Seed Rain, Seed Production, and Seed Dispersal in Four Tropical Forests
Helene C. Muller-Landau, S. Joseph Wright, Jess K. Zimmerman, Yu Yun Chen, I Fang
Sun, Richard Condit, Robin Foster, Stephen P. Hubbell, James V. LaFrankie, Susan J.
Mazer, Supardi Noor, Miles Silman, Jill Thompson, Renato Valencia, Gorky Villa,
[Dalling?]
[Helene: We do not include citations and a bibliography in Inside CTFS; however, you
are welcome to list a few article at the end for the reader to refer to for additional
information.]
Seed rain is critical to plant population and community dynamics. Knowledge of
where seeds arrive is a prerequisite for understanding the influences of post-dispersal
processes such as seed predation, microhabitat requirements for establishment, and
density-dependent survival. This is especially true in tropical forests, where seeds of any
given tree species reach only a small minority of potential regeneration sites.
Seed rain studies are underway within four Forest Dynamics Plots (FDP): Barro
Colorado Island (BCI), Panama; Luquillo, Puerto Rico; Yasuní, Ecuador; and Pasoh,
Malaysia. At each site, an array of 120-300 seed traps is censused weekly or biweekly,
with all seeds, fruits and other reproductive parts in each trap counted and identified to
species. In addition, information on species’ seed masses and seed dispersal syndromes
is being collected or compiled from the literature. These data and the associated seedling
plots compliment the core FDP data, together providing information on the complete life
cycle of trees at each site. Further, the standardized sampling design makes it possible to
compare results not only among multiple species within plots but also among multiple
sites.
The Forest Dynamics Plots offer a unique opportunity to study seed production
and seed dispersal because they provide a large area in which tree locations and sizes are
precisely known. We have combined data on seedfall with data on the sizes of trees and
their locations relative to the traps to estimate seed production and seed dispersal
distances of individual tree species. We have also used the seed trap data alone to
quantify seed limitation (limited seed arrival) and its components.
One striking and consistent result across all four plots is the high degree of seed
limitation in tropical forests. In every site, over 50% of the species that are present as
trees greater than 10 cm dbh have seeds arrive at fewer than 1% of traps per year (Figure
1). At Pasoh and Yasuní, over 80% of the tree species reach fewer than 1% of the traps
per year. Among-species and among-plot differences in seed limitation are explained in
part by differences in adult abundance – more abundant species have higher populationlevel seed production and their seeds reach more traps. However, seed limitation is
stronger at Pasoh and Yasuní than at BCI and Luquillo even after correcting for
differences in abundance.
At all four sites, variation in total seed fall among species is strongly negatively
related to seed mass (Figure 2). Within each site, there is a wide range of seed masses
and seed production among species, indicating species niche differentiation along this life
history axis. The relationship between seed mass and seed fall among species is broadly
similar at different sites, except that the intercept is significantly higher at Yasuní than at
Pasoh and Luquillo, suggesting higher seed production for a given seed mass at Yasuní
(Figure 2).
Estimated mean dispersal distances are not consistently related to seed mass or
other species characters within or among sites. Seed mass and dispersal distance are
negatively related among wind-dispersed species on BCI, consistent with the idea that
heavier seeds have a higher terminal velocity and disperse shorter distances. However, in
Pasoh, there is a positive association between seed mass and dispersal distances, possibly
reflecting the greater importance of variation in other characters influencing dispersal
distances, especially tree height and seed aerodynamic properties. Among animaldispersed species (the majority in tropical forests), there is no relationship between seed
mass and dispersal distance at any site.
In addition to general questions of tree life history strategies that we are
addressing on all plots, we are also pursuing site-specific questions involving seed
dispersal and production. In Puerto Rico, we are investigating to what degree seed
dispersal properties explain which species are common in secondary forests of different
ages. In Pasoh, we are investigating how seed production and dispersal distances differ
between a large and a small masting event. In BCI, we have investigated inter-annual
variation in seed dispersal and its relationship to windspeed and animal abundances, and
we’re now working with collaborators to develop and apply mechanistic models of seed
dispersal by wind.
As the amount of available seed rain data continues to grow with additional years
of study and additional sites, we will be able to increase our understanding of seed
production, seed dispersal, and their roles in tropical forests.
Ideas in this article are discussed in greater detail in the following articles:
Dalling, J. W., H. C. Muller-Landau, S. J. Wright, and S. P. Hubbell. 2002. Role of
dispersal in the recruitment limitation of neotropical pioneer species. Journal of
Ecology 90:714-727.
Hubbell, S. P., R. B. Foster, S. T. O'Brien, K. E. Harms, R. Condit, B. Wechsler, S. J.
Wright, and S. Loo de Lao. 1999. Light-gap disturbances, recruitment limitation,
and tree diversity in a neotropical forest. Science 283:554-557.
Muller-Landau, H. C. 2001. Seed Dispersal in a Tropical Forest: Empirical Patterns,
their Origins and their Consequences for Forest Dynamics. PhD Dissertation.
Princeton University, Princeton, NJ.
Muller-Landau, H. C., Y. Y. Chen, I. F. Sun, S. J. Wright, S. P. Hubbell, and N. S. Noor.
2003. Seed dispersal in a lowland dipterocarp forest during two masting events. in
Proceedings of the Seminar on Ecological Research in Tropical Forests, (CFFPR
series 2003), 19-21 August 2003, FRIM Kuala Lumpur, Malaysia.
Muller-Landau, H. C., J. W. Dalling, K. E. Harms, S. J. Wright, R. Condit, S. P. Hubbell,
and R. B. Foster. 2004. Seed dispersal and density-dependent seed and seedling
mortality in Trichilia tuberculata and Miconia argentea. Pages 340-362 in E. C.
Losos and E. G. Leigh, Jr., editors. Forest Diversity and Dynamism: Findings
from a Network of Large-Scale Tropical Forest Plots. University of Chicago
Press, Chicago.
Muller-Landau, H. C., S. J. Wright, O. Calderón, S. P. Hubbell, and R. B. Foster. 2002.
Assessing recruitment limitation: concepts, methods and case-studies from a
tropical forest. Pages 35-53 in D. J. Levey, W. R. Silva, and M. Galetti, editors.
Seed Dispersal and Frugivory: Ecology, Evolution and Conservation. CAB
International, Wallingford, Oxfordshire, UK.
Nathan, R., and H. C. Muller-Landau. 2000. Spatial patterns of seed dispersal, their
determinants and consequences for recruitment. Trends in Ecology and Evolution
15:278-285.
Westoby, M., D. S. Falster, A. T. Moles, P. A. Vesk, and I. J. Wright. 2002. Plant
ecological strategies: Some leading dimensions of variation between species.
Annual Review of Ecology and Systematics 33:125-159.
Figure 1. Seed limitation among Forest Dynamics Plots species having at least one
individual > 10 cm dbh. The average percent of traps hit per year is calculated for the
last 3 years of seed trap data available. Trap sizes are 0.5 m2 at all sites except Luquillo,
where they are 0.25 m2. The number of traps is 200 at BCI and Yasuní, 247 at Pasoh, and
120 at Luquillo.
Figure 2. Total seed fall per cm2 basal area versus seed mass for tree species in four
tropical forests. Total seed fall per ha for each species is calculated from the seed and
fruit fall into traps, the species-specific number of seeds per fruit, and the trap area per
ha, and is then divided by the total basal area of individuals greater than 1 cm dbh per ha .