Twenty-Year Changes in Biomass and Nutrient Distribution in Forests of Walker Branch Watershed, Tennessee.

摘要

Changes in biomass and nutrient distribution in forests of Walker Branch Watershed from 1967 to 1987 are reviewed. There have been major changes in species composition in certain forest types: yellow-poplar (Liriodendron tulipifera) has largely been replaced by shortleaf pine (Pinus echinata) in what was formerly the pine forest type, and chestnut oak (Quercus prinus) appears to be replacing hickory in the oak-hickory forest type. In both cases, insect attacks (bark beetles and borers) precipitated the changes by inducing heavy mortality in native pines and hickory species in the 1970s. Subsoil (Bt horizon) exchangeable Ca sup 2+ and Mg sup 2+ have decreased markedly over a period of only 11 years in certain poor, upper-slope stands with oak and oak-hickory vegetation. From nutrient flux and mass balance analyses, we conclude that the Ca sup 2+ changes can be attributed primarily to the sequestering of calcium in biomass. The situation with respect to Mg sup 2+ is less clear; leaching was low relative to uptake in the one plot where Mg sup 2+ decreases were statistically significant (95% level, t-test), yet it was clearly the dominant mechanism of Mg sup 2+ export in all other plots where less significant (85% level, t-test) decreases were noted. It is noteworthy that leaching is dominated by SO sub 4 /sup 2/minus//, implying that acid deposition may be a major cause of the subsoil exchangeable Mg sup 2+ decreases in some cases. 31 refs., 11 figs., 3 tabs. (ERA citation 13:056443)