PS 34-36 - How do climatic influences on alpine tree growth vary across different climate zones?

Wednesday, August 9, 2017
Exhibit Hall, Oregon Convention Center
Myesa Legendre-Fixx, Ailene Ettinger, Leander DL Anderegg and Janneke HilleRisLambers, Department of Biology, University of Washington, Seattle, WA

Gaining more insight into climatic influences on tree performance could allow us to better predict how communities will respond to ongoing climate change. Understanding the sensitivity of high elevation tree species to climate is particularly important, because treeline is an important ecotone between the forest and alpine meadow biomes. In this study, I assess whether macroclimate influences the climate sensitivity of four high elevation species: Pacific silver fir (Abies amabilis), mountain hemlock (Tsuga mertensiana), Alaska yellow cedar (Xanthocyparis nootkatensis), and sub-alpine fir (Abies lasiocarpa). I also investigate if this sensitivity varies temporally. To address these questions, I analyzed tree cores from three high elevation sites in Mt. Rainier National Park, with each site located in a distinct climatic zone (i.e. macroclimate). Specifically, our south side site receives the greatest snowfall, the northwest side receives the most spring rainfall, and the east side is the driest. I compared standardized tree growth of each focal species to climate data (growing degree days, precipitation as snow, etc.)---obtained using climateWNA. Using the correlation analysis function in the R package “treeclim”, I identified the most influential variables and then used a collection of multiple linear regressions assess climate sensitivity.


Preliminary results show that snowpack and growing season temperature have the strongest influence on growth for the first three species mentioned above, with greater snowpack depressing growth and warmer summers increasing growth. Some site-specific differences in the strength of the relationship emerged. The identity of climate drivers and magnitude of effects was generally consistent across species growing at the same site, implying that broad-scale climatic constraints might limit tree growth for multiple species. We also found that climate sensitivity changed over time, although trends were species- and site-specific. For example, sensitivity to snowpack has been increasing significantly for silver fir and yellow cedar at some sites (the available datasets span from 1914-2007). Additionally, populations in the snowiest macroclimate had the greatest sensitivity to snowpack. This suggests that a drier climate could result in less tree growth limitation by snowpack. This could result in treeline shifts upward on Mt. Rainier, which could positively increase carbon sequestration while negatively encroaching on the alpine wildflower meadows, a major attraction for tourists. With better predictions of tree-line shifts, we can make more informed management decisions for ecotourism and conservation.