Due to heterogeneous terrain in mountains, a de-coupling of climatic gradients and geographic gradients is likely across the range of alpine plants. If local climatic conditions are variable across range edge populations, or vary orthogonally to geographic gradients, then populations at range edges may not all respond to changing conditions in the same way. Linking demography and microclimate across a species range will refine the underlying mechanisms and enhance predictions of potential range shifts in these complex landscapes. Since 2013, I have monitored 16 populations (over 4,000 individuals) of a long-lived alpine plant, Ivesia lycopodioides var. scandularis (Rosaceae) across the entirety of its altitudinal range in the White Mountains, CA (3350 – 4420m). I asked how the sensitivity of each demographic rate varies across different microclimate gradients (field-measured accumulated degree-days, days of snow-cover, and growing season soil moisture). Observations from this study motivated a field experiment established in 2015 that tested the demographic response to manipulations of summertime temperature and moisture availability in nine populations. Both the sensitivity of each demographic rate and the overall population growth rate to the manipulations were quantified to assess potential range shifts with warmer, wetter conditions.
Variation in all demographic rates across this species range was better explained by microclimatic conditions driven by fine-scale topography than by population position along the elevational gradient. In addition, different microclimate gradients shaped the variability of each demographic rate, and demographic rates exhibited contrasting responses to the same microclimatic gradient. The field experiment confirmed these findings; different demographic rates responded to the manipulations in inverse ways. The magnitude of the demographic rate responses also depended on the relative position of the population within the range (lower edge, range center, upper edge). However, the manipulations predominantly had a minimal effect on the population growth rate. Inverse responses of demographic rates to climate gradients (natural and experimental) may allow populations to be demographically buffered against changes in climatic conditions (demographic compensation). In contrast, in the lowest elevation population heating and watering greatly reduced the population growth rate. The compensatory relationships between survival and germination in this population may have been overwhelmed by the manipulations, leading to predictions of population collapse and subsequent range contraction. These complex relationships between topography, microclimate and demography across a species range will shape the rate and magnitude of biogeographic responses of mountainous species to a changing climate.