from May through October, but begins to decline in November with the transition to the dry season (Fig. 1). The early dry season from December through February is almost totally without rain, but moderate rainfall typically returns in the late dry season well before the beginning of the wet season in late May or June. Mean max imum air temperatures are 30-35"C in the early dry season, but ri se sharp ly to 38-40' C in Apri l and May. As the wet season begins in late May and June, mean maximum temperatures stead il y decline from these high levels to 25-26' C in November and December.

MATERIALS AND METHODS

The population structure of the semi-evergreen forest at Sakaerat was extracted from data collected by other researchers in previous studies . The size distribution of Hopea ferrea and other tree spec ies, the relative distribution of basal area, and biomass distribution in this community were taken from BUNJAVEJCHEWIN (J 986) and SABI-IASRI ET AL. (1968), while population data on sap lings and seedlings at this s ite were measured by Jantanee (1987).

58 PHILIP W. RUNDEL， MARK T. PAITERSON， AND KANsRI BOONPRAGOB

Seasonal measurements of ambient environmental conditions and leaf physiology were made from a 46・mtower出atallowed access to canopy leaves of Hopea ferrea. These canopy leaves occurred in full sun at 32 m height. Diumal physiological measurements of stomatal conductance， transpiration， and leaf temperature were made with a LICOR-1600 porometer as part of a broader study during January， March， April， June， August， and October 1992， and January 1993. Gas exchange measurements of net photosynthesis and field measurements of photosynthetic response to P AR irradiance were made on the s創necanopy branches in December 1991 and May 1993 using a LICOR-6200 photosynthesis

system. Experimental studies of light saturation and stomatal汀iggeringfrom artificialligh出ecks

were made in the forest understory near the research tower using seedlings and saplings of H. ferrea and three associated woody species. These experiments carried out in early moming before sunflecks had struck understory leaves， ambient light intensities of 5-20 μmol m-2 S-1 reaching the leaf chamber were raised to 700μ.mol m-2 S-1 using a focused projector bulb powered by a 12-volt battery. A cold mirror prevented more出ana 2.C increase in temperature over the 30 minutes of these experiments. Leaf temperatures were 30ー34.Cin these understory measurements. Data presented here are from single leaves， but replicate measurements were in the field.

Light saturation curves were measured in both understory and canopy leaves， with PAR irradiance varied by covering the leaf cuvette with neutral density filters calibrated to remove a fixed percentage of incoming irradiance. Natural irradiance in midmoming was used泊 C釦 opyleaf experiments， while PAR irradiance provided by artificial illumination from a projector bulb was used in the understoη. Leaves were allowed to reach a stable 'photosynthetic rate for at least 5 min at each light level before photosynthetic rates were recorded. Replicate measurements were made for each of two leaves on two plants at each level of PAR intensity. Leaf紅 easof plant tissues used for photosynthetic studies were marked in the field and measured using a LICOR portable leaf紅白 meteron the same day so that shrinkage would not occur with drying.

Leaf temperatures and leaf-泊rtemperature gradients were determined using an Everest Instruments infra-red thermometer， focused to measure mean leaf surface temperature over a 1 cm2 area. Leaf water potentials were measured on freshly cut branch tips using a Scholander四 typepressure chamber.

Mean leaf angles were measured on populations of seedling (0.5 m)， sapling (2.0 m) 叩 dc組 opyleaves (32 m) of Hopea 戸rreausing a Suunto inclinometer. Fifty leaves of each population were used for measurements.

Stable carbon isotope analyses were carried out using pooled samples of 3δmature leaves from each study population. Analyses were made using standard technique with a Finnigan isotope ratio mass spectrometer at the Stable Isotope Laboratory of the Department of Biology， Boston University. Carbon isotope ratios can be used as a measure of integrated water use efficiency over the period of time in which carbon was sequestered in leaf strucωres (see RUNDEL ET AL.， 1989 and EHLERINGER ET AL.， 1993 for reviews of stable isotope physiology). A higher (less negative) value of carbon isotope ratio， measured in units of parts per thousand (0/00)， indicates higher water use efficiency than a leaf with a lower (more negative) value.

