Effects of Forest Fragmentation and Flowering Phenology
on the Reproductive Success and Mating Patterns of
the Tropical Dry Forest Tree Pachira quinata
ERIC J. FUCHS,* JORGE A. LOBO,* AND MAURICIO QUESADA†‡
*Escuela de Biología, Universidad de Costa Rica, San José, Costa Rica
†Instituto de Ecología, Departamento de Ecología de los Recursos Naturales, Universidad Nacional Autónoma de
México, Apartado Postal 27–3 (Xangari) 58089, Morelia, Michoacán, México, email mquesada@oikos.unam.mx
Abstract: The results of several studies suggest that forest fragmentation affects the mating patterns and reproductive success of tropical tree species by reducing pollinator activity, pollen deposition, and outcrossing
levels. The flowering synchrony of trees has also been proposed as an additional factor in controlling fruit set
and regulating levels of outcrossing, particularly in disturbed habitats. We examined the effects of forest fragmentation and flowering phenology on the reproductive success and genetic structure of the progeny produced by the tropical tree Pachira quinata. We conducted our study in the dry forest of Costa Rica and compared trees in two density and environmental conditions: (1) isolated trees separated by 500 m from other
adult conspecifics and located in disturbed sites and (2) trees from continuous populations of groups of 20 or
more reproductive individuals per hectare surrounded by undisturbed mature forest. Our study was conducted in the Guanacaste Conservation Area, Costa Rica, and surrounding areas. To evaluate flowering phenology, trees were classified as having synchronous or asynchronous flowering. The phenological stage of individuals was classified according to the proximity of the peak flowering date of each tree with respect to the
mean peak flowering of the rest of the population. Six percent of the flowers produced a fruit in trees from
continuous populations, whereas in isolated trees only 3% of the flowers did so. Fruit set was not affected by
the flowering phenology of trees but was influenced mainly by factors associated with forest fragmentation.
Seed production per fruit was not affected by forest fragmentation or flowering phenology. Overall, total fruit
production per tree was not affected by forest fragmentation, because isolated trees tended to produce more
flowers than trees from continuous populations. Genetic analysis revealed that the progeny of trees from continuous populations experienced lower levels of relatedness, a tendency for higher levels of outcrossing, and/
or more sires than isolated trees. Our results suggest that forest fragmentation can have an effect on the mating patterns of P. quinata, reducing the number of outcross sires represented in the progeny of isolated trees.
Efectos de la Fragmentación de Bosques y la Fenología de Floración sobre el Éxito Reproductivo y los Patrones de
Apareamiento de Pachira quinata, un Árbol de Bosque Tropical Seco
Resumen: Los resultados de varios estudios sugieren que la fragmentación de bosques afecta los patrones de
apareamiento y de éxito reproductivo de especies de árboles tropicales al reducir la actividad de polinizadores, la deposición de polen y los niveles de exogamia. La sincronía de floración de árboles también ha sido
propuesta como un factor adicional en el control de la producción de frutos y la regulación de niveles de exogamia, especialmente en hábitats perturbados. Examinamos los efectos de la fragmentación de bosques y la
fenología floral sobre el éxito reproductivo y la estructura genética de la progenie producida por el árbol
tropical Pachira quinata. Realizamos nuestro estudio en el bosque seco de Costa Rica y comparamos árboles
en dos densidades y condiciones ambientales: (1) árboles aislados separados 500 m de otros adultos
conespecíficos y localizados en sitios perturbados y (2) árboles en poblaciones continuas con grupos de 20 o
‡Address correspondence to M. Quesada.
Paper submitted March 21, 2002; revised manuscript accepted April 17, 2002.
