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, 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 vs. 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 attackers were consistently unresponsive to
variation in seedling density across two levels 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, shedding light on enemy controls over plant recruitment.