Strong congruence in tree and fern community turnover in response to soils and climate in central Panama

Plant species turnover in central Panamanian forests has been principally attributed to the effects of dispersal limitation and a strong Caribbean to Pacific gradient in rainfall seasonality. Despite marked geological heterogeneity, the role of soil variation has not been rigorously examined. We modelled the compositional turnover of trees and ferns in the Panama Canal watershed as a function of soil chemistry, climate and geographical separation, using generalized dissimilarity models (GDMs). Predictability in both plant groups was strong, with 74% of turnover explained in trees and 49% in ferns. Major trends in the two plant groups were strikingly similar. The independent effects of soils, and of climate for trees, were sizeable, but those of geographical distance were minor. In both plant groups, distance and climatic effects on species turnover covaried strongly. Including floristic dissimilarity of the other taxon as a predictor increased explained deviance to 81% in trees and 59% in ferns. Controlling for differences in plant density among plots reduced deviance explained by climate and distance, while soil effects remained strong. Limiting the analyses to soils of volcanic origin increased deviance explained by climate, soils and distance, but their effects covaried strongly. Independent soil effects on tree turnover were reduced, but their effects on fern turnover remained pronounced. Dry season length was the most important climatic predictor for both taxa, and P and pH were the most important soil predictors. Particularly, rapid species turnover was associated with the driest end of the seasonality gradient, linked to declining individual densities and species richness, and with the low end of the phosphorus gradient. Synthesis. While changes in rainfall and seasonality undoubtedly limit plant distributions in this region, soil effects are at least as important, and interactions between the two are sizeable. This is likely to hold elsewhere in the Caribbean region, where mosaics of marine and volcanic soils combined with pronounced rainfall gradients are common. Strong congruence between our focal taxa suggests that our results can be extrapolated to other plant groups, particularly as trees and ferns are distantly related and represent different life-forms.