The mechanisms of forest diversity effects on insect herbivores

Insect herbivory is often reduced in plant species mixtures compared to monocultures. Although several hypotheses have been proposed to explain this pattern of associational resistance, empirical tests of these mechanisms are rare in forests and seldom account for the structural or trophic complexity intrinsic to natural ecosystems. In this thesis, I addressed this knowledge gap by testing both traditional and novel mechanisms underlying forest diversity effects on insect herbivores.

Using a long-term forest diversity experiment in Finland, I explored whether forest diversity effects are driven by concurrent changes in environment or host tree traits, and by interactions with other taxonomic groups (insectivorous birds and mammalian browsers). Reduction in insect herbivory in mixed stands was frequently linked to changes in canopy cover around a focal tree. For instance, associational resistance of Norway spruce to a galling adelgid was driven by increased shading by taller heterospecific neighbours in mixed stands. For silver birch, canopy cover decreased in mixed stands resulting in reduced foliar quality and, thus, associational resistance.

I also showed that interactions between and within trophic levels can influence tree diversity effects on insect herbivores. Winter browsing by moose altered both the magnitude and direction of forest diversity effects on birch insect herbivores the following summer whereas avian insectivory increased with tree species diversity at the neighbourhood scale demonstrating that birds might facilitate associational resistance. Both of the above patterns were linked to differences in canopy cover between pure and mixed stands.

Taken together, this work identifies novel pathways linking plant diversity and insect herbivory and highlights the role of structural and trophic complexity as mediators of forest diversity effects on insect herbivores. My findings also suggest that manipulations of tree diversity and canopy cover may offer an effective management strategy against insect pests in planted forests.