Tree species composition can have a substantial influence on carbon (C) and nitrogen (N)  cycling in forests, and in the Northeastern U.S. tree species composition is changing rapidly due to multiple introductions of exotic insects and diseases.   We examined the impact of one such introduction—beech bark disease (BBD), an insect/disease complex that affects American beech (Fagus grandifolia), a dominant tree of the Northern Hardwood forest.    Our principal objectives were to determine whether BBD caused changes in tree species composition, and if so, whether those changes were influenced by soil calcium (Ca) availability and whether they altered soil C and N cycling.  We took advantage of the “natural” experiment created by the decline of beech and its replacement by other species.   Working in the Catskill Mountains of New York and the White Mountains of New Hampshire, we identified 271 plots in which a mature beech tree had died from BBD and the canopy had closed in the resulting gap.  In each plot we measured the leaf area profile and basal area of trees, and forest floor and mineral soil concentrations of total C and N, extractable NH₄⁺ and NO₃^- , and exchangeable and total Ca.  

Results/Conclusions

Multiple species responded to fill the gaps created by loss of beech.  Across the range of soil Ca represented by these plots, we found that as Ca availability increased, sugar maple (Acer saccharum) was more likely to replace beech.  This effect was stronger in the Catskills (p < 0.0001) than in the White Mountains (p = 0.023).  In both regions, increasing abundance of sugar maple in the plots was associated with significantly lower forest floor depth and C:N ratio, and significantly higher extractable NO₃^-   concentration.    This analysis indicates that the introduced disease is restructuring the northern hardwood forest on a template set by the geological substrate, and that the vegetation response to the disease partially determines the cycling of C and N in the system.