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ECOSYSTEM ECOLOGY - ORIGINAL PAPER
Deciduousness in a seasonal tropical forest in western Thailand:
interannual and intraspecific variation in timing, duration
and environmental cues
Laura J. Williams Æ Sarayudh Bunyavejchewin Æ
Patrick J. Baker
Received: 20 February 2007 / Accepted: 3 December 2007 / Published online: 10 January 2008
Ó Springer-Verlag 2007
Abstract Seasonal tropical forests exhibit a great diver-
sity of leaf exchange patterns. Within these forests variation
in the timing and intensity of leaf exchange may occur
within and among individual trees and species, as well as
from year to year. Understanding what generates this
diversity of phenological behaviour requires a mechanistic
model that incorporates rate-limiting physiological condi-
tions, environmental cues, and their interactions. In this
study we examined long-term patterns of leaf flushing for a
large proportion of the hundreds of tree species that co-
occur in a seasonal tropical forest community in western
Thailand. We used the data to examine community-wide
variation in deciduousness and tested competing hypotheses
regarding the timing and triggers of leaf flushing in seasonal
tropical forests. We developed metrics to quantify the nat-
ure of deciduousness (its magnitude, timing and duration)
and its variability among survey years and across a range of
taxonomic levels. Tree species varied widely in the mag-
nitude, duration, and variability of leaf loss within species
and across years. The magnitude of deciduousness ranged
from complete crown loss to no crown loss. Among species
that lost most of their crown, the duration of deciduousness
ranged from 2 to 21 weeks. The duration of deciduousness
in the majority of species was considerably shorter than in
neotropical forests with similar rainfall periodicity. While
the timing of leaf flushing varied among species, most
(*70%) flushed during the dry season. Leaf flushing was
associated with changes in photoperiod in some species and
the timing of rainfall in other species. However, more than a
third of species showed no clear association with either
photoperiod or rainfall, despite the considerable length and
depth of the dataset. Further progress in resolving the
underlying internal and external mechanisms controlling
leaf exchange will require targeting these species for
detailed physiological and microclimatic studies.
Keywords Dry season flushing Huai Kha Khaeng
Southeast Asia Tropical tree phenology
Introduction
The production of new leaves in a tree requires a significant
investment of carbon, water and nutrients, the availability
of which is directly and indirectly influenced by intra- and
interannual variations in environmental conditions. In the
temperate zone, intra-annual temperature seasonality exerts
the dominant control over the timing of leaf flushing (e.g.
Kramer and Kozlowski 1979; Valentine 1983; Lechowicz
1984). In tropical forests, climatic seasonality is primarily a
function of intra-annual variation in water availability
(Borchert 1994a, 1994b, 1994c; Reich et al. 2004). In the
seasonal or monsoonal tropical forests, where rainfall
seasonality is most pronounced, this has led to a great
diversity of leaf exchange patterns—trees may produce
new leaves during the wet season, the dry season, or the
transition between seasons; trees may lose all of their
leaves, some of their leaves, or none of their leaves; and
these patterns may vary within and among individuals and
species and from year to year.
Communicated by Marilyn Ball.
L. J. Williams P. J. Baker (&)
Australian Centre for Biodiversity, School of Biological
Sciences, Monash University, Melbourne, VIC 3800, Australia
e-mail: patrick.baker@sci.monash.edu.au
S. Bunyavejchewin
Wildlife and Plant Conservation Department, Research Office,
National Parks, Chatuchak, Bangkok 10900, Thailand
123
Oecologia (2008) 155:571–582
DOI 10.1007/s00442-007-0938-1
Variation in the timing and intensity of leaf exchange
among sympatric tree species is influenced by a range of
internal physiological rate-limiting thresholds (e.g. inter-
nal water status, nonstructural carbohydrate supply) and
external environmental cues (e.g. photoperiod, soil water
availability, vapour pressure deficit) that may interact in
complex ways. In seasonal tropical forests around the
world, the timing of leaf exchange varies within and
among regions (Murphy and Lugo 1986). Leaf expansion
requires cell enlargement—a water-intensive process that
is highly susceptible to water stress (Borchert 1994a,
1994b). Intuitively, then, leaf flushing should occur when
water and carbon availability are high (Chabot and Hicks
1982)—that is, during the wet season. Indeed, wet season
leaf flushing is common in dry forests in Africa (Lie-
berman 1982; Lieberman and Lieberman 1984) and the
neotropics (Frankie et al. 1974; Bullock and Solis-Ma-
gallanes 1990). However, the occurrence of leaf flushing
during the dry season, which would appear to contravene
this rationale, is well-documented in tropical savannas of
north Australia (Williams et al. 1997) and Brazil (Rivera
et al. 2002), dry lowland forest in Costa Rica (Borchert
1994a), deciduous forest in India (Elliott et al. 2006), and
seasonal tropical forests in Thailand (Rivera et al. 2002;
Elliott et al. 2006), Argentina and Java (Rivera et al.
2002).
While dry season leaf exchange may seem counterin-
tuitive, the production of new leaves during a period of
high water stress implies that the tree has access to or has
accumulated sufficient water and carbon reserves for leaf
expansion. Given sufficient reserves, the timing of leaf
expansion may then depend upon the appropriate envi-
ronmental cue, such as rainfall (Walter 1971; Lieberman
1982; Reich and Borchert 1984; Borchert 1994b), tem-
perature (Walter 1971; Morellato et al. 2000)or
photoperiod (Rivera et al. 2002; Borchert et al. 2005; El-
liott et al. 2006). Many phenological studies of seasonal
tropical forests have focused on rainfall as a proximate cue
for leaf flushing because of the rate-limiting threshold
imposed by water availability (Reich and Borchert 1984;
Borchert 1994a, 1994b, 1994c). However, the results have
been equivocal. Rainfall is clearly not the only factor
triggering leaf exchange (Wright and Cornejo 1990; Rivera
et al. 2002; Kushwaha and Singh 2005; Elliott et al. 2006)
and irrigation studies have failed to induce leaf flushing
consistently among species within a seasonal tropical forest
community (Rivera et al. 2002). Recent attention has
focused on photoperiod, in particular the rapid change in
day-length
that occurs around the spring equinox, as a
trigger of leaf flushing (Borchert et al. 2005). However,
these studies have also failed to account for a considerable
proportion of the variation in leaf flushing behaviour.
