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

Abstract

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 the tree community context (e.g. diversity, composition) ormeasured 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 tree 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 tree diversity did not affect leaf damage by insects, it had a significant negative effect on leaf necrosis by pathogens whereby, on average, percent leaf necrosis on mahogany seedlings in polyculture was half of that observed in monoculture. We discuss the potential influences of changes in mahogany tree density versus frequency (relative to other tree species) driving this diversity effect. Unexpectedly, we found no effect of seedling conspecific density on either leaf insect or pathogen damage (i.e. density‐independent attack). Likewise, we found no significant tree diversity by seedling density interaction, indicating that plant enemies were consistently unresponsive to variation in seedling density acrosslevels of tree diversity. Overall, this study provides a unique test of the JCH by experimentally evaluating seedling density and tree diversity effects on contrasting plant enemies and, in turn, the controls they exert over plant recruitment.