Measuring and modeling northeastern forest ecosystem response to environmental stresses

Because forest responses to CO$\sb2$ fertilization and associated climate change are likely to be extremely complex, numerical models representing forest response to an integrated set of future conditions can be useful predictive tools. I compared predictions of forest net primary productivity (NPP) made by two ecosystem process models (PnET-II and TEM 4.0) using different climate scenarios, spatial resolutions, and methods of representing land cover and soils. Decreasing spatial resolution did not appreciably change NPP estimates. Input datasets, particularly climate, land cover, and soil water holding capacity, were important sources of variability in NPP estimates. These datasets interacted with model structures to produce significant variability in NPP predictions, but a comparison with predictions made by other models suggested that model-to-model differences might be even more important than input datasets. Both models predicted a substantial increase in regional forest NPP under climate change, with PnET-II predicting an average increase of 38% and TEM 4.0 predicting an average increase of 30%. The parameterization of the CO$\sb2$ response, the inclusion/exclusion of N cycling rates, and the accuracy of input datasets were identified as important items for future attention in modeling efforts. The hemlock woolly adelgid (Adelges tsugae), an aphid-like insect thought native to Japan, is causing significant mortality of eastern hemlock (Tsuga canadensis) trees in forests of the northeastern United States. I measured the impact of hemlock mortality caused by adelgid infestation on forest structure and N cycling rates in Connecticut hemlock forests. With hemlock mortality, light availability, soil temperature, and seedling regeneration increased. Annual net N mineralization rates were higher in stands with adelgid-induced mortality, and annual nitrification rates increased thirty-fold. No change was evident in soil organic matter (SOM) content. N turnover rates increased significantly with hemlock mortality. Increased decomposition rates are changing the quality of the organic matter in the forest floor and mineral soil, but the effect of accelerated decomposition on SOM will not be visible for several more years. The short-term impact of adelgid infestation may be more severe in terms of species composition than in terms of decomposition and nutrient cycling rates