Where sufficient water or carbon reserves are not present,
leaf flushing will not occur despite the occurrence of the
requisite environmental cue. For example, Shorea siam-
ensis in western Thailand experience bud break several
weeks before the spring equinox, but leaf expansion is
limited to those individuals with ready access to subsurface
soil water reserves (Elliott et al. 2006). For those individ-
uals with small root systems or that are distant from a ready
source of subsurface soil water, leaf expansion occurs only
after the first significant rains, when water availability is no
longer rate-limiting.
Reconciling the range of leaf flushing behaviours
among diverse biogeographic regions and tree flora
requires a general mechanistic model of foliar phenology
that integrates both internal and external factors and the
interactions among them. In this study we use ‘‘decidu-
ousness’’ as an organizing concept to evaluate the range
of leaf exchange behaviour from an extensive, decade-
long phenology survey of a seasonal tropical forest in
western Thailand. In this area where forest types domi-
nated by deciduous or evergreen tree species occur in
mosaic fashion across the landscape, leaf exchange pat-
terns at the community level are particularly complex and
poorly documented (Elliot et al. 2006). However, the
wide range of leaf exchange patterns, species-specific
physiologies, and local environmental heterogeneity pro-
vide a unique opportunity to investigate the relative
importance of specific internal and external factors on leaf
exchange.
In this study we examine variation in the timing, mag-
nitude, and consistency of leaf flushing behaviour within
and among conspecific trees, within and among years, and
among different tree species. We use these data to define
the range of variation in leaf flushing behaviour among a
diverse set of species and test the role of photoperiod and
rainfall as potential external cues for leaf flushing.
Methods
Study site
The study was conducted at the Huai Kha Khaeng (HKK)
Wildlife Sanctuary, Uthai Thani Province in central-west
Thailand (15°N, 100°E). Huai Kha Khaeng is dominated
by three forest types that occur in a landscape mosaic:
seasonal dry evergreen forest, mixed deciduous forest, and
deciduous dipterocarp forest. Foliar phenology was moni-
tored in and around a 50-ha forest dynamics plot dominated
by seasonal dry evergreen forest. Seasonal evergreen forest
at HKK is structurally complex, with trees reaching heights
of 60 m, and species-rich, with [250 tree species. While
the seasonal evergreen forest has a complex history of
natural disturbances, the area of forest in which we
572 Oecologia (2008) 155:571–582
123
conducted the phenological study has no history of human
disturbance (Baker et al. 2005).
This part of Thailand has a distinct monsoon climate.
The study area receives ca. 1,500 mm of rainfall each year,
with [90% occurring between April and October. While
the dry season is typically considered to extend from
November to April, there is interannual variation in the
timing of the onset of the monsoons. In some years the first
significant rains may arrive as early as mid-March, while in
other years the rains may arrive as late as mid-May. The
early dry season is comparatively cool (mean minimum
and maximum January temperatures are 18.3 and 32.5°C,
respectively), with maximum daily temperatures reaching a
peak during the late dry season (mean minimum and
maximum April temperatures are 25.6 and 37.9°C,
respectively) (Rundel and Boonpragob 1995).
Data collection
The full dataset includes observations of the phenological
conditions of 1,383 individual trees from 218 species.
Because some species are only represented in the dataset
by one or a few individuals we have restricted our
analysis to the 85 tree species with [5 individuals,
Table 1). These species include all of the common species
and many of the less common species on the 50-ha plot.
For these 85 species, observations were made at two-week
intervals from April 1999 to April 2004. For 28 species,
observations were also made at two-week intervals from
April 1994 to 1997 (Table 1). Phenological activity of
each tree was recorded using an incremental scale
(‘‘phenology score’’) of 0–4 to record the proportions of
the tree crown present as new (light green leaves), mature
(dark-green leaves), old (brown, red, or yellow leaves) or
fallen leaves; 0 = 0%, 0.1 = rare, 1 = up to 25%, 2 = 26–
50%, 3 = 51–75%, 4 = 76–100%, such that each tree’s
phenology score summed to four. Analyses of the spatial
distribution of tree species on the 50-ha plot relative to
known edaphic and topographic variations have shown
little evidence of habitat specialization among the
tree species. All trees surveyed were reproductively
mature individuals and were not suppressed. In those
cases in which a tree died during the study period it was
replaced with a nearby individual of the same species.
However, this was relatively uncommon (\1% of the
study trees).
Data preparation
Phenological observations were tabulated and checked for
missed phenophases where consecutive observations
Table 1 The 85 tree species analysed and observation period(s)
Species Observation
period(s)
Acer oblongum Wall. ex DC. (SAP) 1994–1997,
1999–2004
Afzelia xylocarpa (Kurz) Craib (FAB) 1994–1997,
1999–2004
Aglaia odorata Lour. (MELI) 1999–2004
Aglaia spectabilis (MELI) 1999–2004
Anisoptera costata Korth. (DIP) 1999–2004
Aphanamixis polystachya (Wall.) R. Parker
(MELI)
1994–1997,
1999–2004
Ardisia polycephala Wall. ex A. DC. (MYRS) 1999–2004
Arytera littoralis Blume (SAP) 1999–2004
Baccaurea ramiflora Lour. (EUP) 1994–1997,
1999–2004
Carallia brachiata (Lour.) Merr. (RHI) 1999–2004
Carya sphaerica Roxb. (JUG) 1999–2004
Casearia grewiifolia Vent. var. grewiifolia (FLA) 1999–2004
Cassia fistula L. (FAB) 1994–1997,
1999–2004
Champereia manillana (Blume) Merr. (OPI) 1999–2004
Chionanthus callophyllus Blume (OLE) 1994–1997,
1999–2004
Chukrasia velutina (M.Roem.) C. DC. (MELI) 1999–2004
Croton hutchinsonianus Hosseus (EUP) 1999–2004
Croton roxburghii N.P. Balakr. (EUP) 1999–2004
Cyathocalyx martabanicus Hook.f. & Thoms. var.