149
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Volume 17, No. 1, February 2003
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Fuchs et al.
más individuos reproductivos por hectárea rodeados por bosque maduro no perturbado. Nuestro estudio se
llevó a cabo en el Área de Conservación de Guanacaste, Costa Rica y áreas circundantes. Para evaluar la
fenología de floración, los árboles fueron clasificados en floración sincrónica o asincrónica. El estado
fenológico de los individuos se clasificó de acuerdo con la proximidad al pico promedio de floración del resto
de la población. Seis por ciento de las flores produjeron fruto en árboles en poblaciones continuas, mientras
que en árboles aislados sólo el 3 % de las flores desarrollaron frutos. La producción de frutos no fue afectada
por la fenología de floración de los árboles pero fue fluenciada principalmente por factores asociados con la
fragmentación del bosque. La producción de semillas por fruto no fue afectada por la fragmentación del
bosque ni por la fenología de floración. En general, la fragmentación del bosque no afectó la producción total de frutos por árbol, porque los árboles aislados tendieron a producir más flores que los árboles en las poblaciones continuas. Un análisis genético reveló que la progenie de árboles en poblaciones continuas experimentó menores niveles de afinidad, una tendencia a mayores niveles de exogamia y/o más padres que los
árboles aislados. Nuestros resultados sugieren que la fragmentación de bosques puede tener un efecto sobre
los patrones de apareamiento de P. quinata, reduciendo el número de padres exógamos representado en la
progenie de árboles aislados.
Introduction
The reduction and fragmentation of forests is one of the
major threats to natural tree populations in the tropics
( Whitmore & Sayer 1992; Heywood et al. 1994; Trejo &
Dirzo 2000). A reduction in natural habitat, with the subsequent spatial isolation of populations, is predicted to
have negative consequences for the reproductive success and gene flow of tropical tree species (Templeton
et al. 1990; Young et al. 1993; McCauley 1995; Nason &
Hamrick 1997). A lower density of reproductive trees is
expected to reduce the quantity and quality of pollen
flow and seed dispersal (Nason & Hamrick 1997). This,
in turn, is likely to reduce the genetic variability of and
increase deleterious effects on the progeny as a result
of inbreeding, especially in the case of outcrossing
species (Charlesworth & Charlesworth 1987; Ellstrand 1992; Ellstrand & Ellam 1993).
Empirical evidence shows that forest fragmentation
negatively affects plant reproduction by reducing pollinator activity (Aizen & Feinsinger 1994b; Cunningham
2000a ), pollen deposition ( Aizen & Feinsinger 1994a;
Cunningham 2000a; Ghazoul et al. 1998; Cascante et al.
2002), and fruit and seed set (Aizen & Feinsinger 1994a;
Ghazoul et al. 1998; Cunningham 2000b; Cascante et al.
2002 ). In addition, population genetic studies indicate
that the progeny of fragmented populations or isolated
trees are more likely to be produced via selfing or by a
few outcross pollen donors (Murawski et al. 1994; Hall
et al. 1996; Aldrich & Hamrick 1998; Aldrich et al. 1998;
Cascante et al. 2002). In contrast, high levels of gene
flow between fragmented populations have been proposed as a hindrance to the loss of genetic diversity by
genetic drift, resulting in fragmented populations that
preserve a high proportion of the genetic variability of
the original populations (White et al. 1999). But to estimate the effect of spatial isolation in disturbed habitats,
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Volume 17, No. 1, February 2003
more studies need to integrate genetic and reproductive
information ( Nason & Hamrick, 1997; Aldrich & Hamrick 1998; Cascante et al. 2002). This is of particular importance in plants with complex self-incompatibility systems and phenological patterns (Bawa 1974; Frankie et
al. 1974; Hamrick & Murawski 1990).
In addition to the spatial isolation caused by forest
fragmentation, temporal isolation caused by asynchronous flowering of individuals has been proposed as another important factor that affects the reproduction and
genetic structure of plant populations in disturbed habitats ( Murawski et al. 1990; Murawski & Hamrick 1991;
Murawski & Hamrick 1992b; Chase et al. 1996; Doligez
& Joly 1997; Nason & Hamrick 1997 ). Flowering phenology directly determines the effective number of pollen donors and the density of flowering individuals, both
of which affect the patterns of pollen flow between
trees (Stephenson 1982; Murawski & Hamrick 1992b ).
Trees that overlap in flowering period are likely to receive pollen from more pollen donors than trees with
asynchronous flowering. In contrast, trees with asynchronous flowering may experience a reduction in reproductive output, with the number of pollen donors
and the selfing rate being similar to those of trees found
in isolated fragments. Therefore, the integration of flowering phenology data into forest fragmentation studies is
required to understand variation in the reproductive
success and genetic variability of fragmented populations ( Murawski & Hamrick 1992b; Nason & Hamrick
1997).