PHOTOSYNTHETIC RESPONSES AND DOMINANCE OF HOPEA FERREA 59

RESULTS

Despite relatively high tree diversity, with more than 50 species occuning in a single hectare of semi-evergreen forest at Sakaerat, Hopea ferrea was the strong ecological dominant over much of this area. The upper canopy of this forest was formed almost entirely by tall H. ferrea 30-35 m in height with crowns reaching 15-20 m or more in diameter. Only scattered individuals of Shorea hemyana Piene (Dipterocarpaceae) , Jrvingia malayana Oliv. Ex Benn. (lrvingiaceae) , Lagerstroemia cochinchinensis Pierre (Lythraceae), and Dialium cochinchinensis Pierre ex Gagnep. (Leguminosae: Caesalpinoideae) reached to this stratum. Hopea f errea comprised 32.9% of all trees 10 em or more in dbh in this community, 53 .7% of the basal area of such trees, and 57.0% of total tree biomass (SABHASRI ET AL., 1968, BUNYAVEJCHEWIN, 1986).

The dominance of Hopea f errea was not limited to large canopy trees, however. This species was likewise the ecological dominant in the seedling and sapling pools, and at every size class from saplings to large canopy individuals (Fig. 2). Among the seedling pool less than 1 .3 m in height, H. fen·ea comprised 68% of all individuals, while it made up 44% of woody plants over 1.3 m in height but less than 5 em dbh (JANTANEE, 1987). Forty percent of small trees 5-9.9 em dbh were H. ferrea, and relative density increased steadily with diameter size. This forest community had a mean density of 14 trees ha- 1

above 60 em dbh , and H. f en·ea formed 64% of these individuals. Stomatal conductance in Hopea f errea was strongly impacted by seasonal changes in

leaf water potential. Wet season rates of maximum stomatal conductance for canopy leaves ranged from 220 to 250 mmol m-2 s- 1

, when midday leaf water potentials reached no lower than -1.5 MPa. Dry season maximum rates of stomatal conductance were 40-110 mmol

160

~ 120 I

0 ..c. :>

"0 c: 80

>-..... IJ)

c: Q)

0 40

0 J lil......,..n_..... 7.5 15 25 35 45 55 70 90 >100

Diameter class (em}

Figure 2. Densities (individuals ha- 1) by diameter class for Hopea f errea and all trees in the semi-evergreen

forest of the Sakaerat Environmental Research Station in northeastern Thailand. Data from B UNJAVEJCHEWIN (1986) and JANTANEE (1987). Open squares - all trees; closed squares - H. ferrea

60 PI-I !LIP W . R UNDEL, M ARK T. PATIERSON, AN D K ANSRI B OON PRAGOB

300r-------------------------------------~

len (\J 250 (f) Hopeo-wot

I E 0 E 200 E -Q) 0 c 150 Hopeo-dry season 0 u ::::J

""0 c 100 0 0

0

0 50 E 0 -(/)

0 0 0.5 1.5 2 2.5 3 3.5

Leaf water potential (-MPa)

Figure 3. Seasonal distribution of mimimum midday water potential (MPa) and max imum stomata l conductance (mmol m-2 s- 1

) in canopy leaves of Hopea f errea.

8

canopy len • •

(\J 6 ---· 2.0m sapling IE • /,·--·-· • 0 .. / 0.5m seedling E ::1. 4 II ·-·-· :A:

en

·~-0 Prismotomeris en 0 Q) o-

..c: ..... 2 ~/0 c >. en , 0

0 ..c: • a. 0 ----------------------------..... Q)

z

_2 &0----2~0-0---4~0-0---6~00----8~00---1-0~0-0 __ 1_2~00---1~400

Figure 4. Light response curve of photosynthes is (~-tmol m-2 s- 1) for field seed lings , sap lings , and canopy leaves

of Hopea fen·ea, and understory leaves of Prismatomeris memecyloides. Data were co ll ected in the late dry season on April 30 and May I, 1993.

PHOTOSYNTHETIC RESPONSES AND DOMINANCE OF HOP EA FERREA 61

m-2 s- 1 at a time when midday water potentials fell below - 3.0 MPa (Fig. 3). Data reported here were collected in early May of 1993 under late dry season conditions of low maximum conductance and moderate water stress .

Photosynthetic rates in canopy leaves of H. ferrea peaked at a mean level of 6.4 ~mol m-2 s-1, although- individual leaves had rates as high as 8.0 ~mol m-2 s- 1

. Stomatal conductance in these leaves was 85 mmol m-2 s-1

. Light response curves showed that these leaves reached light saturation at a PAR irradiance of about 700 ~mol m-2 s- 1 (Fig. 4).

