Background/Question/Methods Anthropogenic climate change is causing the rapid redistribution of vegetation as plant species move to track their climatic optima under accelerated warming. Despite a global trend of upward movement in latitude and elevation there is increasing evidence of idiosyncratic responses, including downward shifts in elevation and latitude, highlighting the critical need to better understand vegetation responses to climate change. To understand how recent changes in climate are influencing the distribution of plants and to ask what role species’ functional traits might play in mediating their responses, we resampled a vegetation transect established in 1977 along a steep elevational gradient in Southern California. Gaining 2,438 meters over a distance of 16 kilometers, the Deep Canyon Transect spans multiple habitat types including desert scrub, pinyon-juniper woodland, chaparral, and conifer forest plant communities. In 2019, we resampled 20 400m long vegetation transects along this gradient and compared our findings to surveys completed in 1977 and 2008. We additionally measured key morphological traits of abundant species and related them to individual species’ response to four decades of climate change. Results/Conclusions We found a general trend of increasing total perennial cover and species’ average cover at lower elevations and decreasing cover at higher elevations. Using generalized additive models, we were able to show that historic optimum elevation, rather than relative range position, best predicted these changes in cover across the gradient. We found substantial changes in species’ distributions from 1977 to 2019, with an average leading-edge elevation increase of 116 meters and a trailing edge elevation decrease of 84 meters. Moreover, we found that many species were increasing in abundance at their trailing edge range positions. Lastly, we found that these changes were mediated by plant functional traits. Specifically, species exhibiting downward trailing edge shifts, trailing edge abundance increases, and subsequent increases in total range size had higher specific leaf area (SLA), with general upward and contracting range shifts for lower SLA species. At lower elevations, these tended to be drought-deciduous species, while at higher elevations these species were mostly herbaceous understory forbs. Taken together, our results provide additional evidence for widespread unexpected range dynamics under recent climate change, particularly downward elevation shifts, and suggest that a focus on plant functional traits may help resolve some of these idiosyncrasies.
