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UBC Theses and Dissertations
Mapping the distribution of conifer tree species in response to environmental changes across western North America using a physiologically based approach Mathys, Amanda Sarah
Over the past decade, changes in climate have been sufficient to affect both the composition and function of forest ecosystems. The extent that projected climate change will continue to impact tree species vulnerabilities remains unclear and has been mainly assessed based on simple relationships between the distribution of mature trees and climate variables. The objective of this thesis was to assess the effects of regional climate and soil variations on the current and future distribution of 20 major conifer tree species across western North America and determine the impacts of changing environmental variables on tree species vulnerabilities. The spatial variation in properties of soil water availability and soil fertility was combined in the process-based model 3-PG to provide detailed projections of species shifts in response to changes in environmental conditions. The relative importance of limitations imposed on photosynthesis by suboptimal temperatures, frost, solar radiation, soil water and vapor pressure deficits was ranked in a decision tree analysis based on tree species occurrences across the region. The baseline distributions of the tree species were predicted with an average accuracy of 84% (κ = 0.79), based on their recorded presence and absence on 43,404 field survey plots. Inclusion of soil properties was crucial to improving the overall accuracy of the species distribution models and 75% of the species directly responded to changes in the soil water input. At the ecoregion level, this thesis identified the highest vulnerability of the 20 tree species analyzed to occur within the North American Deserts, particularly in the Thompson-Okanagan Plateau of British Columbia (BC). Comparison of areas suitable for tree species range expansion with a large empirical dataset on tree seedling occurrences in BC agreed on average 79%, serving as indicators of early species responses to climate shifts in the province. Outcomes of this thesis demonstrate species-specific responses to current and future climatic variations and can contribute to informing forest management for climate change adaptation.
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