We sought to determine the effects of flowering phenology and spatial isolation due to forest fragmentation
on the reproductive success and genetic structure of the
progeny produced by the tropical timber tree Pachira
quinata by analyzing the following variables: (1) flower
production, ( 2 ) fruit set, ( 3 ) seed production, ( 4 ) outbreeding frequency ( tm ), ( 5 ) paternity correlation ( rp )
Fuchs et al.
within trees ( i.e., probability of full-sib progeny obtained from outcrossed sires), and (6) mean relatedness
of seeds within and between fruits.
Habitat Fragmentation and Tree Reproduction
151
trees in pastures and a few continuous populations still
exist within protected areas of Guanacaste.
Sampling Design
Methods
Study Area
We studied Pachira quinata in northwestern Costa
Rica in the province of Guanacaste. This region is tropical dry forest according to the life-zone classification system of Holdridge (1969). The average annual rainfall is
1600 mm, and the dry season extends from December
to May. Most tree species are deciduous, and their flowering peak occurs during the dry season. Tropical dry
forests constitute one of the most endangered ecosystems in the world. Today, only 0.09% of the original dry
forest is present in Mesoamerica ( Janzen 1988 ), and
0.1% of Costa Rica is covered with this biome (SánchezAzofeifa 1996 ). Most of the tropical dry forest in Costa
Rica has been destroyed by the timber and cattle industry during the last century, and today much of this land
is covered by pasture and agricultural fields (Quesada
1974; Janzen 1988; Quesada & Stoner 2003).
Selection of Trees
Pachira quinata (Bombacaceae) ( Jacq.) Alverson
[Bombacopsis quinatum ( Jacq. Dugand )] ( Alverson
1994) is a Neotropical species distributed in the dry forest from Honduras to northern South America. It is an
important timber species that has been heavily exploited because of its durable and resistant wood. P.
quinata trees may grow up to 35 m in height and 2.5 m
in diameter under natural conditions. Trees normally
have big buttresses at the base, a gray thorny bark, and a
deep central root system. Leaves are digitate compound,
with glabrous, oblong leaflets ( Alverson 1994 ). P.
quinata is deciduous: leaf drop occurs in late November
and new leaves are produced in early May. The flowering period extends from January to March, immediately
followed by the fruiting period from April to May. Flowers are hermaphroditic and protandrous, with multistaminate white filaments and a single central style. Anthesis takes place at dusk, and flowers last for one night.
Flowers are pollinated by bats (Glossophaga soricina)
and sphingid moths (Quesada et al. 2001). Each ovary contains an average of 160 ovules, and fruits produce an average of 20 seeds, which are small and wind-dispersed
(Quesada et al. 2001). Fruits are dehiscent, dry, and woody.
P. quinata has been classified as a self-incompatible species based on hand-pollination experiments and our previous genetic studies ( Kane et al. 1993; Quesada et al.
2001). A high proportion of adult P. quinata in natural
populations has been extracted for timber, but isolated
To examine the effects of forest fragmentation on the reproductive success and genetic variation of the progeny
of P. quinata, we compared isolated trees with trees in
continuous populations. A tree was considered isolated
if it was separated by more than 500 m from the nearest
conspecific and surrounded by agricultural fields or pastures. Isolated trees were systematically selected in disturbed sites along the Panamerican Road near the Guanacaste Conservation Area. Continuous populations
consisted of groups of 20 or more reproductive individuals per hectare surrounded by undisturbed mature forest. They were located within the Guanacaste Conservation
Area, Costa Rica ( lat. 8437W, long. 945N, 200–250 m).
Trees in continuous populations were selected from
two sites located 15 km from each other within the
study area.
