Forest structure and tree species distribution in relation to topography-mediated heterogeneity of soil nitrogen and light at the forest floor

We investigated the patterns of soil nitrogen (N) and forest floor light availability, forest structure and tree species distribution along a topographic gradient on a 200-m mountain slope in a cool-temperate deciduous broad-leaved forest in Japan. Rates of soil N mineralization and nitrification decreased from lower to upper slope positions, revealing that N availability decreased up the slope. Maximum tree height and above-ground biomass were greater on the lower than the upper parts of the slope. Canopy openness, an index of light availability at the forest floor, increased up the slope. Tree species could be placed into three groups according to their distribution patterns on the slope. 'Ridge' and 'valley' species were distributed on the upper and lower parts of the slope, respectively, whereas 'uniform' species were distributed over the entire slope. Topography-mediated resource gradients of soil N and light may be important determinants of species distribution patterns and forest regeneration, and the results of this study imply that the determinants of the regeneration process differ between the lower and upper parts of a slope. The former may be relatively light limited and the latter may be soil N limited. Valley species may have a greater ability to compete for light, whereas ridge species have a greater ability to compete for soil N. The broad distribution of uniform species probably reflects an ability to effectively compete for both light and soil N.