Local or broad-scale drivers? The role of local climate change and mean climatic conditions that vary over broad spatial scales in determining elevational distribution shifts of 892 plant species across western North America
In a warming climate, the extent and direction of distribution shifts will depend on the identity and strength of climatic factors that currently limit species’ distributions. Although regional-scale studies support this claim, the link between underlying climatic conditions that vary over broad spatial scales and elevational distribution shifts over broad spatial scales has not been considered. Here, we use an extensive network of species’ occurrence records for 892 plant species across western North America and rates of change in seasonal temperature extremes and precipitation over 50 years to evaluate how climatic change has influenced elevational shifts at species’ optima (mean of species’ distributions) over the past 40 years across a climatically diverse geographic range.
As with other studies, we found large variability among species in range shifts – with some species shifting upwards (to higher elevations, as expected), some downwards, and some not shifting at all. Interestingly, regions differed in whether species on average shifted upward or downward, with differences across regions not explained by life form. Whether species’ distribution shifts followed expected trends (upward elevational shift) reflected both local climatic changes (i.e. the rate and magnitude of recent climatic change) and the mean climatic conditions that change over latitudinal or longitudinal gradients and presumably influence what limits plant growth. Interestingly, the local climatic drivers most likely to drive upward or downward range shifts, warming trends in summer temperature extremes and changes in snowfall, were consistently important across western North America. We also uncovered some complex interactions among climate drivers, e.g. upward range shifts in response to warming temperatures depressed when snowpack declines. In all, our results suggest that species’ distribution shifts, upward or downward, will strongly depend on which limiting climatic factors are locally limiting, which varies over the broad geographic area we considered. However, generalizing which climatic factors are limiting is possible, at least for elevational distribution shifts.