?Tree diversity reduces co-infestation of Douglas fir by two exotic pests and pathogens

The number of non-native invasive pests and pathogens has increased dramatically in recent years, with disastrous consequences for the health forests worldwide. Multiple studies have shown that mixed may suffer less damage from insect than single species forest. This “associational resistance” can be notably explained by fact heterospecific neighbours make it more difficult herbivores to locate then exploit their host tree. However, validity these findings case non-native, remains demonstrated. In this study, we monitored over two hundred Douglas firs pure plots a tree diversity experiment assess gall midge Contarinia pseudotsugae needle cast Nothophaeocryptopus gaeumannii . probability Swiss needle-cast infection was lower fir trees surrounded neighbours. Gall on taller neighbours, consistent hypothesis reduced Apparency. were damaged C. also often infected N. Our study thus provides partial support associational resistance against exotic pathogens. Promoting forest at stand level could, therefore, offer interesting prospects reducing impact biological invasions, especially those involving both