Contrasting demographic processes underlie uphill shifts in a desert ecosystem.

Climate change is projected to cause extensive plant range shifts, and, in many cases such shifts already are underway. Most long-term studies of range shifts measure emergent changes in species distributions but not the underlying demographic patterns that shape them. To better understand species' elevational range shifts and their underlying demographic processes, we use the powerful approach of rephotography, comparing historical (1978-1982) and modern (2015-2016) photographs taken along a 1000-m elevational gradient in the Colorado Desert of Southern California. This approach allowed us to track demographic outcomes for 4263 individual plants of 11 long-lived, perennial species over the past ~36 years. All species showed an upward shift in mean elevation (average = 45 m), consistent with observed increasing temperature and severe drought in the region. We found that varying demographic processes underlaid these elevational shifts, with some species showing higher recruitment and some showing higher survival with increasing elevation. Species with faster life-history rates (higher background recruitment and mortality rates) underwent larger elevational shifts. Our findings emphasize the importance of demography and life history in shaping range shift responses and future community composition, as well as the sensitivity of desert systems to climate change despite the typical "slow motion" population dynamics of perennial desert plants.