Plant functional traits predict heterogeneous distributional shifts in response to climate change

1. Climate change is causing the rapid redistribution of vegetation as plant species move to track their climatic optima. Despite a global trend of upward movement in latitude and elevation, there is extensive heterogeneity among species and locations, with few emerging generalizations. Greater generalization may be achieved from considering multidimensional changes in species' distributions as well as incorporating ecologically relevant functional traits into studies of range shifts. 2. To better understand how recent changes in climate are influencing the elevational distribution of plant species and how species' functional traits mediate distributional changes, we resampled a 2438 m elevation transect spanning a distance of 16 km which encompasses desert scrub, pinyon-juniper woodland, chaparral and coniferous forest plant communities. 3. Over the last 42 years, total perennial cover and species' average cover increased at lower elevations and decreased at higher elevations while average elevational leading-edge increased 116 m and elevational rear edge decreased 84 m. Notably, these changes were mediated by species' functional traits, where species exhibiting more conservative traits (lower specific leaf area [SLA], greater delta C-13, larger seed mass) and taller height shifted upward in their leading-edge range limit, average elevation and trailing edge range limit, while declining in abundance at the median and trailing edge of their range. Species possessing more acquisitive traits (higher SLA, lower delta C-13, smaller seed mass) and shorter height shifted downward and increased in abundance at their trailing edge, with increases in their total range size. 4. Our results provide clear evidence that heterogeneous range dynamics under recent climate change can be generalized by considering ecologically relevant plant functional traits, and how they respond to localized climate exposure. Furthermore, by documenting changes across a steep elevational gradient comprising a large aridity gradient, we show divergent patterns for plants occupying contrasting positions along the global spectrum of plant form and function, which provides critical insight into how trait-mediated changes under increasing aridity will impact ecosystem functioning.