To control for the effect of flowering phenology on
the reproductive success and genetic variation of progeny of isolated and continuous populations, trees were
classified according to the time of peak flowering. We
determined the date of peak flowering for each individual
in Julian days by counting the total number of flowers
produced every 15 days during the reproductive season
and then used those data to estimate an average peak
flowering date for the entire population. The date of
peak flowering for the entire population was estimated
as the weighted mean of flowering dates of the population, where each flowering date was weighted by the
number of trees blooming for each date. The date of
peak flowering for each individual tree was estimated as
the weighted mean of the flowering dates of each individual, where each flowering date was weighted by the
number of flowers per tree for each date. Trees with
peak flowering dates within 1 SD of the population
mean date were classified as individuals with synchronous flowering, and the rest were classified as individuals with asynchronous flowering. Therefore, 50% of the
flowers produced by trees with asynchronous flowering
opened at least 3 weeks before ( or after ) the population’s peak flowering date.
To evaluate the reproductive success of P. quinata,
we determined the total flower and fruit production and
fruit set (i.e., number of fruits per flower) of each tree.
Because we counted the number of flowers produced
by each individual every 15 days, we estimated the total
flower production of each tree as the area under the distribution obtained by the number of flowers versus
time. We estimated the production of fruits of each individual tree by counting the total number of fruits produced every 15 days during the fruiting period. The total
number of fruits produced by each individual was esti-
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Habitat Fragmentation and Tree Reproduction
mated as the maximum number of fruits counted during
5 days of sampling. Because the dry fruit capsules of P.
quinata remain attached to the mother tree for several
days after dehiscence, the numbers of fruits counted for
each individual accumulate to a maximum number that
can be used as an estimate of the total production of
fruits per tree. Fruit set was estimated as the proportion
of the total number of fruits over the total number of
flowers produced by each individual. Flowers and fruits
were counted on 31 isolated trees and 24 trees in continuous populations. We used a two-way factorial analysis
of covariance (ANCOVA) (general linear models [GLM];
SAS Institute 1995 ) to analyze the effect of forest fragmentation and flowering phenology on total flower production, total fruit production, and fruit set of P.
quinata. The model we used included the following
main effects: tree condition (continuous or isolated
trees), flowering phenology (synchronous and asynchronous trees), and the interaction term. To control for variation associated with tree size, we used diameter at
breast height (dbh) as a covariate in the ANCOVA
model. To achieve normality, we used a square-root
transformation for total flower and fruit production and
an arcsine transformation for fruit set.
A sample of 20 fruits was collected from each maternal tree, and seeds from each fruit were counted and
classified as either aborted or potentially viable. We collected seeds from 20 isolated trees and 20 trees in continuous populations. Aborted seeds are usually wrinkled
and black, whereas potentially viable seeds are well
rounded and brown. We calculated the proportion of
potentially viable seed as the number of viable seeds
over the total number of seeds per fruit. We used a
mixed-effect analysis of variance ( ANOVA ) (GLM; SAS
1995) to analyze the effects of forest fragmentation and
flowering phenology on the number of potentially viable seeds. Tree condition and flowering phenology were
considered fixed effects, whereas maternal tree, nested
within tree condition, was considered a random term.
Seed number was log-transformed to achieve a normal
distribution.
Genetic Analysis
To determine the effects of forest fragmentation and
flowering phenology on the genetic structure of the
progeny produced by P. quinata, we conducted an allozyme analysis with starch-gel electrophoresis. We collected 10 fruits from each of 15 isolated trees and 15
trees from continuous populations and randomly selected four seeds from each fruit. Seeds were germinated in individual petri dishes. We extracted enzymes
from homogenized hypocotyl tissue obtained 3 days after seed germination. Extraction, gel buffers, and staining
protocols followed those of Weeden and Wendel (1989).
Six polymorphic enzyme systems were analyzed: leucine
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Fuchs et al.
aminopeptidase ( LAP, 3.4.11.1 ), shikimate dehydrogenase ( SKDH, 1.1.1.25 ), phosphoglucoisomerase ( PGI,
5.3.1.9), aspartate aminotransferase (AAT, 2.6.1.1.), esterase (EST, 3.1.1.1.), and alcohol dehydrogenase (ADH,
1.1.1.1). The PGI enzyme system showed two polymorphic loci; so we used seven polymorphic loci to determine the selfing rate and genetic relatedness within P.
quinata progenies. The genotype of adult trees was not
scored because enzyme activity was resolved only in
seedlings.