harmandii Finet & Gagnep. (ANN)
1999–2004
Dalbergia cochinchinensis Pierre (FAB) 1999–2004
Dalbergia oliveri Gamble (FAB) 1999–2004
Dimocarpus longan Lour. subsp. longan var.
longan (SAP)
1994–1997,
1999–2004
Diospyros ferrea (Willd.) Bakh. (EBE) 1994–1997,
1999–2004
Diospyros variegata Kurz (EBE) 1999–2004
Diospyros winitii Fletcher (EBE) 1999–2004
Dipterocarpus alatus Roxb. ex G.Don (DIP) 1999–2004
Dipterocarpus obtusifolius Teijsm. ex Miq. (DIP) 1994–1997,
1999–2004
Duabanga grandiflora (Roxb. ex DC.) Walp
(SON)
1994–1997,
1999–2004
Engelhardtia serrata Blume (JUG) 1994–1997,
1999–2004
Fernandoa adenophylla (Wall. ex G. Don)
Steenis (BIG)
1999–2004
Garcinia speciosa Wall. (CLU) 1994–1997,
1999–2004
Gardenia sootepensis Hutch. (RUB) 1999–2004
Garuga pinnata Roxb. (BUR) 1994–1997,
1999–2004
Gluta obovata Craib (ANA) 1999–2004
Harpullia arborea (Blanco)
Radlk.
(SAP) 1994–1997,
1999–2004
Oecologia (2008) 155:571–582 573
123
showed the crown changing from fallen to mature leaves
without an intervening observation of leaf flushing. In
instances where a leaf-flushing event was missed between
observation periods, an observation of new leaves (of
phenological intensity equivalent to the change from fallen
to mature leaves) was inserted on the date halfway between
two observation periods. Where a tree died during the study
period, data preceding the tree’s death were excluded from
analysis, based on a qualitative assessment of typical
phenological behaviour, to remove atypical phenological
observations related to the health of the tree.
Data analysis
Characteristics of deciduousness
We examined the nature of deciduousness within and among
species in the community using two quantitative metrics: the
magnitude and duration of deciduousness. The magnitude of
Table 1 continued
Species Observation
period(s)
Hopea odorata Roxb. (DIP) 1994–1997,
1999–2004
Irvingia malayana Oliv. ex A.W. Benn. (IRV) 1999–2004
Lagerstroemia macrocarpa Wall. (LYT) 1999–2004
Lagerstroemia tomentosa C.Presl (LYT) 1994–1997,
1999–2004
Lagerstroemia villosa Wall. ex Kurz (LYT) 1999–2004
Lithocarpus ceriferus A.Camus (FAG) 1999–2004
Lithocarpus grandifolius (D.Don) Bigwood
(FAG)
1999–2004
Litsea semecarpifolia Hook.f. (LAU) 1999–2004
Mallotus philippensis Mull. Arg. (EUP) 1999–2004
Mangifera caloneura Kurz (ANA) 1999–2004
Margaritaria indica (Dalzell) Airy Shaw (EUP) 1994–1997,
1999–2004
Markhamia stipulata Seem. (BIG) 1999–2004
Melia azedarach L. (MELI) 1994–1997,
1999–2004
Memecylon ovatum Sm. (MELA) 1999–2004
Mischocarpus pentapetalus (Roxb.) Radlk. (SAP) 1999–2004
Mitrephora thorelii Pierre (ANN) 1994–1997,
1999–2004
Murraya paniculata (L.) Jack (RUT) 1999–2004
Neolitsea obtusifolia Merrill (LAU) 1999–2004
Orophea polycarpa A. DC. (ANN) 1999–2004
Persea sp. (LAU) 1999–2004
Phoebe paniculata (Nees) Nees (LAU) 1999–2004
Phyllanthus emblica L. (EUP) 1999–2004
Polyalthia cerasoides (Roxb.) Benth. ex Bedd.
(ANN)
1999–2004
Polyalthia viridis Craib (ANN) 1994–1997,
1999–2004
Premna villosa C.B. Clarke (VER) 1999–2004
Prismatomeris tetrandra (Roxb) K.Schum. subsp.
