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

1. 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. 2. 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 three months in the early wet season (June – August) and two 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. 3. 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. 4. 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. 5. 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.1. 诸多研究均将真菌与卵菌植物病原菌（fungal and oomycete phytopathogens）视为可影响植物物种组成、推动植物群落多样性形成的自然天敌。然而目前学界对植物-病原菌互作沿区域非生物梯度（abiotic gradients）的变化规律，以及其随树木物种特征产生的差异仍知之甚少，这限制了我们对树木物种丰富度变异成因的理解。 2. 我们于巴拿马中部热带湿润森林中，在湿季早期（6月至8月，共3个月）及随后的旱季（3月至4月，共2个月）沿年平均降水梯度，对272个树种共计10756株幼苗开展病害症状调查。观测到的病害症状中，99%以上由坏死营养型叶部病原菌引发，仅不足1%的症状源自土传病原菌。叶部病害发病率与年平均降水量呈负相关，这一模式或源于旱生林物种更强的病害易感性。 3. 叶部病害发病率随同种幼苗密度升高而增加，但不受同种成年个体邻近程度的影响。尽管叶部病害发病率随年平均降水量升高而降低，但同种密度依赖或距离依赖的强度与降水梯度无关。 4. 常见树种以及由非飞行哺乳动物传播的树种的幼苗，其叶部病原菌侵染风险更高。常见物种的病害发生率升高，或可抑制其种群优势地位。 5. 综合分析表明：叶部病害发病率随同种幼苗密度、物种多度以及传播机制的变化呈现非随机特征，这或有助于热带植物群落的调控与物种共存。此外，叶部病害发病率、传播机制与降水量之间的关联提示，植物-病原菌互作可能会随气候变化以及传播者群落的破坏发生改变。