Specific leaf area for five tropical tree species growing in different tree species mixtures in Central Panama

Leaf traits have been shown to explain a great fraction of differences in growth rates in trees. With this study, we evaluate if differences in growth performance of different species mixtures in a tropical tree plantation can be attributed to the plasticity of the specific leaf area index. We compare the specific leaf area of five native Panamanian tree species growing in monospecific stands, stands containing two or three tree species and stands containing all five tree species. We could confirm the commonly observed interspecific linear relationship between leaf dry mass per leaf area. Despite the general relationship, a plasticity of the specific leaf area was detected for certain species in different mixture types. In general, the plasticity was lower for sun leaves than for shade leaves and the expression of leaf trait plasticity depended largely on the species. We performed an additive partitioning of mixture effects and the analysis resulted in strong complementarity effects on plot level. We speculate that leaf plasticity might increase the ability of certain tree species to use light more efficiently in mixed species stands. The resulting complementarity between species allows mixed stands an optimized light resource use and higher productivity. Accordingly, relative growth rate on plot level scaled linearly with specific leaf area of a given stand. Establishing mixed reforestation stands covering a wider range of tree leaf functional traits, such as assembling tree species characterized by different specific leaf area, might favor the expression of the trait plasticity and thus the overall plantation performance.