Effects of tree species diversity and conspecific seedling density on insect herbivory and pathogen infection on big-leaf mahogany seedlings

The Janzen-Connell Hypothesis (JCH) predicts that attack by specialist enemies on seedlings increases with conspecific seedling density, but studies have rarely experimentally tested for the contingency of such effects on tree community context (e.g., diversity, composition) and measured responses by different enemies (e.g. herbivores, pathogens). We conducted a field study in a large-scale system evaluating tree species diversity and conspecific density effects on leaf damage on mahogany (Swietenia macrophylla) seedlings. We established quadrats of eight levels of seedling density across mahogany tree monocultures and species polycultures including mahogany, and recorded percent leaf damage by insects and percent leaf necrosis by a pathogenic fungus on mahogany seedlings. We found contrasting effects of tree species diversity on insects and pathogens. Whereas diversity did not affect leaf damage by insects, it had a significant negative effect on leaf necrosis by pathogens. On average,..., ,扬森-康奈尔假说（Janzen-Connell Hypothesis, JCH）预测，专性天敌对幼苗的攻击会随同种幼苗密度的升高而增强，但现有研究极少针对该效应如何随林木群落背景（如物种多样性、群落组成）产生变化开展实验检验，也未针对不同类群天敌（如植食性昆虫、病原菌）的响应差异进行量化分析。本研究在大型野外样地系统中开展实验，探究树木物种多样性与同种密度效应对大果桃花心木（Swietenia macrophylla）幼苗叶片损伤的影响。我们在大果桃花心木单优林以及包含大果桃花心木的多物种种混交林中，设置了8个梯度的幼苗密度样方，并记录了大果桃花心木幼苗的昆虫叶片损伤率与病原真菌引发的叶片坏死率。研究发现，树木物种多样性对昆虫与病原菌产生了迥异的调控效应：物种多样性并未显著影响昆虫引发的叶片损伤，但对病原菌引发的叶片坏死具有显著的负向作用。平均而言，……