We estimated two genetic parameters for the population of trees in isolation and in continuous forest: multilocus outcrossing rate ( tm ) and a paternity correlation
( rp ) within trees, which was the proportion of full siblings among outcrossed siblings. The paternity correlation is also equivalent to the probability that any two
randomly chosen outcrossed seeds were sired by the
same father. Ritland (1989) states that the paternity correlation ( rp ) is inversely related to the number of outcross parents ( n ) by rp 1/n, where n is the effective
number of pollen donors ( i.e., unrelated sires ). These
genetic parameters were estimated for the progeny of
populations of maternal trees under different conditions
of forest fragmentation (isolated vs. continuous populations) and flowering phenology ( asynchronous vs. synchronous). We calculated genetic parameters using the
models proposed by Ritland (1989) and the MLTR computer program ( Ritland 1996 ). The standard error was
calculated by bootstrapping with 1000 repetitions (Ritland 1996). To compare the genetic parameters for the
populations of trees in different forest conditions, we
determined the difference in outcrossing rates for each
of the 1000 bootstraps for trees in isolation and trees in
continuous populations. We used the same procedure to
compare the correlation of paternity of the progeny
from trees in isolation with that of trees in continuous
populations. The normality of these differences was verified, and the probability of obtaining a difference value
statistically different from zero was tested with a student’s t test. We used the same procedure to compare
the tm and rp values between trees with synchronous
and asynchronous flowering phenology.
Because the estimation of the mating-system parameters of plant populations can have large variances because of variation between progenies in the number of
sires, selfing rate, and biparental inbreeding, we also calculated mean levels of relatedness for individual fruits.
We calculated two coefficients of mean relatedness for
each fruit: (1) the mean relatedness of seed pairs within
fruits and (2) the mean relatedness of seed pairs between fruits, which consisted of seeds from one fruit
and seeds from other fruits within the same tree. We
used a regression measure of relatedness (Queller &
Goodnight 1989) as an estimate of the standard coefficient of relationship, in which an estimate of 0.25 indicates a half-sibling relationship and 0.5 indicates a full-
Fuchs et al.
sibling relationship. However, natural inbreeding or outbreeding and statistical error may yield estimates of relatedness for half sibs and full sibs beyond this theoretical
range. In addition, natural variation of relatedness within
sibling progenies can result in a continuous distribution,
with values between 0 and 1. In studies with limited
sample sizes, as occurs for the progeny of individual
trees, regression coefficients of relatedness are statistically more robust than relatedness coefficients obtained
by maximum-likelihood methods ( Ritland 1989; Lynch
& Ritland 1999). Regression estimates of relatedness
were calculated with the computer program Relatedness 5.0 (Goodnight & Queller 1990). We used a twoway factorial ANOVA (GLM; SAS 1995) to analyze the
effect of forest fragmentation and flowering phenology
on the mean relatedness of seeds within and between
fruits of P. quinata. The model we used included the
following main effects: tree condition (continuous or
isolated trees), flowering phenology ( synchronous and
asynchronous trees ), and the interaction term. A Kolmogorov-Smirnov test showed that relatedness values
followed a normal distribution (within fruits, d 0.028,
p 0.05; between fruits, d 0.034, p 0.05). A Pearson correlation coefficient was calculated for the relationship of mean relatedness between and within fruits.
Results
The duration of flowering and the mean peak flowering
date were similar between tree conditions (Fig. 1). However, isolated trees ( x 6655, SE 91) produced more
flowers per individual than trees from continuous populations ( x 2357, SE 100; F1,51 5.5, p 0.05; Fig.
1). Total flower production was independent of the
flowering phenology of trees ( F1,51 0.2, p 0.65) and
of the interaction between tree condition and flowering phenology (F1,51 0.04, p 0.8498). Total fruit
production was independent of forest fragmentation
(F1,51 3.56, p 0.07 ), flowering phenology condi-
Habitat Fragmentation and Tree Reproduction
153
tions ( F 1,51 0.1, p 0.75), and the interaction between tree condition and flowering phenology (F1,51
1.5, p 0.22).
