COMPARISON OF SOIL GEOCHEMISTRY BENEATH HEMLOCK AND BLACK BIRCH FOREST, WESTERN MASSACHUSETTS, USA

Ecological forest successions associated with climate change and human disturbance may alter biogeochemical cycles within forested New England watersheds. Spread of the invasive insect hemlock wooly adelgid (Adelges tsungae) to eastern North America is causing decline and mortality of the eastern hemlock (Tsuga Canadensis). To begin an evaluation of whether changes in nutrient cycling could be altered by this disturbance, we investigated differences in soil geochemistry in secondary growth forest located at the Ada and Archibald MacLeish Field Station, West Whately, MA. Eastern hemlock on this property was selectively logged 20 years ago, with black birch regrowth succeeding hemlock. We hypothesize that such a succession could repeat should hemlock on the property experience mortality due to the hemlock wooly adelgid.

We measured soil pH, exchangeable acidity, exchangeable base cations, and nitrogen mineralization rates over an 80 day period of the 2010 growing season using field incubated soil cores placed in hemlock and younger black birch stands. Although soil pH of organic horizons between hemlock (4.3) and black birch (3.8) are both acidic, the concentration of exchangeable base cations in the organic horizon beneath black birch (3.37 meq 100 g^-1 soil) is higher than for hemlock (1.9 meq 100 g^-1 soil). Likewise, the base saturation is 10% greater for black birch (34% versus 24%). These results suggest that the acidity typically associated with soils that support hemlock forests has not been neutralized by black birch regrowth. However, more base cations are becoming available within the cation exchange pool. The soil acidity may be stabilized by exchangeable Al³⁺ , which is similar for the two plots (4.9 meq 100 g^-1 soil), whereas the differences in base saturation may reflect variation in inputs of base cations from throughfall and leaf litter. Nitrate concentrations for soils collected from initial cores are significantly higher in the black birch regrowth stand than in hemlock. Nitrogen mineralization rates over the 80 day incubation period will be presented.