Long-term influence of stream water chemistry in Japanese cedar plantation after clear-cutting using the forest rotation in central Japan

Because both natural and anthropogenic disturbances affect biogeochemical cycles in forest ecosystems, monitoring is needed to separate their influences. Chronosequence is very useful for such studies. In our study area, plantation through forest rotation on a watershed basis resulted in more than 40 adjacent watersheds of between 0 and 87 years of stand age, kind of chronosequence. Here, we examined the biological similarity of the watersheds and the long-term effects of clear-cutting on stream water chemistry. The stream water NO₃⁻–stand age relationship was similar between the two observation years; stream water NO₃⁻ concentrations increased dramatically in the watersheds after clear-cutting and decreased in 7–10-year-old replanted watersheds. The slope of stream water NO₃⁻ concentrations between the different watersheds covered by same age stand was significant, at 1:1. Additionally, stream water NO₃⁻ concentrations were more strongly correlated between the different watersheds covered by same aged stand than between the observations at 4 years intervals within a watershed. These findings indicate that stream water NO₃⁻ concentration is mainly regulated by stand age, i.e., by vegetation regrowth, rather than watershed-specific characteristics. Hence, adjacent watersheds are biologically similar apart from stand age and can be regarded as a chronosequence. While there was a clear relationship between stream water NO₃⁻ concentration and stand age, there was significant correlation with stream water SO₄²⁻, Ca²⁺, Mg²⁺, Cl⁻ or Na⁺ between two observations in the same watershed. This indicates that watershed-specific characteristics, rather than vegetation regrowth, control stream SO₄²⁻, Ca²⁺, Mg²⁺, Cl⁻, and Na⁺ concentrations. After 25 years of clear-cutting Ca²⁺, Mg²⁺ and Na⁺ concentrations significantly increased. It is likely the contribution of forest floor accumulation with stand development. Based on these results, clear-cutting influences stream chemistry, not only NO₃⁻, but also the major cation and the influence of clear-cutting continues for several decades at this study site.