Fruit set was significantly greater in individuals from
continuous populations than in isolated trees (F1,51 4.45, p 0.039). On average, 6% (1.3% SE) of the flowers produced a fruit on trees from continuous populations, whereas only 3% ( 0.5% SE ) developed into fruits
on isolated trees. The flowering phenology of trees did
not affect fruit set (F1,51 0.03, p 0.852). An average
of 3% ( 0.006 SE) of the flowers set fruit in trees with
synchronous and asynchronous flowering. There was no
significant interaction between tree condition and flowering phenology (F1,51 0.30, p 0.585). Differences
in fruit set between trees in isolation and in continuous
populations were influenced mainly by the greater proportion of individuals from continuous populations,
which had more than 10% of their flowers set fruit.
These individuals produced intermediate to low numbers of flowers relative to the highest levels of flower
production in the population ( Fig. 2). Five of the 24
trees from continuous populations showed a fruit set of
10%, whereas only 1 of the 31 isolated trees had an
equivalent fruit set.
An average of 24 (0.9 SE) potentially viable seeds and
9 ( 0.5 SE ) aborted seeds were produced per fruit. The
proportion of potentially viable seeds produced per fruit
was independent of tree condition (F1,815 0.06, p 0.807) and flowering phenology (F 1,815 0.17, p 0.681). The variation in the number of seeds per fruit
was significantly affected by the identity of the maternal
tree (F39,815 160.1, p 0.001).
Allele frequencies from which to estimate genetic parameters were calculated from four alleles for each of two
loci (LAP, SKDH), two triallelic loci (EST, AAT), and three
loci with biallelic variation (PGI-1, PGI-2, ADH). The allozyme genetic analysis showed similarly high levels of expected heterozygosity for progenies of continuous populations (He 0.398, 0.267 SD) and isolated populations
(He 0.400, 0.210 SD).
Figure 1. Mean flower production in
1999 of isolated Pachira quinata and
P. quinata from continuous populations,
estimated as the total number of flowers
produced by each individual every 15
days during the reproductive period. Bars
represent standard errors.
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Habitat Fragmentation and Tree Reproduction
Fuchs et al.
Figure 2. Relationship of the total flower
production and fruit set for Pachira
quinata in isolation and in continuous
populations.
P. quinata was predominantly outcrossing and the
proportion of outcrossed seeds showed a tendency to
be greater for trees in continuous populations than for
isolated trees (Table 1), but this difference was not statistically significant ( t 1.327, df 999, p 0.094 ).
The same trend was observed for synchronous and asynchronous trees, where the proportion of outcrossed
progeny showed a tendency to be greater for synchronous trees, but the difference was not statistically different (t 1.21, df 999, p 0.115). In addition, the paternity correlation estimates suggested that isolated
trees were more likely to produce full-sibling progeny
within trees than were individuals from continuous populations. The difference in the paternity correlation between isolated trees and trees from continuous populations was 0.274 (0.015 SE) and was significantly
different from zero (t 1.851, df 999, p 0.033).
Two to three outcross pollen donors sired the progeny
of trees from continuous populations, whereas one outcross pollen donor sired the progeny of isolated trees.
Similarly, the paternity correlation of asynchronous
trees was greater than that of trees with synchronous
flowering (t 2.36, df 999, p 0.01). The progeny
produced by trees with synchronous flowering was
sired by an average of two outcross pollen donors,
whereas only a single pollen donor was represented in
the progeny of asynchronous flowering trees. Mean relatedness within fruits was significantly greater for the
progeny of isolated trees than for the progeny of trees in
continuous populations (Fig. 3; F1,147 6.96, p 0.01).
Seed relatedness within fruits was independent of flowering phenology (F1,147 0.33, p 0.56) and of the interaction between tree condition and flowering phenology (F1,147 0.18 , p 0.66).
Similarly, the relatedness of seeds from different fruits
within trees was significantly greater for the progeny in
isolated populations than for the progeny in continuous
populations (Fig. 4; F1,148 13.59, p 0.01). Seed relatedness between fruits was independent of flowering
phenology ( F 1,148 3.45, p 0.06) and of the interaction between tree condition and flowering phenology
Table 1. Outbreeding rate (tm ), paternity correlation (rp ) and
number of sires, estimated as n 1/ rp, for the progeny of Pachira
quinata produced under different spatial and phenological
conditions.*
Trees
Continuous
Isolated
Synchronous
Asynchronous
tm
rp
No. of
sires
0.915 (0.043)
0.777 (0.114)
0.919 (0.055)
0.726 (0.117)
0.470 (0.085)
0.740 (0.118)
0.471 (0.099)
0.892 (0.116)
2–3
1–2
2–3
1
* Standard errors are given in parentheses. Number of sires is the
minimum and maximum number of donors.
