Many tropical forests experience periodic, large-scale disturbances that can dramatically alter forest structure. In coastal regions, these disturbance events are typically the result of tropical cyclone activity (i.e. hurricanes, typhoons, cyclones). During these storms, damage to larger trees opens the forest canopy, dramatically increasing the amount of light reaching the forest floor. Increased light levels can result in higher seedling densities in the understory, and may trigger an increase in density-dependent mortality in years following hurricanes. Thus, in hurricane-impacted forests, large variations in abiotic and biotic conditions are likely to be important drivers of seedling dynamics, which in turn will determine patterns of forest recovery. To understand the role of these factors in shaping patterns of seedling dynamics, we quantified changes in understory light levels and seedling densities over a ten year period following Hurricane Georges in a tropical montane forest in northeastern Puerto Rico. Specifically, we asked (1) Are abiotic or biotic variables more important drivers of seedling survival in this forest? (2) Does the relative importance of these variables change over time since hurricane disturbance? (3) Can life history strategy be used to predict the dominant factors shaping seedling survival?
We found large changes in understory light levels and seedling densities over the course of the study. Analysis of 13,836 seedlings of 82 tree and shrub species using generalized linear mixed effects models revealed that both light and seedling density were significant drivers of seedling survival, but the relative importance of these variables changed over time since hurricane disturbance. Light was a significant predictor of survival only in the first census interval following the hurricane. Conversely, the effect of neighbor density on survival was only significant in later census intervals. Among species groups with differing life history strategies, we found clear differences in the effects of biotic and abiotic factors on seedling survival, although there was also considerable variation among species within each group. Our results suggest that hurricanes shape species composition through indirect effects on understory conditions that influence the success of seedlings. Thus, predicted increases in the intensity or frequency of hurricanes in the Caribbean will likely alter transient seedling dynamics and species composition in hurricane-impacted forests.