Abstract
We analyzed the streamwater from 33 forested watersheds in Nara Prefecture, Japan, in 2002 to clarify the influences of forest clearcut and subsequent afforestation on streamwater chemistry. Each watershed is covered with even-aged Japanese cedar (Cryptomeria japonica) stands with ages ranging from 1 to 87-years-old. The highest arithmetic mean of nitrate (NO3−) concentrations in streamwater was observed in the watershed with 1-year-old stands. Mean NO3− concentrations declined sharply as the stand age increased, and reached the pre-cutting low level in the 13 year-old-stand watershed. The results of multiple regression analysis showed that NO3− concentrations were determined mostly by the stand age rather than the geographic factors (area, elevation, relief ratio, or aspect) of each watershed. The rainfall on the sampling day did not affect the relationships between NO3− concentrations and stand age. The effects of clearcut on streamwater NO3− agreed with the results from other studies in the U.S. and Europe. Among 1- to 9-year-old-stand watersheds, a positive correlation was found between NO3− and the sum of base cation concentrations, suggesting that nutrient losses caused by clearcut remain at least for 9 years after the afforestation. In contrast, there were no increases in H+ or total Al concentrations observed in this study. This finding indicates that all the watersheds have the acid-neutralizing capacity to buffer H+ produced in the soil due to the enhanced nitrification after clearcut.
