Pest regulation is one of the services provided by plant diversity in terrestrial ecosystems. Plants growing in species-rich stands often experience fewer attacks by specialist insect pests than plant growing in monocultures (a phenomenon known as associational resistance, which is usually attributed to higher concentration of host plants in pure stands). However, an opposite phenomenon – associational susceptibility – may occur when plants experience an increase in detectability and consequent damage by generalist herbivores in mixed stands, for example in the presence of highly preferred species in a patch. Effects of tree genetic diversity on associated insect herbivore community have been poorly studied, but studies on herbaceous plants suggest that plant genetic diversity effects on herbivores may be more variable than plant species richness effects and may lead to higher herbivory in genetically diverse stands.

We have been monitoring herbivory on all tree species and birch clones in the Satakunta forest diversity experiment since its establishment. The objectives of this research are:

1.  To compare effects of tree species diversity, tree species composition and intraspecific genetic diversity on tree pests

2.  To compare forest diversity effects on insect and mammalian herbivores, different feeding guilds of insect herbivores (chewers, miners, gallers, rollers etc) and herbivores with different degree of feeding specialization (generalists vs specialists)

3.   To establish the mechanisms of forest diversity effects on herbivores (e.g., differences between pure and mixed stands in tree growth, spatial structure, nutritional quality, abundance and diversity of herbivore natural enemies – parasitoids and predators)

4.   To explore ontogenetic changes in tree resistance to herbivores and potential interactions between ontogeny and tree diversity effects   

Insect herbivores

Our studies at the Satakunta tree species diversity experiment have shown that tree species richness significantly affects abundance of most insect herbivore guilds, but the direction of the effect depends on the degree of herbivore feeding specialization. For instance, abundance of specialist herbivores such as silver birch aphid (Euceraphis betulae) and gall mites (Aceria leionotus and Acalitus rudis, Eriophyidae) was higher on birch trees growing in monocultures than in birch trees in mixed stands, as predicted by the resource concentration hypothesis.

In contrast, birch trees in mixed stands exhibited associational susceptibility to leaf chewers and skeletonizers, many of which are generalist herbivores. Similar conclusions have been obtained in a meta-analysis using insect herbivory data from the Satakunta tree species diversity experiment. This study showed that among insect feeding guilds, only densities of leaf-miners – one of most specialised group of herbivores - were consistently lower in mixed stands as compared to tree monocultures regardless of the host tree species. Leaf miners also showed greater year-to-year fluctuations in monocultures than in mixtures, which is in accordance with the prediction of the diversity-stability hypothesis. 

Our studies in the Satakunta birch clone diversity experiment revealed that birch clones varied significantly in their resistance to most types of insect herbivore damage, but genotype resistance rankings often shifted over time. At the plot level, birch genotypic diversity had significant positive additive effect on leaf rollers and negative non-additive effects on chewing herbivores and gall makers. This research highlights the importance of long-term studies including different feeding guilds of herbivores to understand the effects of plant genetic diversity on arthropod communities.

Mammalian herbivores

Herbivory by moose, deer, voles and hare is an important source of tree mortality during the early stages of stand establishment in boreal forest zone. Even when mammalian browsing is moderate and does not kill the tree, it often detrimentally affects timber quality. Therefore, reduction of mammalian herbivory is important in forest management.

Unlike insect herbivores, most mammalian herbivores are generalists with respect to plant species utilisation, although the degree of specialisation varies among species, which may also cause variation in responses to plant species diversity. Our studies in the Satakunta tree species diversity experiment have shown that voles and moose show contrasting responses to stand diversity and species composition. In accordance with the predictions of the associational resistance hypothesis, vole damage was higher in tree monocultures than in mixed stands. Voles also damaged more trees in coniferous than in deciduous stands. In contrast, the intensity of moose browsing per plot increased with tree species richness. Tree species composition of a plot was also an important determinant of intensity of moose browsing. The greatest browsing occurred in plots containing preferred species (pine and birch) while intermediate preference species (larch and alder) experienced associational susceptibility when growing with pine and birch compared with their monocultures or mixtures without pine and birch. In contrast, we found no evidence of associational resistance; the presence of a least preferred species (spruce) in a mixture had no significant effect on moose browsing on other tree species.

Moose browsing alters effects of tree diversity on insect herbivory

Winter browsing by moose has been previously shown to trigger changes in host tree traits that increase insect herbivore damage in the following growing season. To explore whether these browsing-induced changes can modify tree species diversity effects on insect herbivores, we recorded both the presence and intensity of winter browsing by moose on silver birch in the Satakunta tree species experiment and measured insect chewing damage during the following growing season. We have shown that moose browsing reverses the direction of tree diversity effects on insect herbivory on birch. Unbrowsed trees experienced lower insect chewing damage in mixed stands (associational resistance) whilst browsed trees suffered more insect chewing damage in diverse stands (associational susceptibility). Increasing moose browsing intensity also reversed the relationship between tree species richness and insect chewing damage from negative to positive. The observed interactions between moose browsing and tree species richness effects could be explained by lower canopy cover of more diverse stands compared with birch monocultures, leading to increased re-growth capacity and more high-quality foliage of browsed birch trees in more open diverse stands.