Leaf photosynthesis in seedlings and saplings of Hopea f errea did not eKhibit strong adaptations to low light intensity in the understory in comparison to the canopy leaves. In a forest understory with typical midday PAR irradiance of only 5-20 ~mol m-2 s-1, light saturation was reached in leaves of saplings at 2 m height at about 500 ~mol m-2 s- 1

, and at about 400 ~mol m-2 s- 1 for seedlings at 0.5 m height. Light saturated rates of net photosynthesis were 5.4 ~mol m-2 s- 1 in the sapling leaves and 4.1 ~mol m-2 s-1 in the seedlings. By comparison, a common shade-adapted shrub in this understory, Prismatomeris memecyloides (Rubiaceae), had a maximum photosynthetic rate of only 3.2 ~mol m-2 s-1

,

and also became light saturated at a relatively high PAR irrad iance of about 450 ~mol m- 2 s- 1.

Under morning PAR irradiance of 7-20 ~mol m-2 s- 1 the forest understory saplings of Hopeaferrea and three associated woody species, Walsura trichostemon Miq. (Meliaceae) and Ardisia helferi Kurz (Myrsinaceae), and P. memecyloides, exhibited low rates of net photosynthesis of only 0.2- 0.6 ~niol m-2 s- 1 (Fig. 5). Hopea ferrea was intermediate at

-1(/) 6

C\.1 • HOPF 'E

5 • WALT 0 £ARDH E o PRIM :1.

(/)

(/) Q)

.t::. ..... c >- 2 (/)

0 ..... 0

.t::. 0. ..... Q)

z 0 -4 0 4 8 12 16 20 24

Minutes

Figure 5. Stomatal triggering in response 10 artificial light fl ecks in saplings of Hopeaferrea, Walsura trichostemon , Ardisia he!feri , and Prismatomeris memecyloides. Net photosynthes is C~-tmo l m-2 s- 1) was measured continuously over the period of these experiments during the late dry season from May 1-3, 1993. Leaves were exposed to saturating light intensities at time zero.

62 PHILIP W. RUNDEL, M ARK T . PATTERSON, AN D KANSRI BOONPRAGOB

0.06 Hopeo

0.04

0.02 -'en 0.00

N 0.06 I E 0 E

0.04

- 0.02 Q) u 0.00 c 0 - 0.06 u ;:::,

"C c 0.04 0 u

0 0.02 -0 E 0 0.00 -en 0.06

0.04

0.02

0.00 -10

t 0 10 20 30 40

Time (minutes) light on

Figure 6. Response of stomatal conductance to the end of art ifical li ghtflecks in saplings of Hopea fen·ea, Walsura trichostemon, Ardisia helferi , and Prismatomeris memecy/oides. These experiments were carried out under fie ld conditions during the late dry season from May 1- 3, 1993. Light-saturated leaves were placed into complete darkness at time zero .

PHOTOSYNTHETIC RESPONSES AND DOMINANCE OF HOPEA FERREA 63

0.4μmol m-2 S-I.百lesespecies exhibited very dif~erent responses， however， to triggering by a continuous artificial sunfleck of 700μmol m-2 S-1 P AR irradiance. Within 2 ritinutes of exposure， H. ferrea leaves reached a photosyn白eticrate of 4.7μmol m-2 S-I， more than 80% of their maximum rate. The three associated species all responded more slowly to a simulated sun fIeck and had significantly lower rates of light-sa加ratednet photosynthesis than H.舟rrea.After a 2・minuteexposure to saturating light， W. trichostemon and A. he俳rihad reac~ed _only about 60% of their maximum rates of net photosynthesis at 4.3 and2.7μmol m-2 S-I， respectively. Prismatomeris memecyloides was e~en mo~ sluggish in its stomatal response， reaching less than 50% of its steady-state maximum rate of ph，?tosynthesis after 2 min. Photosynthesis in this shrub species rose to a peak of 3.2μmol m-2 S-1 after 10 min and then dropped to a lower steady-state rate of 2:5μmol m-2 S-I.

