Size-specific trends in growth across tree species elevational ranges suggest complex range shift dynamics in response to changes in climate

Background/Question/Methods

What determines species geographic range limits is a fundamental but unresolved issue in ecology, and has taken on new importance in a period of rapid anthropogenic climate change that could induce large range shifts for many species. In the absence of dispersal barriers, declines in demographic performance create range limits, but these declines could be caused by many factors. Furthermore, these responses can differ by life history stage and these differences would likely influence range shift dynamics in periods of environmental change. We address these issues by examining patterns in tree performance (as measured by growth in basal area) of four tree species (Abies amabilis, Tsuga heterophylla, Pseudotsuga menziesii and Thuja plicata) across elevation, and how this relationship varies for trees of different sizes, using a large forest inventory dataset from Mount Rainier National Park in the western Cascade Mountains of Washington State, USA.

Results/Conclusions

Three species showed declines in growth from range cores towards their upper limits, while one showed no significant trend (P. menziesii). For A. amabilis, where we captured the entire elevational range of the species, we also found an increase in growth from the core to the lower limit, so that performance was highest at the lowest elevation. Individuals of different sizes responded differently to changes in elevation. For A. amabilis and T. heterophylla, small trees showed the greatest decline in growth at higher elevations, while for T. plicata, large trees showed the greatest decline. Our results suggest that if dominant tree populations at lower elevations in our study area decline in response to climate change, the response will likely be slow because performance in these locations is currently high. In contrast, population expansion at species upper limits may be relatively rapid because performance there is currently depressed and would likely increase with warming. However, population dynamics at upper range limits will likely vary by species based on which life history stages are most sensitive to differences in the environment. Range shifts of these dominant tree species will likely be idiosyncratic and complex, with demography strongly modulating responses to climate change.