TO SAPLING: AN ONTOGENETICALLY INTEGRATED STUDY OF TREE RECRUITMENT IN
AN AMAZONIAN RAINFOREST (2003-2007)
Varun Swamy, PhD, Duke University
Center for Tropical Conservation
Nicholas School of the Environment and Earth Sciences
Duke University Box 90328, Durham NC 27708, USA.
Web page: http://www.duke.edu/~vs12/
I examined recruitment patterns of multiple tree species in a western
Amazonian floodplain forest at three ontogenetic stages: seed fall, seedling
establishment, and sapling recruitment.
From analyzing a long-term seed rain dataset collected using a high-density
array of seed traps, I confirmed that seed fall decreases sharply with
increasing distance from fruiting trees, with disproportionately large
contributions from a very small fraction of all trees. Patterns of seed
fall, although idiosyncratic for individual species, tended to relate
to dispersal syndrome. Intact seeds were found at significantly greater
distances away from fruiting adults than ripe fruit and almost exclusively
comprised the tail of the seed shadow for most species.
Saplings of all species examined recruited in areas of very low predicted
seed density at significantly higher abundances than expected under a
null hypothesis of "all seeds are equal". The value of a seed
in terms of its potential to produce a sapling recruit - measured as sapling/seed
ratio - initially increased greatly with increasing distance from reproductive
conspecific adults and leveled off at farther distances, in almost all
A parallel experimental study employed >1000 individual seedlings of
common tree species situated near and far from conspecific adults. Overall
survival for all species pooled and for eight out of 11 individual species
was significantly higher at sites located far from versus close to conspecific
adults, with the study design controlling for seedling density at sites.
Survival analysis based on multiple censuses revealed that a "distance
effect" persisted and intensified over time, although the timing
of onset of distance-related differential mortality differed amongst species.
The role of host-specific invertebrate herbivores and microbial pathogens
in causing seedling mortality near conspecific adults was confirmed by
the use of mesh exclosures.
Overall, my results provide community-level support for the influence
of distance-dependent processes on recruitment patterns. Seed dispersal
appears critical for successful recruitment and undispersed seeds make
a minimal contribution. When de-coupled from distance-dependence, effects
of competition-based density-dependent processes on recruitment were weak
or undetectable. I conclude that community-level tree recruitment processes
and patterns in western Amazonian lowland rainforests that harbor intact
floral and faunal assemblages conform closely to predictions of the Janzen-Connell
hypothesis of tropical tree recruitment.