Tree shape, forest structure and diversity of drosophilid community: Comparison between boreal and temperate birch forests

Some models, based on the latitudinal variation in sun angle distribution, predict that trees at high latitudes have narrowly conical crowns and constitute simple-layered forests, whereas trees at low latitudes have shallowly dome-shaped and form more structurally complex multilayered forests. There is a hypothesis that structurally complex habitats can harbor potentially more species than simple ones. In this study, we examined latitudinal correlations between tree shape, forest structure and diversity in drosophilid communities, comparing boreal and cool-temperate forests. We selected secondary birch forests with a common canopy tree species, white birch (Betula platyphylla Sukatchev), as study sites. The crown shape of white birch tended to be spherical in the cool-temperate forest, but narrowly conical in the boreal forest. The foliage structure differed between the two forests. The cool-temperate forest was characterized by a clearly two-layered structure, whereas foliage in the boreal forest was less clearly stratified, being distributed somewhat continuously from the ground to the canopy at lower densities. The structural complexity expressed by foliage height diversity was greater in the cool-temperate forest than in the boreal forest. Various measures of drosophilid diversity were higher in the cool-temperate forest than in the boreal forest, probably resulting from the impoverishment of the canopy subcommunity in the boreal forest. Thus, a physical environmental factor (i.e. the angle of solar inclination) could be a potentially important factor in structuring latitudinal patterns of sylvan animal communities through changes in plant structure at the individual and community levels.