Vertical stratification collapses under seasonal shifts in climate

Aim Tropical forests are vertically complex, and offer unique niche opportunities in the form of climate, habitat and resource gradients from ground to canopy. Rainforest species organize within this vertical spatial gradient and recent macroecological research suggests that the highest levels of vertical stratification occur in structurally complex and climatically stable tropical rainforests. However, although the classical view of the tropics is that of aseasonality, particularly in temperature, there is strong seasonality in rainfall. Thus, we predict considerable variation in vertical stratification in time, characterized by a seasonal restructuring of communities across vertical space. Location Sierra Llorona, Colon Province, Panama. Taxon Amphibians. Methods We performed 121 ground-to-canopy surveys across the wet and dry seasons for amphibians. Using a bootstrap simulation method we calculated species-specific and community-wide vertical height and abundance shifts between seasons. Separately, we tested the importance of vertical height and season on the vertical distribution of species using a redundancy analysis, and employed linear models to explore turnover in species composition across vertical height between seasons. Results Our results show a clear downward shift of 5 m in height in amphibian communities from the wet season to dry season. We also observe significant changes in species composition across vertical strata in both seasons, driven primarily by nestedness in the dry season (loss of species over height) and genuine turnover in the wet season (loss and addition of species over height). Main conclusions The exploitation of canopy microhabitats and resources in the wet season resulted in complex patterns of stratification, whereas drying flattened the distribution and simplified the composition of arboreal communities. As such, pattern and process in the vertical dimension is not static in time but rather exists as a dichotomy with inverse patterns between wet and dry seasons. Complex, multidimensional distributions of diverse rainforest communities can be simplified by climatic constraints - an important consideration as anthropogenic climate change increases the magnitude of seasonal swings in temperature and precipitation.