If climate shapes species distributions, then the response of local populations to short-term climate variation should vary in a predictable way across the species' geographic range. For instance, populations in the warmer parts of a species range should decrease after hotter than average years, whereas in colder areas populations should increase after hotter than average years. To test this hypothesis, we used an autoregressive population model to estimate the effects of annual temperature and precipitation anomalies on the abundance of a woody shrub species, sagebrush (Artemisia tridentata), in permanent plots across its range. Our dataset includes 5709 observations of year-to-year change in sagebrush cover or production from 944 monitoring sites across western North America. We tested the effects of annual anomalies in spring through fall temperatures and fall through spring precipitation. We included lag effects and allowed the effects of annual climate anomalies to vary with the long-term mean climate of each site.
Of the climate covariates tested, average temperature of the preceding three growing seasons had the strongest effect on changes in sagebrush abundance. The effects of temperature supported our hypothesis: in hotter sites sagebrush cover decreased after periods of above average temperature, whereas in colder sites sagebrush cover increased after periods of above average temperature. The effects of precipitation, however went against our hypothesis: sagebrush responded positively to precipitation only in the wetter parts of its range. Our analysis allows us to make quantitative predictions about the effect of climate change on this species. In response to 2°C of warming, our model predicts that sagebrush cover would increase in the coldest sites and decrease in the warmest sites. However, there was high uncertainty in these predictions and 95% confidence intervals around predicted changes overlapped zero for most sites. Our analysis shows how multi-year monitoring data can be used to learn more about how climate has shaped the large-scale distribution of species and how climate change might affect species in the near future.