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Volume 17, No. 1, February 2003
Figure 3. Mean relatedness within fruits of the progeny of Pachira quinata for trees in isolation and in continuous populations. Bars represent standard errors.
Fuchs et al.
Figure 4. Mean relatedness between fruits of Pachira
quinata of the progeny of isolated trees and trees in
continuous populations. Bars represent standard
errors.
(F1,148 1.69, p 0.19). Estimates of mean relatedness
between and within fruits were significantly correlated
(rp 0.245, t146 3.03, p 0.0020).
Discussion
Our results indicate that in P. quinata, isolated trees
produced twice the flowers of trees in continuous populations. Although trees from both density conditions
were similar in stem diameter, isolated trees tended to
develop crowns with more reproductive branches than
individuals in the forest because they were found in
open areas where there was no competition from neighbors. However, total fruit production was similar between tree conditions. This apparent contradiction is
explained by the fact that fruit set was negatively affected by forest fragmentation. Two trees in continuous
populations produced fewer flowers (900–1200) but
were able to develop a high proportion (15–25%) of
them into mature fruits. Trees with similarly high levels
of fruit set were found in another naturally continuous
population of P. quinata in Costa Rica (Quesada et al.
2001). A possible explanation for these results is that
pollination with compatible pollen is hindered in fragmented habitats, thereby limiting the ability of individuals to achieve high levels of fruit set. In agreement with
our results, other researchers have found a positive relationship between fruit set and forest-fragment size, (Aizen & Feinsinger 1994a; Ghazoul et al. 1998; Cunningham 2000b). For example, Shorea siamensis, another
tropical dry-forest tree, shows a reduction in fruit set as
a result of limited transfer of compatible pollen between
Habitat Fragmentation and Tree Reproduction
155
distant individuals located at low-density, disturbed sites
(Ghazoul et al. 1998). Similarly, in isolated P. quinata,
the low density of reproductive donors might be hindering
the ability of pollinators to transfer pollen between trees,
which in turn might be negatively affecting fruit set.
An additional explanation for a decrease in fruit set in
isolated trees might be related to limited resources for
the development of fruits. In isolated trees, the proportion of flowers setting fruit might be restricted by a
threshold in the total number of fruits a tree is able to
produce (Fig. 2). Evidence from other species indicates
that an upper limit to fruit production is set by resources
rather than flower number ( Lloyd 1980; Stephenson
1981, 1992; Lee 1988).
In P. quinata, the degree of relatedness of the progeny of isolated trees was greater than that of the progeny of trees in continuous populations (Figs. 2 & 3). In
contrast, the degree of relatedness was not affected by
the level of flowering synchronization between trees.
Levels of relatedness between fruits were positively
correlated with levels of relatedness within fruits. This
correlation indicates that spatial isolation affects pollen
flow within individual trees and the number of pollen
donors within individual flowers because uniparental
paternity is more likely to occur within and between
fruits of isolated trees (Table 1; Figs. 3 & 4).
Studies on the mating systems and incompatibility
mechanisms of the family Bombacaceae suggest that
population estimates of outcrossing rates can be influenced by different density conditions ( Murawski &
Hamrick 1992a, 1992b; Gribel et al. 1999). A previous
study indicates that a natural population of P. quinata
located in a continuous forest is self-incompatible (Quesada et al. 2001). But the results of our genetic analysis
of trees from different populations located in continuous forests and in isolation suggest that partial self-compatibility in P. quinata may depend on the degree of isolation. In P. quinata and other species of the family
Bombacaceae, the incompatibility reaction apparently
occurs at the base of the flower style or at the ovary, but
self-fertilization is not completely discarded (Baum
1995; Quesada et al. 2001). One possible explanation of
our results is that P. quinata has a cryptic self-incompatibility system that allows self-fertilization when crosscompatible pollen is limited. A similar flexible-breeding
system might be operating in other Bombacaceous trees
(Murawski & Hamrick 1992a, 1992b; Gribel et al. 1999).
