| Summary: | Climate change will affect the regeneration, growth, survival and distribution of trees. Here, I use common gardens to empirically test establishment, growth and the potential for persistence, adaptation and migration for two iconic North American trees, whitebark pine (Pinus albicaulis) and lodgepole pine (Pinus contorta ssp. latifolia). Whitebark pine is of conservation concern due to range-wide diebacks, while lodgepole pine is critical to forest productivity and carbon sequestration. Whitebark seeds were planted north of the current range in areas predicted to be climatically suitable through the 2050s; these germinated and survived in varying proportions at all locations. Establishment and growth were positively affected by moderate snow-cover durations, heavier seed weights, and warmer provenance temperatures. Whitebark pine seedlings grown from seeds sown in growth chambers spanning current and predicted-future temperatures demonstrated positive responses to warmer growing seasons. Lodgepole pine seedlings in the same chambers outgrew the whitebark pine seedlings at all but the coldest temperatures. Together, these results suggest that whitebark pine may lose its competitive advantage to other species within its narrow alpine-treeline niche as the climate warms, but that it is capable of establishing in climatically-suitable areas north of its current range. Using tree-ring data from long-term lodgepole pine common garden trials, I built universal growth-trend response functions to forecast future growth trends relative to genetics, climate and tree age. The models predict growth reductions for all populations by the end of the 21st century based on middle-of-the-road climate models, except in far northern areas near and within Yukon, Canada. Analogous models built using summer and winter climate indices indicate that the growth declines are primarily caused by warmer summers, and may be offset by growth increases resulting from warmer winters. I found that populations are most sensitive to annual temperatures and summer aridity, but that sensitivity to climate varies due to local adaptation. Overall, my research will help forest professionals and conservationists forecast changes in forest productivity and species growth and survival under warming temperatures.
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