Tropical forests are vulnerable in terms of functional redundancy

Many vegetation properties may depend upon the distribution of species among functional entities (unique combinations of functional traits). Understanding these distributions is fundamental to conserve biodiversity and maintain ecosystem functions. Here, we employed the functional indices based on trait probability density and functional entities in comparison with null model to assess the response of tree community to potential species loss in 17 1-ha forest dynamics plots across six old-growth forest types in a tropical nature reserve. We found that functional diversity, functional redundancy and functional over-redundancy were positively, while functional vulnerability was negatively related to species richness. Both functional redundancy and functional overredundancy were low, while functional vulnerability was high across the six old-growth tropical forest types. The null model tests revealed that species in each of the tropical forest type were packed into a few functional entities, leading to functional over-redundancy and resulting in functional vulnerability. Our result highlighted that although species-rich tropical forests had high probability of functional redundancy, the insurance effect provided by that could not offset functional vulnerability in the ecosystems. Reducing loss of species with unique trait combinations and low local abundance is an effective and necessary way to avoid the loss of functions in tropical forest ecosystems.