官leresponse of light saturated leaves to sudden darkness， simulating the end of a sunfIeck， was investigated in saplings of H. ferrea and the same three associated species by illuminating understory leaves to a point where maximum rates of phoωsynthesis were reached and then covering the leaf chamber to reduce all incoming P AR irradiance. All four species showed similar temporal responses in their rates of stomatal closure under these experiments. While net photosyn曲目isrates dropped to negative levels almost immediately， stomatal conductance dropped to only 70-80% of maximum rates after 4 min and remained at 20ー32%of maximum after 20 min in the dark (Fig. 6). Moderate rates of stomatal conductance of 25-42 mmol m-2 S-1 were present at midmoming in all of these species under shaded understory conditions with P AR irradiance below 20阿nolm-2 S-I.

Leaf temperatures of canopy leaves ranged from 35-40・Cduring canopy measurements， never rising more than 30C above ambient temperatures. However， expos町 'eto full sun泊

a long sunfIeck produced signific叩 teffects on leaf energy balance泊 understorysaplings and seedlings.百lemoderate rates of transpir，剖ionmeasured in shaded saplings of H. ferrea， W. trichostemon， and A. he俳ri(data not shown) produced midmoming leaf temperatures of 0.5ー1.5・Cbelow ambient. Even though exposure to sunfIecks with full irradiance signific佃 tlyincreased stomatal conductance and thus transpirational cooling， this cooling effect was not sufficient to maintain leaf temperatures at ambient levels. Under these conditions， all three species experienced leaf temperatures of 4ーデCover ambient， heating them in midmoming to 36ー38・Cwhen sun fIecks were present.

百lemore limited over~eating of c佃 opyleaves of Hopea j甘rea，only 2-3・Cabove ambient air temperature resulted from low to moderate winds 鉱山islevel in the forest and the associated reduction in boundary layer resistances. Leaf orientation was also an impo託antcomponent of the observed energy balance in H.戸rrea.Canopy leaves were sharply inclined with a median angle of about 目。 fromhorizontal (Fig. 7)， thereby decreasing midday heat loads. Nearly 50% of canopy leaves had佃 glesfrom 50ー70・.Leaves of saplings at 2.0 m height had a medi佃 angleof about 150， wi由70%of these leaves having angles of 0-00

• Nearly 80% of seedling leaves at 0.5 m height were horizontal. Presumed differences in rooting architecture (root excavations have not been carried

out) produced different levels of midday water potential in sapling species. Hopea ferrea reached -1.71 MPa， compared to a more negative -2.28 and -2.78 MPa in W. trichostemon and A. he俳ri，respectively (Table 1).百leshade-adapted P. memecyloides reached only -1.37 MPa. Seedlings of H. ferrea， presumably with shallow root systems， reached -2.51 MPa at midday， while upper canopy leaves at 29 m height dropped to -2.01 MPa even though moderately high conductance rates of 60ー90mmol m-1 were maintained.

64 PHILIP W. R UNDEL, M ARK T. P ATTERSON, AN D K ANSRI B OONPRAGOB

80

- 60 Seedlings - 0.5m ~ 0 ->-(,) c 40 Q) :::::1 0" Q) .... 20 u..

0 80

- 60 Saplings - 2.0m ~ 0 ->-(,) c 40 Q) :::::1 0" Q) .... u.. 20

0 80

- 60 Canopy ~ 0 ->-(,) c 40 Q) :::::1 0" Q) ....

20 u..

0 5 15 25 35 45 55 65 75 85

Leaf angle (0)

Figure 7. Relative di stribution of leaf angles Cfrom horizontal) in seedlings (0.5 m), sapling (2.0 m), and canopy leaves (32 m) of !-Iopea f en ·ea in the semi-evergreen forest of the Sakaerat Environmental Research Station in northeastern Thailand.

PHOTOSYNTHETIC RESPONSES AND DOMINANCE OF HOPEA FERREA 65

Table 1. Ecophysiological characteristics of midday water potential， maximum photos戸出eticrate， leaf nitrogen content， and carbon isotope ratio of study species 泊 thesemi-evergreen forest of the Sakaerat Environmental Research Station， northeastem官lailand.Data here were collected in the late dry season from April

30 to May 6， 1993. ND = no data.

