Climate determines upper, but not lower, altitudinal range limits of Pacific Northwest conifers

Abstract

Does climate determine species' ranges? Rapid rates of anthropogenic warming make this classic ecological question especially relevant. We ask whether climate controls range limits by quantifying relationships between climatic variables (precipitation, temperature) and tree growth across the altitudinal ranges of six Pacific Northwestern conifers on Mt. Rainier, Washington, USA. Results for three species (Abies amabilis, Callitropsis nootkatensis, Tsuga mertensiana) whose upper limits occur at treeline (> 1600 m) imply climatic controls on upper range limits, with low growth in cold and high snowpack years. Annual growth was synchronized among individuals at upper limits for these high-elevation species, further suggesting that stand-level effects such as climate constrain growth more strongly than local processes. By contrast, at lower limits climatic effects on growth were weak for these high-elevation species. Growth-climate relationships for three low-elevation species (Pseudotsuga menziesii, Thuja plicata, Tsuga heterophylla) were not consistent with expectations of climatic controls on upper limits, which are located within closed-canopy forest (< 1200 m). Annual growth of these species was poorly synchronized among individuals. Our results suggest that climate controls altitudinal range limits at treeline, while local drivers (perhaps biotic interactions) influence growth in closed-canopy forests. Climate-change-induced range shifts in closed-canopy forests will therefore be difficult to predict accurately.

Non-climatic constraints on upper elevational plant range expansion under climate change

“…high elevation or latitude) [15,38,54]. Observational studies often provide suggestive evidence of a role for biotic interactions (most often competition and facilitation) in controlling species' distributional limits [44,55], but without direct experimentation, the mechanisms often can only be speculated upon [10,12,54]. Here, we have conducted one of the few experiments to date (see also [14,18]) demonstrating that a dominant species is constrained by biotic factors at what is typically presumed to be climatically determined range limit [25], in a region that is currently experiencing climate warming [24].…”

Site- and Species-Specific Influences on Sub-Alpine Conifer Growth in Mt. Rainier National Park, USA

“…We believe that this algorithm largely maintains similar interannual anomalies in temperature and precipitation across sites (once variables were z-score standardized), and provides the best possible estimate for derived variables such as precipitation as snow and growing degree days that have nonlinear relationships with temperature and precipitation lapse rates. Both the absolute magnitude and interannual variability of precipitation and temperature variables from climateWNA compare favorably to station observations from the Paradise meteorological station, corrected to the location of the long-term forest demography site on the south side, which is the closest to the station (see Ettinger et al, 2011). This suggests that climateWNA, while not perfect, captures interannual climate variability without major biases (See Figure A1 in Appendix A).…”

“…These sites were established in 2010, and described in a recent publication on seedling recruitment [35]. One site was also described in two recent publications about growth-climate sensitivities [36,37]. These sites are located at elevations of 1603 m (south), 1635 m (northwest), and 1750 m a.s.l.…”

“…Variables were tested for the previous May to the current October. Previous studies have indicated that snow negatively impacts and warmth positively impacts the growth of high elevation conifers [17,18,31,36,37]. Rather than test only these climatic variables, our procedure enabled us to identify additional variables that might have unexpected yet significant influences on growth.…”

“…Because we had found no evidence of interactions among climate variables for three of our focal species (A. amabilis, T. mertensiana, C. nootkatensis) at the high elevation site on the south side of Mt. Rainier [36], we included only additive effects in our model of RWI (ring width indices).…”