PS 7-64
Responses of tree populations to recent climatic trends in northeastern mountain forests: Thinking beyond range margins
Recent climatic changes have been associated with plant species abundance shifts in many ecosystems. Montane spruce-fir forests in the northeastern United States are expected to be vulnerable to rapid climate changes. These boreal forest communities are important sources of timber and support regionally unique species assemblages. Recent studies suggest that climatic changes are already affecting two foundational tree species of this forest, red spruce (Picea rubens) and balsam fir (Abies balsamea). To determine spatial and temporal trends in forest vegetation change we resampled historic mountain vegetation plots from the 1960s and 1980s on Whiteface Mountain, New York, and combined them with a new region wide vegetation survey from 11 additional mountains in New York, Vermont, New Hampshire, and Maine. We studied changes in range limits, abundance, and growth rates of red spruce and balsam fir along climatic (elevational) gradients. Growth rate analysis of red spruce and balsam fir on Whiteface Mountain combined tree cores (54 trees per species) with tagged and re-measured trees (158 spruce and 626 fir measured three times since 1980s). We hypothesized that recent climatic warming has resulted in upslope shifts of lower range margins and increased tree abundance and growth at historically climatically stressed high elevations.
Results/Conclusions
We found that the range margins of red spruce and balsam fir appear stable across the elevation gradient in the Northeast. Abundance changes within a species range appear primarily driven by the well documented red spruce decline and response of associated balsam fir. Tree core analysis and forest plot resurveys revealed that spruce growth rates were historically greater at low than high elevations. Since the 1970s however, we found that growth rates no longer differed across the elevation gradient. More recently (last 10 years) spruce has synchronously increased growth rates across the mountain. This supports our hypothesis that a large scale factor, likely climatic warming, is now acting to equalize and enhance spruce growth across elevations. Contrary to our hypothesis, balsam fir growth at high elevations has remained stable since the 1960s while low elevation fir has shown steadily increasing growth rates since the 1990s. This suggests that different factors may be limiting fir growth at low and high elevations. These marked changes in tree growth provide new insight into individual tree species responses to climatic changes that may not yet be easily observable when studying population level shifts across elevational climatic gradients.