Species Midday ψ P-max Leaf nitrogen

(0，δ/1o3Co)| | (MPa) (μ.mol m-2 s-11 (mg g-I)

Canopy Hopea ferrea -2.01 6.7 14.4 -27.5

Sapling

Hopea戸rrea -1.71 5.5 27.8 -32.2

Walsura trichostemon -2.28 4.4 28.2 -31.9

Ardisia he俳ri -2.78 3.2 14.3 -32.2

Understory shrub Prismatomeris memecyloides -1.37 3.2 19.6 -30.0

Seedling

Hopea ferrea -2.51 4.1 ND ND

Despite the different levels of photosynthetic rate and midday water potential泊 the

出r関 sapl泊gsp民 iesstudied， the integrated water use efficiency as indicated from carbon isotope ratios of leaf tissue w出 virtuallyidentical (Table 1). For H.ferrea， W. trichostemon， andA. he俳rithese values were -32.2， -31.9， and -32.20/00， respectively. Better stomatal con甘01and the maintenance of higher water potentials were reflected in a higher water use efficiency for the shade-adapted P. memecyloides， as indicated by its O13C of -30.00/00. 官leo13C for canopy leaves of Hopeaferrea was -27.50/00， but this value was not dire氾tlycomparable with the understory leaves since rl巴spiredCO2 undoubtedly affected the O13C

of the latter.

DISCUSSION

Tree species diversity in the semi-evergreen forest of Sakaerat is high. A complete

census of woody pl佃 tsfor a 1.0-hectare qua的 tat白issite found 55出especies rちaching

5 cm or more in dbh (S班 聞ANDET AL.， 1968). Samples of 20 1000-m' plots in出esame area by BUNYA VEJCHEWIN (1986) reported 59 woody species of血issize. An adjacent semi-evergreen forest community dominated by another Dipterocarpaceae， Shorea henryana， had 75 woody species， with 92 woody species in the two communities combined. Nevertheless， single species dominance w出 acharacteristic feature of both community typ回 .Hopea ferrea made up 54% of the basal area of tr，田sover 10 cm dbh in its community， while S. henryana comprised 25% of basal ar四回出eadjacent community

where H.戸rreawas absent. How由enis a single species such as H.ferrea able to atぬ泊 suchecological dominance

over its potential competitors? One explanation put fo凶1by KANZAKI ET AL. (1995) is白紙

H.戸rreais highly shade-tolerant and白usable to maintain its position in small forest gaps.

66 PHILIP W. RUNDEL, MARK T. PATTERSON, AND KANSRI BOONPRAGOB

This explanation is inconsistent with our ecophysiological observations on seedlings and saplings of H.ferrea, which appear highly adapted to using relatively long-lasting sun:flecks on the forest floor.

Our explanation for this monodominance lies in the relatively open canopy structure of this forest that allows the light-adapted seedlings and saplings of H. ferrea to efficiently utilize frequent sunflecks for growth. High ambient air temperatures and low water availability during the dry season may restrict the ability of other light-adapted tree species to effectively compete with H. fen·ea. Although the aspect of ecological dominance and drought tolerance is very different, this situation with H. ferrea is analogous in many respects to a light-adapted C

4 tree species that utilizes sunflecks to grow successfully in

the understory of Hawaiian forests (PEARCY, 1983). Canopy structure of the semi-evergreen forest at Sakaerat is notably separable into two

strata. The large canopy trees produce dense foliage cover at about 20-30 m height which intercept 80-90% of incoming radiation (YODA ET AL., 1983). Smaller trees form an irregular second canopy stratum centered 5-15 m above the ground. The density of large trees (over 60 em dbh) of only 14 individuals ha- 1 in the semi-evergreen forest is not sufficient to allow a closed coverage of leaves in the upper canopy of the forest. Large tree crowns more than 10 m in diameter covered only about 60-70% of the upper canopy (Fig. 8). Similar results were found by KANZAKI ET AL. (1995) who determined that 74% of the H. ferrea forest canopy was mature, compared to 20% in a building stage and 6% in gaps and

Figure 8. Horizontal profile of canopy trees with maximum crown width of 10 m or more in the SW quadrant of a surveyed !-hectare in the semi-evergreen forest at the Sakaerat Environmental Research Station in northeastern Thai land. Redrawn from SMITINAND ET AL. (1968).

PHOTOSYNTHETIC RESPONSES AND 00加sNANCEOF HOPEA FERREA 67

百lIsbr'Oken upper c佃'Opyand irregul釘 c'Overageby the l'Ower紅白 S回 taall'Owed frequent

sunflecks 'Of relatively l'Ong durati'On t'O pene回 tet'O gr'Ound level. Alth'Ough the shaded f'Orest underst'Ory typically experienced 'Only 5-20阿n'Olm-2 c1 'OfPAR irradiance， sunflecks

∞curred with sufficient合'equencyand intensity t'O all'Ow an average 'Of 7-8% 'Of t'Otal s'Olar irradiance t'O pene回 tet'O sapl泊gsat 5 m height (YooA Ef AL. 1983).百lisunderst'Ory light envrr'Onment w部 出ussignificantly m'Ore 'Open白組曲atdescribed f'Or wet l'Owland rainf'Orest

c'Ommunities such as th'Ose in C'Osta Rica (CHAZOON &日TC阻 R，1984; MONTGOMERY & CHAZI氾N，2002).

