Impact of climate and biotic interactions on growth of Abies lasiocarpa at altitudinal range limits

Background/Question/Methods:

A current concern within the field of ecology is the extent to which climate change will affect species distributions. Due to global warming, current models forecast species ranges shifting both poleward and upward. In order to better understand these range shift dynamics we need to understand the interplay of both abiotic and biotic factors. This study examines the relative importance of biological and climatic determinants in establishing the altitudinal range limits of the conifer species, Abies lasiocarpa on Mt. Rainier (WA, USA).  Previous work suggests that climate largely influences tree species’ upper range limits, but that biotic interactions such as facilitation and competition can respectively extend or contract a species' altitudinal range limits. We compare annual growth using increment cores from A. lasiocarpa trees at high and low density sites at the periphery of the species’ range (1400m and 1800m) against a 90-year climate series in order to determine 1) if competition constrains growth at lower range limits 2) if facilitation moderates climatic constraints on growth at upper range limits 3) whether physiological sensitivity to climate varies at the altitudinal range limits.

Results/Conclusions:

We found that A. lasiocarpa annual growth was sensitive to climate, with reduced growth in high snowfall years. However, results additionally suggest biotic interactions are important. Specifically, competition, but not facilitation is the biotic interaction that largely influences annual growth for A. lasiocarpa trees, regardless of elevation. By examining the past 10 years of annual growth, our data indicate that on average A. lasiocarpa trees grew more in low-density stands than in high-density stands irrespective of elevation (p = 0.013).  In addition, on average A. lasiocarpa trees grew more in areas of low rather than high elevation (p=0.013). Further studies are still necessary to explore the relative influence of competition and facilitation in determining this species distribution, especially as these interactions may shift in importance according to the age structure of the tree. For instance, facilitation may still influence the A. lasiocarpa range by increasing the rate of establishment and colonization of A. lasiocarpa seedlings. Furthermore, since competition appears to affect annual growth throughout the range of A. lasciocarpa, this study emphasizes the need to incorporate biotic interactions into climate models of a species distribution.