Temperature is positively associated with tree mortality in California subalpine forests containing whitebark pine

Whitebark pine (Pinus albicaulis Engelm.) is a keystone high-elevation tree species occurring across much of western North America, yet it has been listed as threatened under the US Endangered Species Act due to rapid population declines and extensive ongoing pressures from white pine blister rust (Cronartium ribicola), mountain pine beetle (Dendroctonus ponderosae), and increasing temperature and aridity associated with climate change. Past research has shown that whitebark pine mortality is more likely in hotter and drier sites, but no broad-extent analyses spanning multiple ecoregions have directly quantified the relationship between mortality probability and its environmental drivers at a fine spatial scale. To address this gap, we used spatially continuous high-resolution (30 m) maps of mortality in California produced by a remote sensing-based change detection algorithm, and we developed generalized additive models to explain this mortality variability using relevant biophysical variables. We found that maximum daily temperature was strongly positively associated with mortality intensity and that mortality was also generally more intense in sites with greater solar exposure (e.g., south-facing slopes). Our results support the interpretation that as climate warms and aridity increases, populations in "trailing edge " (i.e., hot, dry) sites are most vulnerable. The environmental relationships we quantify will aid in identifying the locations and environments with the greatest risk of future mortality as well as those likely to serve as refugia.