AMONG-TREE VARIATION IN LEAF PHENOLOGY AND MORPHOLOGY AND ITS CORRELATION WITH INSECT PERFORMANCE IN THE MOUNTAIN BIRCH

We studied variation in leaf morphology and phenology in a population of the mountain birch (Betula pubescens ssp. tortuosa) in northernmost Finland over two summers. We tested tree quality with bioassays using several leaf-eating insect species to study the impact of plant variation on herbivore performance. Leaf size and phenology varied considerably between years. Differences among trees were significant, but individual trees retained their leaf-character-ranking within and between seasons. Positive correlations between tree height and leaf size and differences in bud growth patterns indicate that the mountain is genetically related to the dwarf birch, B. nana. On tall, large-leaved trees, which flushed early in spring, herbivores grew fast, whereas lower nana-like trees with smaller leaves leafing out late represented poor quality food. This pattern varied between years, being stronger in a summer when the trees produced relatively small poor quality leaves than in a summer when the trees produced larger leaves on which the herbivores grew faster. However, the mean growth rates of herbivores on individual trees were significantly correlated between the years. In general, tree quality seems to depend on the temperature conditions of the previous summer, although a current year's high temperatures may mould leaf quality by altering the maturation pattern of the leaves. Polymorphism within a mountain birch population may be partially maintained by conflicting selection pressures: periods with advantageous climatic conditions and low herbivore pressure should favour taller, large-leaved trees, whereas the episodes of high herbivore pressure should favour low, small-leaved trees.