| Abstract: | Eastern North American boreal forests shifted poleward and upslope during the Holocene period, and anthropogenic disturbance and changing climate have accelerated that shift. To better understand the causes of northward shift, I performed a two-part analysis. First, I determined structure and composition across 45 plots in an ecotone between the temperate-deciduous forest community and a relict boreal-conifer community within undisturbed areas of Great Smoky Mountains National Park (GRSM). Analysis of those data suggested boreal forests are undergoing succession in structure and composition, from boreal-conifer dominated forests to a mixed composition that includes deciduous trees from the temperate-deciduous community. Under continued anthropogenic stress and a changing climate, temperate-deciduous species will continue to increase in dominance. Although boreal-conifer species will decrease, they will retain an understory presence. I then expanded analysis to the full continent. I created an ensemble Species Distribution Model (SDM) from paleo-pollen, forestry inventory, and gridded climate data. That analysis identified the climatic drivers of the ecotone between temperate-deciduous and boreal-conifer forests. Winter temperatures were important across all models, a result of frost damage caused by frost hardening and breaking of deciduous species. Growing season temperatures were significant in the paleo-pollen and relict boreal-conifer forest models, suggesting growth strategy and competition play a key role in maintaining community composition. Finally, I extended the ensemble SDMs to both past and future climate scenarios. Combining paleo-climate data with current locations of relict boreal-conifer forest to hindcast extent suggested that the relict boreal-conifer forest could have had a larger presence throughout the Appalachian Mountains prior to European colonization. In the forecast to future climate scenarios predicted by the International Panel on Climate Change (IPCC), the models predicted northward shifts. Under the two most extreme scenarios, the models forecast that boreal-conifer forest will be extirpated from eastern North America. Our results show a link between the poleward retreat of the boreal-conifer forest from pre-colonization extents and specific temperature-related climate factors. Using a multi-faceted approach to examine forests under threat from climate change improved understanding of response to past stressors, and how forests are likely to respond to predicted future climate changes. The electronic version of this dissertation is accessible from https://hdl.handle.net/1969.1/198565 |
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