Interspecific variation in responses of tropical trees to interannual climate variation and their relationships to species traits

Background/Question/Methods

Tropical climates vary year to year due to both natural cycles such as El Nino and long-term global climate change.  This climate variation can affect tropical tree growth, survival, seed production, and seedling establishment, with differential effects among species.  We expect the strongest effects on seed production and seedling establishment.  We investigated how tree species vary in their responses to interannual climate variation on Barro Colorado Island, Panama, using 28 years of data on seed production and 20 years of data on seedling recruitment combined with long-term climate data on temperature, precipitation, relative humidity, soil moisture, solar radiation, and wind.  For each species and each year, we estimated seed production per unit reproductive basal area and seedling establishment probability (seedling recruit density as a proportion of the corresponding seed arrival density).  For each species and metric, we then regressed annual performance deviations against climate variables.  Focal climate variables included total solar radiation, total diffuse solar radiation, mean soil moisture in the dry season.  We also conducted hierarchical analyses in which species climatic responses were modeled as a function of species traits.  The traits we examined were gap-dependence, an index of association with dry vs. wet habitats, wood density, seed mass, adult stature, dispersal syndrome, and leaf mass per area.  We tested various pre-existing hypotheses; e.g., that seed production and seedling establishment responses to dry season soil moisture would be best predicted species association with dry vs. wet habitats.      

Results/Conclusions

Among years and within species, both seed production and seedling establishment probability were highly variable; of the two, seed production was more variable.  Seed production was lognormally distributed among species, was positively related to solar radiation at the community level, and species responses to interannual variation were not synchronous, consistent with previous work.  For wind-dispersed species, seed dispersal distances were higher in years with higher windspeeds, but this was not associated with higher establishment probability. Only a small minority of species showed individually significant relationships of interannual variation in seed production or seedling establishment with any climate metric.  The results of hierarchical analyses supported hypothesized relationships of climate responses to traits, but statistical power was limited and most variation remaining unexplained.  Overall, we found strong interspecific variation in response to interannual climate variation, suggesting that ongoing climate change will change relative performance and ultimately the species composition of tropical forests.