Mixed effects of landscape structure, tree diversity and stand's relative position on insect and pathogen damage in riparian poplar forests

Tree diversity has long been described as a major driver of insect herbivory and pathogen damage, which can cause defoliation and sometimes stunted growth of forest trees. However, as few studies have simultaneously considered tree diversity effects on insect pests and fungal pathogens and sufficiently captured other ecological drivers, such as landscape structure and forest stand's relative position, of their damage acting at multiple spatial scales, the strength and direction of tree diversity effects on insect and pathogen damage under non-outbreak conditions are largely unknown. Here, we measured insect herbivory and Marssonina leaf blight of poplar infection in 30 mixed and pure forest stands along the Irtysh River in northwestern China. We addressed the effects of forest isolation at landscape scale, tree species richness, stand position as well as tree size of focal host trees on insect and pathogen damage and assessed interaction between leaf damage agents. Both insect herbivory and pathogen damage constantly increased with landscape isolation in buffers of 300 m radius centered on all forest stands. Increasing tree species richness independently reduced insect and pathogen damage, and tree diversity effects were stronger than the effect of stand position in explaining a tree's damage degree, which tended to be higher in stands at nearer distance from river. Moreover, we found a positive association between insect herbivory and pathogen damage. Our results have implications for understanding a complex combination of multiscale drivers in determining insect and pathogen damage under real-world conditions. Therefore, our findings suggest that forest management to trigger resistance to insect and pathogen damage requires taking into account both tree species richness in a forest stand and landscape-scale forest isolation.