To explain high tropical forest tree species diversity, ecologists have hypothesised that pests, pathogens and resource limitation keep the abundances of common tree species in check, thereby allowing rare tree species to persist. If this hypothesis is true then individual trees surrounded by more neighbours of the same species should have lower survival. This is known as conspecific negative density dependence (CNDD). Past tests of the CNDD hypothesis have been criticised for statistical flaws, mainly arising from the use of static data (see here and here). In a study led by Lisa Hülsmann of the University of Bayreuth and just published in Nature, we conducted a comprehensive test of the hypothesis using a dynamic data set comprising repeated censuses of 23 forest plots from the global ForestGEO network.
We found that average CNDD across tree species is generally weak and exhibits little relationship to latitude, contrary to the hypothesis that CNDD explains high tropical tree diversity. Moreover, CNDD was extremely variable across species within a forest, which may limit its ability to maintain diversity. However, CNDD was higher for rare than common species in the tropics, suggesting that CNDD plays a stronger role in structuring abundances in the tropics. Our results paint a more nuanced picture of the role of CNDD in forest community ecology, and point to future experimental and theoretical studies that will be needed to clarify this role further.
The idea for this study arose when Ryan visited Lisa Hülsmann and Florian Hartig at the University of Regensburg, Germany in 2018. The paper is a collaboration between 52 authors, primarily associated with the different ForestGEO plots around the world.
Chisholm Lab 