Limited long-term changes in tree physiological function despite shifts in forest stand structure following moth (Lymantria dispar L.) outbreak and drought

Introduction – In this study, we investigated forest stand dynamics and leaf gas exchange of co-occurring oak and pine species in an oak-pine forest in the Atlantic Coastal Plain in the Northeastern United States following defoliation by a moth (Lymantria dispar L.), and severe drought. Methods – We used forest stand inventory and leaf gas exchange data spanning from 2005-2015 to analyze the effects of drought and defoliation. Results and discussion – The defoliation and subsequent drought caused great mortality in a selected group of species and moderate mortality in others. Investigating these dynamics on a decadal time scale (2005 – 2015) revealed a shift in species composition and recruitment in this forest. There was no enhancement of resource use efficiency of the remaining forest species. Long-term water use efficiency in leaves as determined through carbon isotopic analysis declined over the study period. Photosynthetic nitrogen use efficiency of pines and oaks, however, remained relatively unchanged for all species over the period investigated, signaling a preservation of photosynthetic resource use. Also, quantum yield, as a proxy for light use efficiency, decreased over the study period for all species investigated signaling higher light availability. However, the leaves are progressively constructed with less carbon, thus still signaling leaf trait changes. Therefore, with a possible change in species dynamics and given the dynamics in resource use efficiency this forest may not be progressing toward recovery to pre-defoliation status, but rather toward a different, homeostatic, condition.