Understanding how species and communities shift locally and regionally poses a great challenge as we manage for resilience in the face of a changing climate. Shifts in species distributions are expected to be one of the largest biological effects of climate change and alpine plants are considered early indicators of these biographic responses. However, in montane systems, highly heterogeneous terrain results in a decoupling of climatic gradients complicating straightforward expectations of polar or upslope distributional shifts of plants in response to warmer, drier conditions. Species range shifts will be driven by how these interlaced climate gradients shape current and future population performance across species ranges. This work examines how the differential responses of life history transitions that shape population performance (demographic rates) may mediate range shifts in a changing climate across topographically heterogeneous landscapes.

The focal species for this work is Ivesia lycopodioides A. Gray var. scandularis (Rydb.) Ertter & Reveal (Rosaceae), an iteroparous alpine plant with an approximate 20-year lifespan. I first explored the importance of multiple microclimatic gradients in shaping individual demographic rates and population growth rate in sixteen populations across the elevational distribution of this species in the xeric White Mountains, CA USA. I found that multiple microclimate gradients drove variation in demographic rates across this species range, and that complementary and compensatory relationships between demographic rates lead to stable range-wide population growth through multiple demographic pathways. This work motivated a range-wide multi-year field experiment manipulating summertime temperature and precipitation in nine of the study populations to investigate the degree to which climate change may perturb this population stability.

Building integral projection modeling based on experimental demographic data, I found a negative effect of experimentally increased summertime temperature on population growth rate in all populations across this species range. This universal reduction is population growth in both trailing and leading range edge populations was due to size-dependent and variable relationships between the climate manipulation and demographic rates, and lead to predictions of population contractions at mid elevations of the species range. These results highlight that differential and size-dependent responses of life history transitions to changing climate influence the rate and magnitude of species range shifts and can lead to unexpected shifts.

In order to place the experimental responses of the focal species in a community context, I quantified shifts in abundances for the entire alpine plant community under manipulated climatic conditions. Under experimentally warmer conditions, I observed an increase of hot, dry adapted species relative to their surrounding community members and this effect was not ameliorated by experimental additions of summertime precipitation. Concordantly, I found that overall plant abundance increased and species richness decreased with experimental heating. Together, these results indicate that, with warmer conditions, the White Mountain alpine zone will comprise less diverse plant communities dominated by species associated with hotter, drier conditions.