Aboveground production and nitrogen utilization in nitrogen-saturated coniferous plantation forests on the periphery of the Kanto Plain

摘要

The Tsukuba Forest Experimental Watershed on the periphery of the Kanto Plain receives high nitrogen deposition and has high nitrogen loss in stream water, indicating a nitrogen saturated condition We investigated the soil properties, aboveground production and nitrogen utilization in hinoki cypress and Japanese cedar plantation forests located in the watershed. There was alow accumulation of organic horizon in the both forests, indicating rapid decomposition and nitrogen release from the organic horizon Nitrogen input by litter fall was similar to the rate of annual soil nitrogen mineralization. Although the aboveground net primary production was relatively high in both forests, the Japanese cedar forest hac a higher proportion of pollen cones. Inaddition, taller trees in the Japanese cedar forest had lower leaj biomass as indicated by the lower ratio of crown depth to tree height. The stem growth of these taller trees was lower than that of smaller trees. These results suggest a symptom of decline for taller trees in the Japanese cedar forest. Previously, the decline of Japanese cedar was mainly observed within the Kantc Plain at lower altitudes but the results of this study suggest the presence of declining Japanese cedar or the periphery of the Kanto Plain. Further study is required on the spatial distribution of declining Japanese cedar forests in order to determine the mechanism.