Is there more insect folivory in warmer temperate climates? A latitudinal comparison of insect folivory in eastern North America

P>1. It is generally believed that warmer climate forests suffer more herbivory, as a proportion of leaf area, than cooler climate forests. However, standardized studies using the same methodology have rarely been performed. 2. We carried out a study on scattered forest-edge populations of four widespread tree species (Quercus alba, Acer rubrum, Fagus grandifolia and Liquidambar styraciflua) spanning 17 degrees of latitude in North America. 3. We sampled early summer sun leaves (12 weeks after bud break) at each latitude in 2 years. Total insect folivory damage was estimated from the percentage area damaged in fresh leaves on forest edges, using a scanner-linked software. 4. The percentage area damage per leaf of all four species in both years shows a significant latitudinal trend, with less damage in lower latitude areas of eastern North America. This is contrary to what would generally be expected according to current ecological thinking. Among the four studied species, only A. rubrum showed a significant difference between the 2 years. 5. Synthesis. The observation of an inverse latitudinal trend may have wider implications for the study of community functioning, suggesting that strength of 'biotic' interaction between plants and herbivores might actually be no less important, or in fact more important, in cool temperate climates compared with warm temperate climates.