malayana (Ridi.) Johans. (RUB)
1999–2004
Protium serratum Engl. (BUR) 1994–1997,
1999–2004
Prunus arborea (Blume) Kalkman (ROS) 1994–1997,
1999–2004
Pterocarpus macrocarpus Kurz (FAB) 1994–1997,
1999–2004
Pterospermum grandiflorum Craib (STE) 1999–2004
Radermachera ignea (Kurz) Steenis (BIG) 1999–2004
Saccopetalum lineatum Craib (ANN) 1994–1997,
1999–2004
Schleichera oleosa (Lour.) Oken (SAP) 1999–2004
Senna garrettiana (Craib) Irwim & Barneby
(FAB)
1999–2004
Shorea obtusa Wall. Ex Blume (DIP) 1999–2004
Shorea roxburghii G.Don (DIP) 1999–2004
Table 1 continued
Species Observation
period(s)
Shorea siamensis Miq. (DIP) 1999–2004
Spondias pinnata (L.f.) Kurz (ANA) 1994–1997,
1999–2004
Syzygium cf. hancei Merrill & Perry (MYRT) 1999–2004
Syzygium cumiui (L.) Skeels (MYRT) 1999–2004
Terminalia bellirica (Gaerth.) Roxb. (COM) 1999–2004
Terminalia glaucifolia Craib (COM) 1999–2004
Terminalia mucronata Craib & Hutch (COM) 1999–2004
Tetrameles nudiflora R.Br. (DAT) 1994–1997,
1999–2004
Trewia nudiflora L. (EUP) 1994–1997,
1999–2004
Vatica cinerea King (DIP) 1994–1997,
1999–2004
Vitex glabrata R.Br. (VER) 1999–2004
Vitex peduncularis Wall. ex Schauer (VER) 1999–2004
Xanthophyllum virens Roxb. (POL) 1999–2004
Zanthoxylum limonella (Dennst.) Alston (RUT) 1999–2004
Family codes are provided in brackets following species names
Family codes: ANA Anacardiaceae, ANN Annonaceae, BIG Bignon-
iaceae, BUR Burseraceae, CLU Clusiaceae COM Combretaceae, DAT
Datiscaceae, DIP Dipterocarpaceae, EBE Ebenaceae, EUP Euphor-
biaceae, FAB Fabaceae, FAG Fagaceae, FLA Flacourtiaceae, IRV
Irvingiaceae, JUG Juglandaceae, LAU Lauraceae, LYT Lythraceae,
MELA Melastomataceae, MELI Meliaceae, MYRS Myrsinaceae,
MYRT Myrtaceae, OLE Oleaceae, OPI Opiliaceae, POL Polygala-
ceae, RHI Rhizophoraceae, ROS Rosaceae, RUB Rubiaceae, RUT
Rutaceae, SAP Sapindaceae, SON Sonneratiaceae, STE Sterculiaceae,
VER Verbenaceae
574 Oecologia (2008) 155:571–582
123
deciduousness for each species was determined by calcu-
lating the maximum leaflessness of each individual during
each phenological year (November–November) and aver-
aging these values for the species to give the mean maximum
magnitude of deciduousness (D
m
) for each species. We
estimated the duration of deciduousness for those species
with D
m
scores [ 2 (i.e. moderate to high magnitudes of
deciduousness). For each species, the number of weeks that
each tree was at or near the species-specific D
m
was calcu-
lated for each phenological year and then averaged to obtain
the mean duration of deciduousness (D
d
). For both metrics,
standard errors were calculated to provide an index of
interannual and intraspecific variability.
Timing of leaf flushing
Circular statistics were used to analyse the timing of leaf
flushing (Batchelet 1981; Milton 1991; Davies 1999;
Morellato et al. 2000). Initially, dates of leaf flushing were
converted to angles from 0 to 360°. Observations of leaf
flushing were weighted according to their intensity (i.e.
phenology score of 0.1, 1–3, or 4). As data were collected
on a linear scale, statistical weighting was performed by
substituting intensity with frequency—for example, an
observation of phenology score 2 was presented as two
observations of leaf flushing on that date (Dan Bebber,
personal communication). The mean angle ð
hÞ; which
represents the mean date of leaf flushing, and the vector
length (r), which represents the temporal concentration of
data around the mean angle, were then calculated using the
formulae:
h ¼ arctan ðy=xÞ if x [ 0
h ¼ 180 þ arctan ðy=xÞ if x\0
where; x ¼
1
n
X
cos h
i
; and y ¼
1
n
X
sin h
i
then; r ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffi
x
2
þ y
2
p
where n is the number of observations of phenological
activity and h
i
is the angle representing the date of pheno-
logical activity (Davies and Ashton 1999). The vector length,
r, is a value between 0 and 1; 0 if all data are evenly dis-
tributed around the circle and 1 if all data occur on a single
point (Batchelet 1981; Milton 1991; Zar 1999). We tested
whether the mean angle was nonrandom using Rayleigh’s
test (Batchelet 1981; Zar 1999). We then back-transformed
the mean angle to determine the mean date of flushing.
Proximate causes of leaf flushing
To determine the relative importance of rainfall and pho-
toperiod as triggers of leaf flushing, we compared the
timing of leaf flushing of 24 species with sufficient phe-
nology data to daily rainfall data for the period 1994–1997.
In principle, distinguishing between these two factors as
leaf flushing triggers should be relatively straightforward.
Rainfall varies substantially in timing and intensity during
the dry season and the onset of the monsoons, whereas
photoperiod is invariant at temporal and spatial scales
relevant to community ecology. Thus if photoperiod trig-
gers leaf flushing, there should be both intraspecific and
interannual synchrony in the emergence of new leaves. In
contrast, if rainfall triggers leaf flushing, new leaves should
not emerge before the first substantial rains (defined as
C 20 mm; Rolf Borchert, personal communication).
However, this assumes that the internal water status is
directly correlated with rainfall availability. Where trees
have access to deep soil water (e.g. Nepstad et al. 1994)or
are capable of storing water in their stems during the dry
season (e.g. Chapotin et al. 2006), this correlation may be
weak or nonexistent. Nonetheless, it is reasonable to expect
that the internal water status of the tree, which is a function
of species-specific physiology, soil–water availability,
rainfall, temperature, and relative humidity, will show
considerably more interannual variation than photoperiod.
Results
Characteristics of deciduousness
The magnitude of deciduousness (D
m
) among the 85 spe-
cies covered the full range of potential scores, from no loss
of foliage (D
m
= 0) to complete canopy loss (D
m
=4)
(Fig. 1). Most species displayed either a low magnitude of
deciduousness (49% of species had a D
m
\ 1) or a high
magnitude of deciduousness (32% of species had a D
m
[3;
Fig. 1). For species with an intermediate magnitude of
deciduousness there was a gradient of D
m
scores (Fig. 1).