0町 seas'Onaldata 'On plant w蜘 p'Otentialcle的 dem'Ons帥 da str'Ong influence 'Of this fact'Or 'On 'Operati'Onal limits f'Or leaf st'Omatal c'Onductance.τ'hese results are c'Onsistent

with the fmdings 'Of Pitman (1996) wh'O rep'Orted出atwater p'Otential w酪 sec'Ond'Only t'O incident s'Olar radiati'On as an envir'Onmental variable influencing st'Omatal c'Onductance 'Of

H. ferrea. He f'Ound '0凶ya small influence 'Of temperature and vap'Or pressure deficit.

Can'Opy leaves 'Of H. ferrea became light saturated at a PAR irradiance 'Of ab'Out 700 阿'01m-2 S-1 (Fig. 4). The can'Opy leaves 'Of s'Ome early successi'Onal位opical岡崎 have

been rep'Orted t'O saωrate at m'Ore白血 1500J..tm'Ol m-2 S-1 (HOGAN Ef AL.， 1995). H'Owever，

the l'Ow light saturati'On p'Oint 'Of H.戸rreais n'Ot unexpected，部 itis s泊lilart'O l'Ow light saωrati'On levels伽 t'we have measured泊 d'OminantShorea species fr'Om恥叫ace蹴

decidu'Ous dipter'Ocarp f'Orest at Sakaerat (RUNOEL Ef AL.， unpublished data)， and in can'Opy

leaves 'Of Pentaclethra macroloba in C'Osta Rica (OBERBA田 R&STRA町 1986).百lesedata

suggest that the dipter'Ocarp can'Opy leaves are adapted f'Or the cl'Oudy wet seas'On c'Onditi'Ons 'Of relatively l'Ow s'Olar irradiance rather血anf'Or the clear sky c'Onditi'Ons 'Of出e命yseas'On.

N'One 'Of the f'Our species白紙westudied白血.eunderst'Ory dem'Onstra旬dstr'Ong selecti'On

'Of ph'Ot'Osynthetic仕組tsf'Or extreme shade t'Olerance. Even Prismatomeris memecyloides， an 'Obligate underst'Ory shrub， light saturated at 450μ.m'Ol m-2 S-1 in 'Our studies， c'Ompared t'O

light saturati'On at P AR irradiances 'Of 200ー350μ.m'Olm-2 S-1泊 shade-adapted加 pical

species (OBERBAUER & S百 A町， 1986; CHAZDON， 1986; RUNDEL Ef AL.， 1998). Other shade adaptati'Ons were present in P. memecyloides， h'Owever，卸 i民 selecti'On'Of shaded

micr'Osites where l'Ow rates 'Of st'Omatal c'Onductance c'Onserved water supplies and res出 ctedthe 'Occurrence 'Of l'Ow midday water p'Otentials during the dry se部'On.Stable carb'On is'Ot'Ope

rati'Os dem'Onstrated白紙ぬisspecies had a higher water use e伍ciency(less negative O13C value)白anthe 'Other sp配 iesstudied (Table 1).

Seedlings and saplings 'Of Hopea 戸rreaachieved m'Oderately high rates 'Of net

ph'Ot'Os卵白esis泊 c'Omparis'Ont'O 'Other species in the underst'Ory when sunflecks br'Ought high s'Olar irradiance (Table 1， Fig. 5).官lehigh frequency and l'Ong average durati'On 'Of such sunflecks (pers'Onal 'Observ組'Ons)w'Ould be expected t'O all'Ow a significantly higher

daily carb'On gain f'Or H.戸rreathan c'Ompet泊.gspecies. Since the water use efficiency 'Of

曲isspecies was n'O higher出佃 that'Of typical sapling c'Ompetit'Ors such 'Of Walsura trichostemon and Ardisia he俳ri(Table 1)， higher ph'O旬syn出eticrate must c'Ome at血ec'Ost 'Of greater water use. Midday water p'Otentials measured in these sapl血.gsduring the late dry seas'On established出atH.戸rreaw出 ablet'O白ld血.enecessary water res'Ources t'O maintain such a strategy 'Of higher ph'Ot'Osyn血eticcapacity (Table 1).

