Within the tropics there is a strong positive relationship between annual precipitation and plant species diversity, but the cause of this relationship remains unknown. It has been proposed that precipitation intensifies plant-fungal interactions, particularly at the seedling stage, such that density and distance responsive fungal pathogens can increase plant species coexistence as annual precipitation increases. To test whether infection probabilities increase with precipitation, we conducted disease surveys on seedlings across the precipitation gradient in Panama (1800 - 3200 mm precip/year) during the 2018 rainy season and 2019 dry season. Seedlings were identified to species and monitored for symptoms of fungal disease in five forest sites. Dataloggers were placed in each site to account for realized microclimate variation among sites.
Results/Conclusions
We do not find evidence that precipitation impacts the probability that a seedling acquires disease. However, microclimate data show that the forest sites do not vary significantly in temperature, humidity, or soil moisture during the period sampled. Therefore, although these sites represent a precipitation-diversity gradient on an annual scale, there was no gradient at the time of sampling. If plant-pathogens are impacted by precipitation as predicted, it is likely due to the difference in duration of the dry season rather than the intensity of the wet season. We do find that seedlings are more likely to be symptomatic when occurring at high conspecific densities, providing community-wide evidence that fungal pathogens attack seedlings in a density dependent manner.