Most species showed interannual variation in D
m
(rep-
resented by the error bars in Fig. 1). While eight species
lost their entire crown each year, Persea sp. was the only
species to remain completely evergreen. Interannual vari-
ability in D
m
was lowest for species with high scores. For
example, Pterocarpus macrocarpus had a D
m
of 3.9 and
showed high synchrony among years in the magnitude and
timing of crown loss (Fig. 2). In contrast, interannual
variability in D
m
was greatest for species with intermediate
scores. For example, Acer oblongum had a D
m
of 1.6 and
showed considerable variability among years in the mag-
nitude and timing of crown loss (Fig. 2). The highest
interannual variability was observed in Shorea obtusa, with
a D
m
of 2.4 and a standard deviation of 1.9; individuals in
some years lost their entire crown and in other years did not
lose any of their crown (Fig. 1).
Oecologia (2008) 155:571–582 575
123
For the 34 species with a D
m
[ 2, the duration of
deciduousness (D
d
) ranged from \3 weeks in Shorea ob-
tusa,to*5 months in Tetrameles nudiflora (Fig. 1). The
majority of species (70%) had D
d
values of \2 months
(Fig. 1). Only four species were deciduous for more than
2.5 months (T. nudiflora, Spondias pinnata, Garuga pin-
nata and Vitex glabrata). Interannual variation in D
d
was
evident, with standard deviations ranging from 0.1 months
in V. glabrata to 1.5 months in Lithocarpus ceriferus.
However, the variation in D
d
scores was not correlated with
the means of the D
d
scores.
In general, species with low D
m
scores had low D
d
values (R
2
= 0.39; Fig. 3); however, this relationship was
not consistent. For example, both S. pinnata and V. pe-
duncularis generally lost their entire crown. However, S.
pinnata on average was leafless for four months, whereas
V. peduncularis was leafless for 1.3 months.
Timing and patterns of leaf flushing
In all species, the timing of leaf flushing was significantly
nonrandom (Rayleigh’s test, P \ 0.005). Within the com-
munity, the timing of leaf flushing varied widely among the
85 species (Fig. 4); however, the majority of species
(*70%) had their mean flushing date during the dry season
and 90% of these flushed during the last two months of the
dry season (Fig. 4). Of the species flushing new leaves
during the wet season, most flushed in the early wet season,
from mid-April to mid-May. Only four species, Diptero-
carpus obtusifolius, Cyathocalyx martabanicus, Persea sp.,
and Chionanthus callophyllus, flushed after June (Fig. 4).
All species with mean flushing dates at less common times,
such as the late wet season and early dry season, were
evergreen species.
The degree of interannual synchrony in leaf flushing
varied among species (error bars, Fig. 4). In some species,
the mean date of leaf flushing occurred within the same 14-
day period in all years of observation (e.g. Dalbergia co-
chinchinensis, D. oliveri and Pterospermum grandiflorum).
In other species the mean date of leaf flushing differed
among years by[5 months (e.g. Champereia manillana, L.
ceriferus, and Memecylon ovatum). In general, greater
interannual synchrony was observed in species with a
greater D
m
(Fig. 4).
The degree of intraspecific synchrony in the timing of leaf
flushing varied among species. Some species, for example
Premna villosa, S. obtusa and Ardisia polycephala, had
synchronous leaf flushing, with all conspecific individuals
flushing within a 14-day period. Other species, for example
Duabanga grandiflora and Memecylon ovatum, showed
asynchrony in leaf flushing with the mean date of leaf flushing
differing among conspecific individuals by[5 months.
Four genera were examined that each contained three
species (Lagerstroemia, Terminalia, Diospyros, and Sho-
rea). Terminalia and Lagerstroemia showed relatively high
Carya
Lagerstroemia v.
Melia
Senna
Spondias
Terminalia g.
Tetrameles
Vitex g.
Garuga
Vitex p.
Cassia
Terminalia m.
Pterocarpus
Fernandoa
Afzelia
Premna
Terminalia b.
Dalbergia o.
Shorea s.
Lagerstroemia t.
Lagerstroemia m.
Zanthoxylum l.
Trewia
Schleichera
Margaritaria
Polyalthia c.
Markhamia
Phyllanthus
Protium
Engelhardtia
Shorea r.
Shorea o.
Lithocarpus c.
Saccopetalum
Croton h.
Gardenia
Dalbergia c.
Murraya
Chukrasia
Acer
Polyalthia v.
Diospyros w.
Dipterocarpus a.
Anisoptera
Litsea
Radermachera
Duabanga
Chionanthus
Mallotus
Dipterocarpus o.
Mangifera
Diospyros f.
Hopea
Syzygium c.
Harpullia
Aglaia s.
Memecylon
Aphanamixis
Xanthophyllum
Pterospermum
Aglaia o.
Irvingia
Lithocarpus g.
Vatica
Croton r.
Mitrephora
Prismatomeris
Casearia
Syzygium h.
Gluta
Garcinia
Carallia
Orophea
Dimocarpus
Prunus
Baccaurea
Champereia
Phoebe
Cyathocalyx
Arytera
Neolitsea
Ardisia
Diospyros v.
Mischocarpus
Persea
Fallen leaves
(mean phenolo
g
y score)
0 1 2 3 4 5
0 1 2 3 4
Months at/near maximum leaf fall
Fig. 1 Mean magnitude of deciduousness (D
m
)—the mean propor-
tion of crown loss for each of the 85 species (solid symbols)—and
mean duration of deciduousness (D
d
)—the mean number of consec-
utive months spent at or near maximum leaf fall, for the 34 species
showing an average maximum leaf fall of phenology score [2(open
symbols). Error bars represent standard error
576 Oecologia (2008) 155:571–582
123
congeneric synchrony in the timing of leaf flushing
(r = 0.999; Fig. 5a, b). The Lagerstroemia species (L.
macrocarpa, L. tomentosa, and L. villosa) had mean
flushing dates within five days in mid-April, while the
Terminalia species (T. bellirica, T. glaucifolia and T. mu-
cronata) had mean flushing dates within eight days in early
April (Fig. 5a, b). The three Diospyros species were less
synchronous, with mean flushing dates spread over
*1.5 months (Fig. 5c). Despite relatively high intraspe-
cific synchrony in Shorea species (r C 0.87), flushing of
Shorea congeners was generally asynchronous with mean
flushing dates spread over *2 months (Fig. 5d).
