| Summary: | <p> Tree species are expected to respond to recent and future changes in climate and other environmental factors such as pollution. In this dissertation I studied environmental controls on the growth and distributions of forest tree species in mountains of the northeastern United States, focusing particularly on spruce-fir forests. To determine if mountains could provide climate change refugia for tree species, I collected <i>in situ</i> microclimate measurements to quantify elevational shifts in suitable temperature regimes over time. To determine how species elevation distributions have changed since the 1960s, I resampled historical vegetation plots on Whiteface Mountain, New York. I also surveyed current species distributions along elevation gradients on 11 additional mountains in New York, Vermont, New Hampshire, and Maine to determine if smaller size classes were found at higher elevations than larger size classes of the same species (suggesting an ongoing upslope migration). Finally, I collected tree cores on these mountains to determine how climate change and acid rain affect tree growth. I found that the temperature regimes in the northeastern U.S. have already moved upslope hundreds of meters since the 1960s and are expected to continue shifting upslope suggesting that mountains in the northeastern U.S. may not represent climate change refugia for spruce-fir forest species. Elevational distributions of spruce-fir and lower elevation northern hardwood trees did not show evidence of a synchronous upslope shift in elevation as expected from recent climate change. Instead, species showed varied (individualistic) shifts with elevation that may be partly driven by other factors such as land-use history. Tree growth was found to be increasing, primarily due to reductions in precipitation acidity and only partially due to climate warming. Thus, elevational distributions of tree species appear to lag recent climate change although tree growth rates respond to environmental change more immediately. This dissertation provides some of the first evidence suggesting that reductions in acid rain have had direct, measurable impacts on the growth of a terrestrial organism. Managers should continue to monitor tree species growth and distributions especially at range margins as they respond to multiple environmental drivers of change.</p>
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