Previ'Ous studies 'Of Hopea ferrea at Sakaerat have rep'O巾dsignific組.tlyhigher ra旬S

'Of ab'Out 300--500 f'Or midday st'Omatal c'Onductance 'Of can'Opy leaves during the wet se邸'On(PITMAN， 1996).百lesehigh rates may be a functi'On differing c'Onditi'Ons 'Of me酪 urement'Or may have resul加 1合om血enature 'Of the instrumentati'On used in 也i~ earlier s加dy.

68 PHlLIP W. RUNDEL， MARK T. PATIERSON， AND KANSRI BOONPRAGOB

Measurements with a null balance p'Or'Ometer such as we used are generally c'Onsidered

m'Ore acc町atethan th'Ose made with a甘ansientp'Or'Ometer as used by町TMAN(1996).

Ph'Ot'Os戸出eticresp'Onses t'O light variati'On泊血eunderst'Ory is an加p'Ortantc'Omp'Onent

'Of adaptati'On in rainf'Orest spec同(陀ARCY1987; CHAZDON & PEARCY 1991; AL回 N&

PEARCY 2000). In underst'Ory envir'Onments where a 1釘'gepr'Op'Orti'On 'Of the t'O旬1daily

irr柑iancec'Omes泊出，ef'Orm 'Of sh'Ort sunflecks， m'Orph'Ol'Ogical and physi'Ol'Ogical adaptati'Ons

t'O e百'ectivelyutilize such irradiance are凶p'Ortantc'Omp'Onents 'Of species adaptati'On

(VALLAD成田ETAL.， 2000， 2002). Ph'Ot'Osynthetic pr'Oducti'On in saplings 'Of H. ferrea was aided by its rapid st'Omata1 resp'Onse t'O suof1ecks. St'Omata 'Opened faster泊出isspecies由加

泊 the'Other saplings studied (Fig. 5)， all'Owing a m'Ore e筒cientuse 'Of sh'Ort durati'On

sm由ecks.百leserates 'Of st'Omatal inducti'On泊 Hopeaare m'Ore rapid than the highest ra飽s

rep'Orted in previ'Ous studies with underst'Ory saplings in tr'Opical f'Orest trees fr'Om C印刷I

America (KURSAR & COLEY， 1993; POOR'百 R& OBERBAUER， 1993; ALLEN & PEARCY，

2000). Adaptati'On t'O f'Orest underst'Ory environments c'Ould p'Otentially inv'Olve n'Ot 'Only sωmata1

inducti'On in血em'Orning by sunf1ecks， but the st'Omata1 resp'Onse ，'Of light-saturated leaves

t'O a sudden reducti'On泊 PAR irradiance槌 sunflecksend. N'O significant difference w部

'Observed in relative rates 'Of st'Omata1 cl'Osure am'Ong H.舟F・reaand 'Other underst'Ory species

studied， sugges曲19that血isresp'Onse h邸 n'Otbeen subject t'O di首erentialselecti'On between

tree saplings and 'Obligate underst'Ory species in the semi-evergreen f'Orestc'Ommunity.τ'he

rat，田'Ofst'Omatal cl'Osure 'Observed here f'Or H. ferrea and 'Other species were similar t'O th'Ose rep'Orted previ'Ously f'Or sapl泊gs'Of tw'O tree sp即 ies泊 C'OstaRica (POOR'百 R&

OBERBAUER 1993). Limited water availability泊 thedry se槌'Onmay als'O represent a s釘ongselective

pressure 'On tree s田 dl泊gs.PITMAN (1996) ])叩'Ortedmean s'Oil water p'Otentials at 30-47 cm

s'Oil dep白 t'Oreach ab'Out -3.0 MPa under unusually dry August c'Onditi'Ons at Sakaerat.