The degree of synchrony in the timing of leaf flushing
varied among families. In some families, such as the Me-
liaceae, species all had mean flushing dates within
1.5 months (Fig. 5e). In contrast, other families showed
much less synchronous interspecific flushing. For example,
the eight species of Dipterocarpaceae had mean dates of
leaf flushing ranging from October to April, spanning the
entire dry season (Fig. 5f).
Proximate causes of leaf flushing
The synchrony of flushing among conspecifics, across
years and relative to the timing of rainfall, varied among
species. The first substantial rainfall occurred later in 1995
than in 1996 and 2000. Six of the 24 species analysed had
mean dates of leaf flushing that were relatively asynchro-
nous among years and conspecifics, but that were
consistent with interannual variation in the timing of
rainfall and always occurred after the first rain of C20 mm.
Nine species had mean dates of leaf flushing that were
relatively asynchronous among years and individuals, and
did not correlate with rainfall patterns. Nine species
showed consistency among years and conspecifics in the
timing of leaf flushing, despite the varied rainfall patterns.
Discussion
The nature of deciduousness in a seasonal
tropical forest
Substantial within-stand variation in the nature of decidu-
ousness has been reported in many forests, including
tropical deciduous forest in India (Kushwaha and Singh
2005) and subtropical deciduous forest in Argentina
(Rivera et al. 2002). In this study, the magnitude, duration
and interannual variation of deciduousness was found to
vary greatly among species. Only a weak positive associ-
ation was observed between duration and magnitude of
deciduousness. Accordingly, there was a continuum in
the magnitude, duration and interannual variability of
deciduousness among the 85 species that we examined
from this forest community. Within this forest, the duration
of deciduousness was generally shorter than in many
0
0.5
1
1.5
2
2.5
3
3.5
4
30-Nov
30-Dec
29-Jan
28-Feb
30-Mar
29-Apr
29-May
28-Jun
Fallen leaves
(phenology score)
Pterocarpus
macrocarpus
20-Aug
19-Sep
19-Oct
18-Nov
18-Dec
17-Jan
16-Feb
18-Mar
2000
2001
2002
2003
Acer oblongum
Fig. 2 Interannual variation in
patterns of leaf fall for
Pterocarpus macrocarpus and
Acer oblongum. The broken line
represents the spring equinox
0
1
2
3
4
5
6
1.5 2 2.5 3 3.5 4
Fallen leaves (mean phenolo
g
y score)
Months at/nearmaximum
deciduousness
Fig. 3 Interannual variation in the duration and mean magnitude of
deciduousness for the 34 species showing an average maximum leaf
fall score [2; error bars represent standard error
Oecologia (2008) 155:571–582 577
123
neotropical forests with comparable rainfall periodicity.
Only four species were observed to be deciduous for
[2.5 months, compared with many species in the seasonal
dry tropical forest of Costa Rica (Frankie et al. 1974;
Borchert 1994a) and Mexico (Bullock and Solis-Magall-
anes 1990) that are deciduous for[4 months. This suggests
that foliar phenology is not reliably predicted by rainfall
periodicity (Elliott et al. 2006).
Timing and patterns of leaf flushing
Among species within HKK, leaf flushing was not ran-
domly distributed throughout the year. Leaf flushing during
the dry season, and especially the late dry season, was
predominant. The presence and prevalence of dry season
leaf flushing is not a uniform feature of tropical deciduous
forests. However, the prevalence of dry season leaf flushing
in this study is comparable to a subtropical forest in
Argentina where 94% of the 50 species analysed flushed
new leaves during the dry season (Rivera et al. 2002), and a
Northern Australian savanna (Williams et al. 1997) where
80% of dominant species flushed leaves during the dry
season.
In general, less synchrony in the timing of leaf flushing
was observed within and among species that had a lower
magnitude of deciduousness. A lack of synchrony in foliar
phenology within and among species and years has been
15-Nov
30-Nov
15-Dec
30-Dec
14-Jan
29-Jan
13-Feb
28-Feb
15-Mar
30-Mar
14-Apr
29-Apr
14-May
29-May
13-Jun
28-Jun
13-Jul
28-Jul
12-Aug
27-Aug
11-Sep
26-Sep
11-Oct
26-Oct
10-Nov
Trewia
Afzelia
Protium
Melia
Shorea r.
Phyllanthus
Engelhardtia
Shorea o.
Dalbergia o.
Saccopetalum
Pterocarpus
Zanthoxylum l.
Margaritaria
Terminalia m.
Schleichera
Terminalia b.
Polyalthia c.
Lagerstroemia v.
Terminalia g.
Vitex p.
Lagerstroemia m.
Senna
Lagerstroemia t.
Carya
Shorea s.
Premna
Garuga
Fernandoa
Cassia
Lithocarpusc.
Markhamia
Vitex g.
Spondias
Tetrameles
Anisoptera
Dipterocarpus a.
Acer
Hopea
Syzygium h.
Carallia
Misch ocarpus
Croton r.
Arytera
Diospyros v.
Chukrasia
Vatica
Ardisia
Polyalthia v.
Xanthophyllum
Murraya
Syzygium c.
Dalbergia c.
Mangifera
Irvingia
Pterospermum
Harpullia
Phoebe
Champereia
Orophea
Diospyros f.
Casearia
Garcinia
Aglaia s.
Neolitsea
Croton h.
Mitrephora
Prismatomeris
Aglaia o.
Radermachera
Diospyros w.
