Tree growth responses to snowpack and competition across an elevational gradient at Mount Rainier National Park, Washington, USA

Background/Question/Methods Global climate change is expected to cause continued warming and reduced snowpack in the Pacific Northwest. These changes are likely to have dramatic impacts on mountain environments, where the majority of annual precipitation falls as snow. Snow pack controls the length of the growing season, affects soil moisture levels, and could therefore have significant impacts on tree growth and distributions. Climate envelope models use current distributions of species and the climatic conditions where they occur to predict future distributions, based on projected changes in climate. These models are often criticized for their omission of biotic interactions, such as interspecific competition, which may also affect distributional ranges. We utilized tree increment cores and data from permanent forest plots at Mount Rainier National Park to answer the following questions: 1) How do climate-growth relationships vary across altitudinal ranges of trees? 2) Do species composition and competitive environment affect growth rate across the range? Increment cores and permanent plot data were collected across an elevational gradient, which provided corresponding gradients in temperature, snowfall, and other climatic variables. Competitive environment was measured by stand density, and identity and distance of neighboring trees to cored focal trees. We used climate data from stations within the Park, and estimates from the PRISM system for areas where no measurements are available. Results/Conclusions Snow water equivalent (SWE) was correlated with the relative abundance of all five study species (Pseudotsuga menziesii, Thuja plicata, Tsuga heterophylla, Abies amabilis, and Tsuga mertensiana), and the sign and strength of this relationship varied by species. SWE was also correlated with annual growth of Abies, Pseudotsuga, and T. heterophylla. Responses to competitive environment differed by species as well. Results from this project suggest that snow is an important climate variable for trees at Mount Rainier, and that tree responses to snow and competitive environment are species specific. Thus, it is likely that tree distributions will shift asymmetrically, leading to forest communities that differ in composition from those observed at present. These analyses warrant further exploration of how changes in climate will affect tree distributions in the Pacific Northwest. Results from our study provide insight into how forest communities will be altered by climate change, thereby allowing conservation planning and forestry practices to better prepare for the effects of global climate change.