Changes in forest productivity and nitrogen cycling associated with decline of eastern hemlock
The impact of invasive insects threatens communities and ecosystems. In northeastern forests, the hemlock woolly adelgid (HWA; Adelgis tsugae Annand.) has influenced the decline of the eastern hemlock (Tsuga canadensis L. Carr.). However, the effect of the HWA on community and ecosystem change has been understudied, especially across varying levels of hemlock mortality and dominance. To address community and ecosystem change, I measured the effects of hemlock mortality and dominance on forest community composition, net primary production (NPP), net N mineralization, and net N nitrification. Twenty-seven plots were established in three regional forests. Plots were categorized into hemlock-dominated, mixed, and hardwood-dominated stand-types based on hemlock relative importance. Nested-repeated measures ANOVA and regression analyses were used to assess the effects of stand-type and mortality on communities and ecosystems. There were significant effects of stand-type on basal area, density, diversity and equitability, with the greatest decreases (basal area, density) and increases (diversity, equitability) occurring in hemlock-dominated plots. Also, as hemlock mortality increased, the change in basal area and density decreased, and the change in diversity increased. Between 2001 and 2002, there was a significant change over time of live tree biomass with the greatest change occurring in hardwood-dominated stands. This was contrary to my hypothesis that there would be greater NPP in stand-types with greater hemlock dominance or decline. Species litterfall significantly varied across stand-types with the greatest litterfall increase occurring in hardwood-dominated stands. NPP and biomass did not vary across stand-types over any other time interval. The lack of significant stand-type effects except from 2001 to 2002 was perhaps because of a delayed growth response of trees co-occurring with hemlock. There was a significant effect of stand-type on net N nitrification. Mortality also significantly affected net N nitrification with the greater rate occurring in plots with greater hemlock mortality. Thus, the significant effects of hemlock dominance and mortality on communities and ecosystems indicated that forests are changing with the loss of eastern hemlock. Community and ecosystem level experiments must continue assess short- and long-term effects of invasive insects, because the impacts of invasive insects will continue to influence forest dynamics.
Mates-Muchin, Jonathan Todd, "Changes in forest productivity and nitrogen cycling associated with decline of eastern hemlock" (2006). ETD Collection for Fordham University. AAI3201134.