Whitebark pine at the northern edge : current constraints and future potential northern distribution under a changing climate

Understanding the role of climate in shaping species ranges will be critical in forecasting range shifts with changing climate. The factors influencing the niche and distribution of species include climate at a broad scale; topography, land-use, dispersal and many other biotic and abiotic interactions will further constrain species distributions within a spatially-hierarchical ecological system. Whitebark pine (Pinus albicaulis) is an endangered high elevation tree species of western North America. Anticipated climate change effects predict the migration of whitebark pine to suitable habitat north of its current range limits. However, the cause of the current northern range limit in north-central British Columbia is previously unresolved, so it is unclear whether there will be northward migration with changing climate. I used a hierarchically-structured statistical model across regional (climate) and local (climate and topography) scales and found that occurrence and abundance of whitebark pine is best predicted by cold temperatures and associated low growing degree-days. This analysis also indicates ample suitable climatic habitat north of the current distributional limit of whitebark pine. By including seed dispersal via Clark’s nutcracker (Nucifraga columbiana), I showed that the northern range is likely directly limited by low seed dispersal and by an indirect interaction with Douglas-fir (Pseudotsuga menziesii), the dominant alternative food source for Clark’s nutcracker. I applied future potential scenarios of climate change, mortality and active restoration to evaluate the consequences on whitebark pine species distribution, and my results indicate that a range contraction to core habitat areas further south appears to be a potential outcome of climate change and ongoing mortality. The northern-most populations were the most vulnerable, but most responsive to restoration (seedling planting) and Douglas-fir management scenarios. Overall, my research provides a better understanding of how abiotic and biotic processes interact to shape species distributions across scales. Ultimately, these findings can inform conservation planning for whitebark pine, a highly endangered and ecologically valuable tree species.