Relative influences of biotic and abiotic factors on long-term seedling survival in a tropical forest

Background/Question/Methods

Future forest composition is dependent on the dynamics and identity of tree seedlings during recruitment into the understory. The realized regeneration niche of tree species can be influenced by both biotic factors and abiotic factors that may change over time. The relative importance of those factors has not been well delineated. We sampled the seedling layer in 20,000 1m² plots in a 50 ha forest dynamics plot repeatedly for more than ten years in a moist lowland tropical forest in central Panama. In combination with spatially-explicit, long-term tree data from the site, we characterized the biotic neighborhood that seedlings experienced through time as a function of the density and species identity of neighboring seedlings and trees. We also assessed the impact of abiotic factors that were static across time, such as habitat types, slope, elevation, and convexity, as well as, factors that varied with time, such as precipitation and light availability. We analyzed seedling survival for 249 species using a mixed Cox proportional hazards model with time dependent covariates to understand the relative contributions of each of these biotic and abiotic factors.

Results/Conclusions

We found that increases in the density of neighboring conspecific trees had the greatest negative influence on seedling survival relative to other significant factors in the model. Additionally, taller seedlings had better odds of surviving. Increasing density of both conspecific and heterospecific seedlings had a negative influence on seedling survival, however increasing conspecific density had a greater effect. In terms of abiotic variables, increasing shade and increasing water deficits both had a negative effect on the probability of seedling survival. Individual species varied greatly in their median survival time, from less than a year to over a decade. While it is fairly intuitive that seedlings with more light, more water and less competition would fare better, our analyses point towards conspecific negative density-dependent mortality as a key driver of the dynamics of this tropical forest and the maintenance of species diversity.