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Title: Foliar disease incidence in a tropical seedling community is density dependent and varies along a regional precipitation gradient

Many studies identify fungal and oomycete phytopathogens as natural enemies capable of influencing plant species composition and promoting diversity in plant communities. However, little is known about how plant‐pathogen interactions vary along regional abiotic gradients or with tree species characteristics, which limits our understanding of the causes of variation in tree species richness.

We surveyed 10,756 seedlings from 272 tree species for disease symptoms along a mean annual precipitation gradient in the tropical wet forests of Central Panama for 3 months in the early wet season (June–August) and 2 months in the following dry season (March–April). Over 99% of observed disease symptoms were caused by necrotrophic foliar pathogens, while less than 1% of symptoms were attributed to soilborne pathogens. Foliar disease incidence was inversely related to mean annual precipitation, a pattern which may be due to greater disease susceptibility among dry forest species.

Foliar disease incidence increased with conspecific seedling density but did not respond to the proximity of conspecific adults. Although foliar disease incidence decreased as mean annual precipitation increased, the strength of conspecific density‐ or distance‐dependence was independent of the precipitation gradient.

Seedlings of common tree species and species dispersed by non‐flying mammals had a higher risk of foliar pathogen incidence. Increased disease in common species may help reduce their dominance.

Synthesis. The increases in foliar pathogen incidence with conspecific seedling density, species abundance, and dispersal mechanism indicate that foliar disease incidence is non‐random and may contribute to the regulation of tropical plant communities and species coexistence. Furthermore, the relationships between foliar disease incidence, dispersal mechanism and precipitation suggest plant‐pathogen interactions could shift as a response to climate change and disruption of the disperser community.

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Journal of Ecology
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National Science Foundation
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  1. 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.

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