Responses of plant–pathogen interactions to precipitation: Implications for tropical tree richness in a changing world

Abstract

Tropical tree-species richness is positively correlated with annual precipitation, but the mechanisms remain unclear. Phytopathogens promote tree-species coexistence by disproportionately afflicting seedlings of locally abundant species, generating a rare species advantage. We consider whether increased plant–pathogen interactions in humid conditions favourable for phytopathogens could drive the precipitation-richness relationship by accentuating the rare species advantage. Support for this mechanism requires that increases in disease under humid conditions disproportionately affect locally abundant species without spreading to rarer species. This criterion would be augmented by either increased phytopathogen host-specificity under humid conditions, or increased asynchronicity in germination of different tree species. Research suggests that precipitation increases the rare species advantage. Increased precipitation enhances phytopathogen transmission, making escape from specialist pathogens more difficult. Additionally, drought stress predisposes plants to disease, especially by opportunistic pathogens. As seasonality in wet forests decreases, scope for asynchronous germination among species increases, potentially concentrating disease transmission within species. Synthesis. The pathways we identify could drive the precipitation-richness relationship, but finding direct evidence for them remains a priority. Researching these pathways is especially important because decreasing precipitation due to climate change could disrupt key species coexistence mechanisms and erode tree-species richness.