XVI International Botanical Congess

We studied responses of mountain birch to sequential defoliations by simulating progress of a natural 2-3 year defoliator outbreak. Tree resistance against geometrid larvae remained in the range induced by single defoliation event in other studies, and its efficacy was most affected by previous-year defoliation. Previous-year defoliation decreased seed weight probably due to exhaustion of nutrients required for their development, but earlier defoliation limited catkin number by affecting bud type determination. Number of both catkins and long shoots was reduced due to defoliation two years earlier. Trees maintained leaf biomass at the expense of growth and reproduction. This may be a tolerance strategy for coping with herbivory that is at least as important to a long-lived plant as is induced resistance.