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 population-level 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 animal-dispersed 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 .