Dimocarpus
Aphanamixis
Gluta
Gardenia
Mallotus
Lithocarpusg.
Baccaurea
Duabanga
Litsea
Prunus
Memecylon
Chionanthus
Persea
Cyathocalyx
Dipterocarpus o.
Leafless
Flushing
0
50
100
150
200
250
300
Rainfall (mm/14 da
y
s)
Deciduous
Deciduous stem succulent
Evergreen
Spring equinox
Timin
g
of phenolo
g
ical behaviour
Fig. 4 Summary of foliar
phenology for the 85 species
analysed and average rainfall in
mm at the study site (presented
as cumulative over the previous
14 days) for four years during
the study period (1995, 1996,
1999 and 2000; error bars
represent standard error).
Species are grouped as either
deciduous (magnitude of
deciduousness of phenological
score [2) or evergreen, with
deciduous stem succulent
species (sensu Borchert 1994a;
Borchert and Rivera 2001)
grouped separately. Mean date
of leaf flushing is indicated by a
filled square and error bars
represent the standard deviation
of interannual variation in the
mean flushing date (note that the
standard deviation for
Dipterocarpus obtusifolius is 72
and is truncated in the figure).
Hatched lines represent the
duration of deciduousness
(shown only for species grouped
as deciduous). The broken line
represents the spring equinox
578 Oecologia (2008) 155:571–582
123
observed in aseasonal tropical rainforest (e.g. Frankie et al.
1974; Reich 1995; Reich et al. 2004). Selection for syn-
chrony in a species’ foliar phenology may relate to the
intensity of seasonality (Reich 1995; Reich et al. 2004) and
therefore be strongest in species with marked intra-annual
changes in phenological attributes, namely more deciduous
habits.
Given the severity of seasonal drought and the physi-
ological dependence of leaf flushing upon water, the
existence of dry season leaf flushing—let alone its prev-
alence—seems counterintuitive. Physiologically, this
phenomena implies that the tree must have access to or
have accumulated sufficient reserves of water for leaf
expansion. For some individuals subsoil water reserves
may provide the requisite water source for flushing leaves
during the dry season (Borchert 1994a; Nepstad et al.
1994; Elliott et al. 2006). Despite flushing during the dry
season, most species in this study did not flush leaves
before the first significant rainfall event. This rainfall is
unlikely to influence soil water availability for the trees,
but may facilitate leaf flushing by increasing relative
humidity and reducing vapour pressure deficit (Duff et al.
1997; Myers et al. 1998). Water storage within plants
may also influence phenological patterns (Borchert 1994a;
Borchert and Rivera 2001; Rivera et al. 2002; Chapotin
et al. 2006). In this study, however, water storage ability
was not observed to facilitate dry season leaf flushing,
with the two stem succulent species (Spondias pinnata
and Tetrameles nudiflora) observed to consistently flush
leaves during the early wet season. Ecologically, the
prominence of dry season leaf flushing suggests an
adaptive advantage of this phenological behaviour over
evolutionary time. Dry season leaf flushing may be
advantageous in avoiding or minimising damage from
herbivores because young, vulnerable leaves are not being
produced during the wet season when insects may be
abundant (Aide 1992; Rivera et al. 2002). Leaf flushing
during the late dry season may also be advantageous in
maximising the length of the growing season by produc-
ing young, photosynthetically efficient leaves before the
onset of the wet season (Rivera et al. 2002; Elliott et al.
2006).
Proximate causes of leaf flushing
The proximate cause of leaf flushing varied among the 24
species that we examined in detail. Of these species, 38%
0
0.5
1
1.5
2
2.5
T. bellirica
T. glaucifolia
T. mucronata
S. obtusa
S. roxburghii
S. siamensis
0
0.5
1
1.5
2
2.5
D. ferrea
D. variegata
D. winitii
L. macrocarpa
L. tomentosa
L. villosa
New leaves
(phenology score)
(a)
(c)
(d)
(b)
0
0.2
0.4
0.6
0.8
1
1.2
12-Sep
12-Oct
12-Nov
12-Dec
12-Jan
12-Feb
12-Mar
12-Apr
12-May
12-Jun
12-Jul
Aglaia o.
Aglaia s.
Aphanamixis
Chukrasia
Melia
12-Sep
12-Oct
12-Nov
12-Dec
12-Jan
12-Feb
12-Mar
12-Apr
12-May
12-Jun
12-Jul
Anisoptera
Dipterocarpus a.
Dipterocarpus o.
Hopea
Shorea
Vatica
(f)
(e)
Fig. 5a–f Mean patterns (from
September 1999 to 2003) in leaf
flushing for congeneric species
from a Terminalia,
b Lagerstroemia, c Diospyros,
and d Shorea, and for
confamilial species from
e Meliaceae and
f Dipterocarpaceae. Note that
two Dipterocarpaceae species,
S. roxburghii and S. siamensis,
are excluded from Fig. 5f for
clarity but are shown in Fig. 5d.
Note also that the scale of
the y-axis varies among figures.
The broken line represents
the spring equinox
Oecologia (2008) 155:571–582 579
123
had intraspecifically and interannually synchronous phe-
nological patterns indicating photoperiodic control of leaf
flushing and 25% had phenological patterns indicating
rainfall as the proximate cause. The remaining 37% of the
species did not have phenological patterns consistent with
either photoperiod or rainfall as the proximate trigger of
leaf flushing. The phenological behaviour of these species
requires further analysis. However, phenological patterns
in some of these species may be opportunistic and
consistent with the ‘‘irregular leaf exchangers’’ described
in Elliott et al. (2006). In irregular leaf exchangers,
leaf flushing is primarily controlled by internal factors,
with the timing of leaf flushing related to leaf aging and
soil water availability (Rivera et al. 2002; Elliott et al.
2006).
