Spatial patterns of insect herbivory within a forest landscape:the role of soil type and forest stratum

Background:Insect herbivory has profound impacts on ecosystem processes and services.Although many efforts have been made to recognize the main drivers of insect herbivory at different scales,the results are inconsistent.One likely reason is that studies have insufficiently captured the spatially heterogeneous factors such as soil type and forest stratum within the stand that may significantly affect insect herbivory.In particular,there is a lack of studies that address the detailed spatial patterns of insect herbivory which are influenced by these factors.Methods:We measured the detailed spatial patterns of insect herbivory on cork oak(Quercus variabilis Bl.)in response to soil type(gravel soil and loam)and forest stratum(the upper,lower,and sapling stratum),and correlated these patterns with a set of influencing factors(litter coverage,coverage of shrubs and herbs,soil nutrients,soil moisture,and leaf traits)in a forest landscape.Results:Generally,insect herbivory was spatially heterogeneous within stands.Herbivory was significantly lower in gravel soil areas than in loam soil areas and the highest herbivory occurred in the lower stratum.However,there were also 41 individual plots in which the highest herbivory occurred in the upper stratum and 29 plots in which the highest herbivory occurred in the sapling stratum.There were significant differences in soil nutrient and water status between soil types,but no significant differences in leaf traits.The effects of forest stratum on leaf traits were also inconsistent with those on insect herbivory.Conclusions:Leaf traits may not be the main factors influencing insect herbivory in the field.Soil type may have major effects on herbivory patterns by influencing litter coverage while higher coverage of shrubs and herbs may reduce herbivory in the sapling stratum.These findings may advance our understanding of tree-herbivore interactions in real-world situations and have important implications for the sustainable management of forest ecosystems.

Effects of tree diversity on insect herbivory

Tree diversity has long been considered a key driver of insect herbivory in forest ecosystems.However,studies have given contradictory results:increased tree diversity can have positive,negative or neutral effects on insect herbivory.Since many issues can complicate the tree-herbivore interactions,the descriptor‘tree diversity’per se actually has only limited explanatory power for insect herbivory.Particularly,in addition to the direct bottom-up effects on insect herbivores,tree diversity may have stronger indirect top-down effects via natural enemies of insect herbivores.However,most research has addressed only direct bottom-up impacts.In fact,insect herbivory is the result of complex interactions(food webs)among all the species in a community.Although it is hard to integrate all functionally important species and dynamic information into food webs,the effects of tree diversity on insect herbivory are highly dependent on the dominant species at different trophic levels.It is important and practical to simultaneously consider the characteristics of main trees,herbivores,and natural enemies when attempting to predict the overall effects of forest tree diversity on insect herbivory.In order to ensure comparability between studies,it is necessary to classify them according to the descriptors of insect herbivory and tree diversity,and to compare results within each category.These measures might enhance our understanding of the mechanisms by which tree diversity drives insect herbivory and,in turn,help to develop sustainable pest management strategies for forests.