The effects of nutrient variation and age on the resistance of three winter-dormant white birch species to mammalian herbivores

In this study, we investigated variation in the resistance of winter-dormant white birch species (Betula resinifera Britt., Betula pendula Roth, and Betula platyphylla Sukatchev var. japonica Hara) at different ontogenetic phases to mountain hares (Lepus timidus L.) and field voles (Microtus agrestis L.). The birches were grown for 4 years in outdoor pots and in a field experiment at different nutrient levels. The species differed in their overall levels of resistance to mountain hares and in their resistance responses at different nutrient levels. The exotic species, B. resinifera and B. platyphylla, were more resistant than the native B. pendula. Variation in resistance to voles between and within species at different fertilizer levels was less pronounced. The resistance response of white birches to hares seems to be related to their ontogeny, as well as to the nutrient status of the soil. At high nutrient supply, young birch seedlings achieve good growth and high resistance, with no tradeoff between growth and resistance, but the effect of fertilization on resistance stabilizes in aging trees. Before we can arrive at any general conclusions about the resistance responses of trees in variable environmental conditions, the effect of ontogeny must be considered, as well as differences between plant genotypes.