Background/Question/Methods Beech bark disease (BBD), caused by an introduced pest, has demonstrable ecosystem consequences for eastern U.S. forests. In the Catskill Mountains, NY, BBD causes substantial mortality of American beech (Fagus grandifolia), which leads to increased dominance by sugar maple (Acer saccharum) on sites previously co-dominated by beech and maple.  Beech and sugar maple differ markedly in foliar chemistry, especially with regard to calcium, as well as in their site requirement for nutrients and tolerance of acidity.  We hypothesized that the BBD-induced shift in tree species composition would lead to changes in geochemistry, primarily soil acid-base chemistry. To test this hypothesis, we established 19 forest plots to represent a chronosequence of BBD infestation and subsequent beech decline, and developed a ‘BBD score’ for quantitatively expressing both the degree of infestation and beech decline.  Species compositional shift from co-domination by beech and maple to monospecific stands increasingly dominated by maple was recorded.  Mineral soil and forest floor were sampled in each plot and analyzed for soil pH, exchangeable cations (Ca²⁺, Mg²⁺ K⁺, Na⁺, Al³⁺); mineral soil samples were also analyzed for total Ca, Mg and K using X-ray fluorescence.   We used stepwise regression to examine the influence of shifts in species composition on forest floor cation chemistry.
Results/Conclusions

The chronosequence of BBD infestation and mortality revealed increasing beech stumps and logs, as well as increasing dominance by sugar maple in leaf litter, with increasing BBD score.  We found that the BBD gradient of infestation and beech decline was somewhat confounded by a gradient in substrate Ca, which explained the largest proportion of variance in soil acid-base status.  The shift in species composition, measured as the percent of leaf litter comprised of beech leaves, explained a small but significant proportion of the variance in forest floor exchangeable Ca, effective cation exchange capacity, and percent base saturation across the chronosequence.  For plots with total mineral soil Ca>0.1 %, sugar maple litter had higher Ca concentrations compared to beech, whereas beech litter had higher Ca at low substrate Ca (<0.1%).  Furthermore, a greater shift from beech to sugar maple occurred on sites with CaO >0.1%, compared to sites with <0.1% CaO.  Thus, in the Catskill Mountains, higher foliar Ca in sugar maple at high substrate Ca concentrations may be a mechanism that amplifies the shift from beech to sugar maple following BBD-induced mortality of American beech.