Herbivory modifies the genetic structure of birch populations

We studied the effects of selective herbivory on genetic diversity in seedling populations of silver birch (Betula pendula), a light-demanding pioneer tree species. The experimental populations consisted of 20 families competing with each other in dense stands. The seedling stands were randomly assigned to insect and vole exposure and fertilization treatments. The study covered the first two growing seasons, during which considerable thinning occurred within the experimental populations. We compared the observed mortality to a simulated control representing random mortality. Our results show that mortality resulted in decreased diversity compared to the simulated control. When insect and vole herbivory and nutrient enrichment treatments were compared to each other, taking into account differences in mortality due to the treatments, vole herbivory led to increased diversity among the tallest seedlings (i.e. canopy layer). This is noteworthy, since the canopy layer includes those with the best prospects of surviving in competition for light. When mortality was accounted for, 2-year natural exposure to insects had no effect on diversity; but cluster analysis showed that insect herbivory changed the genetic composition of the experimental populations. Our results demonstrate the importance of herbivores in shaping the genetic structure of plant populations during the thinning phase.