Nitrogen and phosphorus economy of the riparian shrub Salix gracilistyla in western Japan

Abstract

Salix gracilistyla is one of the dominant plants in the riparian vegetation of the upper-middle reaches of rivers in western Japan. This species colonizes mainly sandy habitats, where soil nutrient levels are low, but shows high potential for production. We hypothesized that S.␣gracilistyla uses nutrients conservatively within stands, showing a high resorption efficiency during leaf senescence. To test this hypothesis, we examined seasonal changes in nitrogen (N) and phosphorus (P) concentrations in aboveground organs of S. gracilistyla stands on a fluvial bar in the Ohtagawa River, western Japan. The concentrations in leaves decreased from April to May as leaves expanded. Thereafter, the concentrations showed little fluctuation until September. They declined considerably in autumn, possibly owing to nutrient resorption. We converted the nutrient concentrations in each organ to nutrient amounts per stand area on the basis of the biomass of each organ. The resorption efficiency of N and P in leaves during senescence were estimated to be 44 and 46%, respectively. Annual net increments of N and P in aboveground organs, calculated by adding the amounts in inflorescences and leaf litter to the annual increments in perennial organs, were estimated to be 9.9 g and 0.83 g m−2 year−1, respectively. The amounts released in leaf litter were 6.7 g N and 0.44 g P m−2. These values are comparable to or larger than those of other deciduous trees. We conclude that S. gracilistyla stands acquire large amounts of nutrients and release a large proportion in leaf litter.