Plant trait differences and soil moisture jointly affect insect herbivory on seedling young leaves in a subtropical forest

Abstract

Insect herbivory on seedling leaves is one of the most important factors driving seedling growth and mortality in natural forests, which sets the pivotal roles of insect herbivory in affecting natural forest regeneration and species composition. The intensity of herbivory can be influenced by multiple biotic and abiotic factors affecting leaf detection or consumption by insect herbivores. However, the relative importance of these biotic and abiotic factors in affecting insect herbivory and their interactions remain largely unknown in species-rich forests. We measured insect herbivory on seedling young leaves across 600 1 m × 1 m seedling plots in a subtropical forest. Effects of nine seedling traits, neighboring plant compositions, soil moisture, and light availability on young leaf damage by insect herbivores were assessed with generalized linear mixed-effects models. We found that seedling traits associated with plant size (young leaf area and seedling height), leaf nutrition or defense (leaf thickness, carbon content, and stem specific length) explained more of the variation in insect herbivory than the biotic and abiotic environmental factors. Young leaf coloration (delayed greening degree) exhibited contrasting effects on leaf damage at wet vs. dry sites, so did seedling trait diversity (functional dispersion, FDis). The risk of insect herbivory increased with leaf redness and decreased with seedling FDis at wet sites, while the opposite trends were observed at dry sites. Our study advances the understanding on the driving factors of insect herbivory on seedling leaves in species-rich forests and suggests that the alleviating effects of plant diversity on insect herbivory were stronger at wet conditions.