Disentangling the proximate cause of leaf flushing is
complicated by the difficulty of defining synchrony and
confounding environmental factors. The degree to which
an external trigger may drive intraspecific and interannual
synchrony is dependent on the physiological requirements
of an individual being met (Reich 1995; Elliott et al. 2006).
For all species, the availability of water acts as a physio-
logical threshold for leaf flushing. Environmental variation
(e.g. variation in soil and topography) may influence water
availability; therefore, the degree of synchrony across the
population and among years may be lower than theoreti-
cally expected for photoperiodic control. Correlation
between phenology and rainfall patterns may also be
complicated by environmental heterogeneity and micro-
site-specific variation that may influence the association
between rainfall and the availability of water to an indi-
vidual (Reich and Borchert 1984; Bullock and Solis-
Magallanes 1990; Rivera et al. 2002; Elliott et al. 2006). In
addition, cloud cover associated with rainfall may reduce
the availability of nonstructural carbohydrates that are
necessary for leaf formation (Graham et al. 2003). These
complications may indicate the need to integrate intensive
physiological and microclimatological studies with cor-
relative analyses to determine the proximate causes of leaf
flushing.
Phenology and phylogeny
An examination of phenological patterns at successive
phylogenetic levels may provide a framework for exam-
ining community dynamics and the evolution of
phenologies. Rivera et al. (2002) suggested that functional
phenological types that include characteristics of decidu-
ousness and the proximate triggers of leaf flushing might
be conserved within congeneric taxa. Among the 24 spe-
cies analysed to determine their proximate cause of leaf
flushing, phenological patterns of confamilial species
varied in similarity. For example, the three Fabaceae spe-
cies (Afzelia xylocarpa, Cassia fistula and Pterocarpus
macrocarpus) all appeared to have leaf flushing triggered
by photoperiod. In contrast, the three Dipterocarpaceae
species (Dipterocarpus obtusifolius, Hopea odorata and
Vatica cinerea) showed no consistent pattern in the timing
of their leaf flushing. The existence of phylogenetic simi-
larities in proximate triggers may provide an indication of
the physiological basis of external cues. However, in our
study phylogenetic patterns in proximate triggers were
inconsistent despite the large number of related sympatric
species that we examined.
Further research
To further understand dry season leaf flushing, there is a
need to integrate the findings of observational and phys-
iological studies. Several recent studies have investigated
aspects of the physiological controls and constraints on
foliar phenology. For example, studies of trees in seasonal
dry tropical forest have investigated water relations and
gas exchange (Choat et al. 2006), hydraulic architecture
(Choat et al. 2005), and relationships between hydraulics
and photosynthesis (Brodribb et al. 2002) across seasons
and in species with differing foliar phenologies. An
extensive field survey such as ours can provide the nec-
essary context for species selection to target specific
phenological comparisons or contrasts that may be
informative and help bridge the gap between correlative
and physiological analyses. Our dataset identifies a
numb
er
of species that are brevideciduous (e.g. L. ce-
riferus, S. obtusa, Litsea semicarpifolia) but show
considerable interannual and intraspecific variation in the
timing of leaf flushing. A more intensive study focusing
on microsite conditions and stem and leaf physiology
could be developed to address the underlying causes of
this variability.
Developing a conceptual model of functional types
that encompasses deciduousness and other phenological
patterns may advance understanding of community pat-
terns and processes (Borchert 1994a; Williams et al.
1997). However, the diversity of phenological patterns
observed within this study indicates the need to develop
an integrated view of foliar phenology using a mecha-
nistic model consistent with the underlying physiological
principles of carbon, water, and nutrient investment in
leaf exchange and the continuum of phenological attri-
butes at a community scale (Wurth et al. 2005). For
example, the apparently continuous spectrum of decidu-
ousness among sympatric species observed in this study
is inconsistent with the conventional approach of delin-
eating discrete phenological groups and with associating
580 Oecologia (2008) 155:571–582
123
a degree or type of deciduousness with a particular cli-
matic regime. Instead, our data suggest that developing a
functional typology based around continuous trait varia-
tion as suggested by Wright et al. (2004) would be more
appropriate.
Conclusions
We observed a wide range of foliar phenologies within a
seasonal tropical forest community in western Thailand.
The character of deciduousness, as defined by the mag-
nitude, duration and interannual variability of crown loss,
varied among the study species in an almost continuous
spectrum from completely deciduous to completely
evergreen. Foliar phenology is controlled by intra-annual
patterns of drought and water availability—leaf flushing
cannot occur unless the underlying physiological
requirements of cell expansion are met. However, the
complex interactions among species-specific physiologies,
local edaphic conditions, and intra-annual variation in the
timing of rainfall lead to considerable variation in the
timing, duration and magnitude of leaf exchange among
the tree species within this community. This within-
community variation in deciduousness, combined with
marked differences in the duration of deciduousness when
compared to neotropical forests of similar rainfall sea-
sonality, demonstrate that foliar phenology is a poor
indicator of climate. Further advances in understanding
the internal and external factors that influence decidu-
ousness within forest communities must integrate detailed
physiological studies to complement the growing body of
correlative and descriptive studies of foliar phenology in
tropical forests.
Acknowledgments We would like to thank the various staff at the
Khlong Phuu Research Station and Kapook Kapiang Ranger Station
at Huai Kha Khaeng who have taken the phenology measurements
every fortnight over much of the past decade. The management staff
at Huai Kha Khaeng have supported the 50-ha plot and the research
associated with it since its inception in 1991. This research has been
funded by USAID, the National Science Foundation (USA), and the
Smithsonian Institution’s Center for Tropical Forest Science Small
Grants program. We thank Dan Bebber for his suggestion regarding
circular statistics. Comments from Christian Ko
¨
rner, Marilyn Ball,
Jenny Read, and two anonymous reviewers improved the quality of
the manuscript. The research complies with the current laws gov-
erning the conductance of research in Thailand.
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