Opposing effects of density dependence and shading shape long-term forest diversity
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Explaining how hundreds to over a thousand tree species stably coexist in a few hectares of tropical forests continues to challenge ecologists. Conspecific negative density dependence (CNDD) has long been considered as a primary mechanism driving these hyperdiverse forests, with little understanding of how other resource-limiting factors may regulate the performance of both conspecifics and heterospecifics. Using 41 years of spatially explicit data from 451,921 trees representing 331 species in a forest from Panama, we show that although CNDD contributes to promoting local diversity of the Panamanian forest, the role of CNDD is substantially offset by the demographic disadvantages imposed by shading on light-demanding species. Shading from large trees exerts a stronger effect on neighborhood diversity than CNDD, greatly undermining the role of CNDD in maintaining the diversity of the forest. Our study reveals the inadequacy of CNDD in explaining tropical forest diversity and show that the tropical megadiversity emerges from the joint and often opposing effects of density-dependent biotic interactions and density-independent abiotic filters.
