Demographic responses of tree communities relative to recent environmental change in northeastern mountain forests
Recent environmental changes have been associated with plant species shifts in many ecosystems. Montane spruce-fir forests in the northeastern United States are expected to be vulnerable to rapid environmental changes. These forest communities are important sources of timber and support regionally unique species assemblages. Recent studies suggest that climatic changes are already affecting three foundational tree species of this region: sugar maple (Acer saccharum), red spruce (Picea rubens), and balsam fir (Abies balsamea). To determine spatial and temporal trends in climate and forest vegetation change we sampled 83 forest plots on 12 mountains in New York, Vermont, New Hampshire, and Maine. By linking regional climate trends with spatial climate data from our research sites we calculated potential elevational shifts in temperature envelopes during the last 50 years. We studied relative distributions of dominant tree species size classes along climatic (elevational) gradients to see if they suggest upslope shifts consistent with recent climate change. We hypothesized that smaller size classes of trees would be at relatively higher elevations compared to larger size classes suggesting an upslope shift.
We found that maximum and minimum annual average temperature envelopes may have shifted upslope by as much as 100 and 312 meters respectively over the last 50 years. However, vegetation surveys suggest that sugar maple and red spruce saplings are at relatively lower elevations than larger size classes (105 and 81 meters respectively). Balsam fir distributions appear stable across elevation gradients in the Northeast. Further examination of other environmental variables suggests that the increasing pH of rainwater and forest management history of low elevation sites may partially explain why some saplings are occupying lower elevations. Recent climatic change may not yet be able to overwhelm other important factors affecting trees species spatial distributions at these scales or distributional responses may lag behind changing climate. These results highlight the importance of considering multiple interacting factors in the face of ongoing climatic changes in the northeastern United States.