Ceiba pentandra and Cavanillesia platanifolia have
low outcrossing rates similar to those of the isolated and
asynchronous trees of P. quinata (Murawski & Hamrick
1992a, 1992b). Murawski and Hamrick (1992b) suggest
that C. platanifolia and C. pentandra have high levels
of selfing (possibly geitonogamy) because of the high
density of flowering per individual, the amount of pollen
available from other trees, and the number of pollen donors available. More generally, our results are supported by
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Volume 17, No. 1, February 2003
156
Habitat Fragmentation and Tree Reproduction
findings that a flexible breeding system of dipterocarps is
affected by forest fragmentation and logging (Murawski et
al. 1994; Ghazoul et al. 1998; Lee 2000).
Pollinator behavior might also help explain the mating
patterns of P. quinata. In fragmented habitats or in trees
with an early or late flowering peak, bat pollinators are
more likely to promote selfing within trees (i.e., geitonogamy) because they will tend to forage within the
same tree where nectar is concentrated. One of the main
pollinators of P. quinata, the long-tongued bat (Glossophaga soricina ), adopts a territorial behavior within a
single plant in disturbed, isolated environments with limited resources (Lemke 1984, 1985). This same behavior is
likely to restrict the number of donors carried in the pollen loads that bats move between flowering trees. Our results indicate that trees in spatial isolation have a tendency
to produce singly sired fruits, whereas in undisturbed natural forests multiple paternity is more common.
Our results suggest that forest fragmentation reduces
the number of pollen donors represented in the progeny
of P. quinata, an important tropical tree species of rare
and diminishing dry-forest habitat. Total fruit production
was not affected by forest fragmentation or flowering
phenology. Although isolated trees produced more flowers, the total production of fruits per individual was
equivalent between the two tree conditions because fruit
set was higher in continuous populations. The progeny
produced by trees in continuous populations were less
related to each other than progeny from isolated trees
because there were more sires and higher levels of outcrossing in continuous populations. Even though P.
quinata is a relatively common tree in the dry tropical
forest in Costa Rica, forest fragmentation has an effect
on the distribution of the genetic variability within populations. Fragmentation is reducing genetic variability
within the progenies of isolated trees, and thus increasing the variation between progenies of different trees in
the same populations. Trees in isolation appear to be producing a comparable quantity of viable seeds compared
with trees in continuous undisturbed habitats, but the
progeny of isolated trees represent a limited sample of the
genetic diversity of the original natural populations, which
will have unknown consequences on the fitness and the
regeneration ability of seeds produced by these remnant
populations. In using the genetic resources of the tropical
forest and in implementing forestry-management plans,
managers need to consider an assessment of complex spatial and temporal factors that affect the reproduction and
breeding system of tropical tree populations.
Acknowledgments
We thank K. Stoner and T. Robles for making valuable
comments that significantly improved previous versions
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Volume 17, No. 1, February 2003
Fuchs et al.
of this paper. We are grateful to T. Robles, A. Quesada, J.
Breitling, P. Aguilar, and K. Stoner for their valuable help
and assistance in the field and in the laboratory. We also
thank M. M. Chavarría, F. Chavarría, R. Blanco, and S.
Marín from the Area de Conservación Guanacaste for logistical support. This study was performed in partial fulfillment of the requirements of the M.S. degree of E.J.F.
at the University of Costa Rica. This research was partially funded by Consejo Nacional de Ciencia y Technología, Mexico (CONACyT) (research grant 31826N);
Programa de Apoyo a Proyectos de Investigación e Innovación Technológica (PAPIIT), Universidad Nacional Autónoma de México (research grant IN213999); Universidad
de Costa Rica (research grant 111–99–319); Programa de
Cooperación México–Costa Rica (research grant 302CR075
awarded to M. Quesada, K. Stoner, and J. Lobo); International Foundation for Science (grant D/2617–2 awarded
to M.Q.; and the Idea Wild Organization (grant awarded
to E.J.F.).
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