τ'hese釘'ec'Onsistent with '0町 measuredwater p'Otentials 'Of -2.5 MPa泊 seedlings'Of Hopea 舟rrea.Full irradiance 'Of seedlings at midday under印刷ghambient air tem戸:ra回 e白紙

typically 'Occ町加 April加 May勘助isf'Orest produced leaf temperaωres白紙 reached

3ι380C泊 H.戸rrea泊 thef'Orest underst'Ory. Such tempera仰向slikely reduce maximum

ph'Ot'Os卵白eticcapacity. Tr佃 spirati'Onalc'O'Oling加 preventheat damage t'O leaves under

such c'Onditi'Ons requires water res'O町ces佃 dH. ferrea seedlings were m'Ore limited泊

water avai1ability白血 sapl泊gsin 'Our study (Table 1). It w'Ould be interesting t'O investigate

t'Olerance t'O high temperature in H. ferrea in relati'On t'O 'Other species. A study 'Of

ph'Ot'Osynthetic resp'Onse t'O低mperaturein a related genus泊 thedecidu'Ous dipter'Ocarp

f'Orest at Sakaerat， Shorea siamensis， has sh'Own good t'Oler血 ceωhightempera伽res(RUND且

ET AL.， unpublished data). O町 ec'Ophysi'Ol'Ogicalappr'Oach， 'Of C'O即時， does n'Ot c'Onsider life hist'Ory住aits血atmay

be imp'Ortant in determining the ec'Ol'Ogical success 'Of H.ferrea. F，民間dity，s田 ddispersal，

predati'On .levels， and 'Other dem'Ographic traits may als'O be c'Omp'Onents 'Of血issuccess.

O町 data'On Hopea ferrea are n'Ot yet sufficient t'O p釘 ameterizeec'Ophysi'Ol'Ogical

m'Odels 'Of住'eegr'Owth and devel'Opment， but血eyd'O all'Ow us t'O raise significant questi'Ons relevant t'O c'Onsiderations 'Of the e質問ts'Of gl'Obal ch佃 .ge佃 dbi'Odiversity恒 semi-evergreen

f'Orests. While the semi-evergreen f'Orest 'Of Sakaerat is relatively undisturbed， selective l'Ogging 'Of large住eeshas 'Occurred in血.epast. Is the open upper can'Opy s住ucωreat

Sakaerat and perhaps血ed'Ominance 'Of H戸rrea佃 artifact'Of such disturbance? A cl'Osed

PHOTOSYNτ百ETICRESPONSES AND DOMINANCE OF HOPEA FERREA 69

upper can'Opy might well pr'Ovide a very different light envir'Onment in the underst'Ory加 dselect against the current ec'Ol'Ogical success 'Of the light-adapted seedlings and sapl泊gs'Of H. ferrea. Such a change w'Ould be expected t'O pr'Om'Ote m'Ore mixed ec'Ol'Ogical d'Ominance， 組 dperhaps higher bi'Ol'Ogical diversity. W'Ould gl'Obal changes resulting in sh'Orter 'Or less arid合yseas'Ons fav'Or c'Ompe出 gspecies by reducing the dr'Ought s紅白spresent泊 theunderst'Ory and thus eliminate s'Ome 'Of the advantages 'Of H.戸rrea'Over its c'Ompetit'Ors in water relati'Ons characteristics? W'Ould l'Onger 'Or m'Ore severe dry seas'Ons likely stress出esemi-evergreen c'Ommunity 'Overall and all'Ow m'Ovements 'Of gr'Ound frres fr'Om adjacent c'Ommunities int'O血.eunderst'Ory and血uspr'Om'Ote replacement by decidu'Ous dipter'Oc紅pf'Orest species? The present c'Ommunity s町uc佃re'Of dry dipter'Ocarp f'Orests may well be

highly sensitive t'O small changes in f'Orest can'Opy s加 C佃 e'Or regi'Onal climatic c'Onditi'Ons. Future studies will h'Opefully all'Ow m'Ore direct c'Onsiderati'On 'Of such questi'Ons.

ACKNOWLEDGMENTS

We仕1ank出eUniversity 'Of Calif'Omia Pacific Rim Pr'Ogram f'Or supp'Ort 'Of出isresearch. C'O'Operative studies with官1aiscientists were aided greatly by a US AID Fell'Owship t'O PWR伽'Oughthe US/ Asia Envir'Onmental P蜘 m均.Successful field studies w'Ould n'Ot have been p'Ossible with'Out the able assistance 'Of Mak:amas Suttacheep. We釘 egrateful t'O 出estaffs 'Of the Sak:aerat Experimental Research Station and the nCA Ref'Orestati'On Center f'Or their assistance and f'Or all'Owing us t'O use their facilities.

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