Additive and non-additive effects of birch genotypic diversity on arthropod herbivory in a long-term field experiment

Herbivores are important drivers of plant population dynamics and community composition in natural and managed systems. Intraspecific genetic diversity of long-lived plants like trees might shape patterns of herbivory by different guilds of herbivores that trees experience through time. However, previous studies on plant genetic diversity effects on herbivores have been largely short-term. We investigated how tree genotypic variation and diversity influence herbivory of silver birch Betula pendula in a long-term field experiment. Using clones of eight genotypes, we constructed experimental plots consisting of one, two, four or eight genotypes, and measured damage by five guilds of arthropod herbivores twice a year over three different years (four, six and nine years after the experiment was established). Genotypes varied significantly for most types of herbivore damage, but genotype resistance rankings often shifted over time, and none of the clones was more resistant than all others to all types of herbivores. 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. In contrast, leaf-mining and leaf-tying damage was not influenced by birch genotypic diversity. Within diverse plots, the direction of genotypic diversity effects varied depending on birch genotype, some having lower and some having higher herbivory in mixed stands. 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. Different responses of various feeding guilds to genotypic diversity and shifts in resistance of individual genotypes over time indicate that genotypic mixtures are unlikely to result in overall reduction in herbivory over time.