Comparison of changes in organic matter dynamics due to stand age between artificial Japanese cedar (Cryptomeria japonica D. Don) forests and Japanese cypress (Chamaecyparis obtusa Sieb. et Zucc.) forests

The purpose of this study was to clarify the effects of tree species on organic matter dynamics according to stand age in Kanto region of Japan. We investigated forest floor cover percentages, amounts of forest floor and litterfall, and soil carbon content for cedar and cypress forests. Study-plots in the forests planted with Japanese cedar and Japanese cypress were adjacent, on the same slope, with different stand ages (13, 21, 34, 48, 66, 93 years old). Comparisons of the results were made between the cedar forests and the cypress forests. The amount of annual litterfall in the cedar forests decreased with stand age, but in the cypress forests it was unchanged with stand age. The needle leaves in the soil were 1.5-7.3 Mg hasup(-1) in the cypress forests. There were more needle leaves in the cypress forest soil than in the cedar forest soil. The quantity on the forest floor added to the needle leaves in the soil in the cedar forests was almost the same for each cedar forest stand age. The quantity on the forest floor added to the needle leaves in the soil in cypress and cedar forests increased in stands from 13 years old to 34 years old, and was unchanging for stands which were older than 34 years. The residence times of litter C (standing stock of litter/annual litterfall) shortened with stand age in the cypress forests, and lengthened with stand age in the cedar forests. The soil carbon content did not change with stand age in the cedar and cypress forests. The organic matter decomposition rates on the forest floor in the cypress forests were larger than those of the cedar forests. However, the soil carbon contents in the cypress forests were almost the same as those of the cedar forests. This suggested that the carbon of the litter in the cypress forests had been more easily mineralized, relative to the cedar forests.