Increasing fire frequency and severity will increase habitat loss for a boreal forest indicator species.

Climate change will lead to more frequent and more severe fires in some areas of boreal forests, affecting the distribution and availability of late-successional forest communities. These forest communities help to protect globally significant carbon reserves beneath permafrost layers and provide habitat for many animal species, including forest-dwelling caribou. Many caribou populations are declining, yet the mechanisms by which changing fire regimes could affect caribou declines are poorly understood. We analyzed resource selection of 686 GPS-collared female caribou from three ecotypes and 15 populations in a ~600,000 km2 region of northwest Canada and eastern Alaska. These populations span a wide gradient of fire frequency but experience low levels of human-caused habitat disturbance. We used a mixed-effects modeling framework to characterize caribou resource selection in response to burns at different seasons and spatiotemporal scales, and to test for functional responses in resource selection to burn availability. We also tested mechanisms driving observed selection patterns using burn severity and lichen cover data. Caribou avoided burns more strongly during winter relative to summer and at larger spatiotemporal scales relative to smaller scales. During the winter, caribou consistently avoided burns at both spatiotemporal scales as burn availability increased, indicating little evidence of a functional response. However, they decreased their avoidance of burns during summer as burn availability increased. Burn availability explained more variation in caribou selection for burns than ecotype. Within burns, caribou strongly avoided severely burned areas in winter, and this avoidance lasted nearly 30 years after a fire. Caribou within burns also selected higher cover of terrestrial lichen (an important caribou food source). We found a negative relationship between burn severity and lichen cover, confirming that caribou avoidance of burns was consistent with lower lichen abundance. Consistent winter avoidance of burns and severely burned areas suggests that caribou will experience increasing winter habitat loss as fire frequency and severity increase. Our results highlight the potential for climate-induced alteration of natural disturbance regimes to affect boreal biodiversity through habitat loss. We suggest that management strategies prioritizing protection of core winter range habitat with lower burn probabilities would provide important climate-change refugia for caribou.

Spring phenology drives range shifts in a migratory Arctic ungulate with key implications for the future.

Annual variation in phenology can have profound effects on the behavior of animals. As climate change advances spring phenology in ecosystems around the globe, it is becoming increasingly important to understand how animals respond to variation in the timing of seasonal events and how their responses may shift in the future. We investigated the influence of spring phenology on the behavior of migratory, barren-ground caribou (Rangifer tarandus), a species that has evolved to cope with short Arctic summers. Specifically, we examined the effect of spring snow melt and vegetation growth on the current and potential future space-use patterns of the Porcupine Caribou Herd (PCH), which exhibits large, inter-annual shifts in their calving and post-calving distributions across the U.S.-Canadian border. We quantified PCH selection for snow melt and vegetation phenology using machine learning models, determined how selection resulted in annual shifts in space-use, and then projected future distributions based on climate-driven phenology models. Caribou exhibited strong, scale-dependent selection for both snow melt and vegetation growth. During the calving season, caribou selected areas at finer scales where the snow had melted and vegetation was greening, but within broader landscapes that were still brown or snow covered. During the post-calving season, they selected vegetation with intermediate biomass expected to have high forage quality. Annual variation in spring phenology predicted major shifts in PCH space-use. In years with early spring phenology, PCH predominately used habitat in Alaska, while in years with late phenology, they spent more time in Yukon. Future climate conditions were projected to advance spring phenology, shifting PCH calving and post-calving distributions further west into Alaska. Our results demonstrate that caribou selection for habitat in specific phenological stages drive dramatic shifts in annual space-use patterns, and will likely affect future distributions, underscoring the importance of maintaining sufficient suitable habitat to allow for behavioral plasticity.

Critical summer foraging tradeoffs in a subarctic ungulate.

Summer diets are crucial for large herbivores in the subarctic and are affected by weather, harassment from insects and a variety of environmental changes linked to climate. Yet, understanding foraging behavior and diet of large herbivores is challenging in the subarctic because of their remote ranges. We used GPS video-camera collars to observe behaviors and summer diets of the migratory Fortymile Caribou Herd (Rangifer tarandus granti) across Alaska, USA and the Yukon, Canada. First, we characterized caribou behavior. Second, we tested if videos could be used to quantify changes in the probability of eating events. Third, we estimated summer diets at the finest taxonomic resolution possible through videos. Finally, we compared summer diet estimates from video collars to microhistological analysis of fecal pellets. We classified 18,134 videos from 30 female caribou over two summers (2018 and 2019). Caribou behaviors included eating (mean = 43.5%), ruminating (25.6%), travelling (14.0%), stationary awake (11.3%) and napping (5.1%). Eating was restricted by insect harassment. We classified forage(s) consumed in 5,549 videos where diet composition (monthly) highlighted a strong tradeoff between lichens and shrubs; shrubs dominated diets in June and July when lichen use declined. We identified 63 species, 70 genus and 33 family groups of summer forages from videos. After adjusting for digestibility, monthly estimates of diet composition were strongly correlated at the scale of the forage functional type (i.e., forage groups composed of forbs, graminoids, mosses, shrubs and lichens; r = 0.79, p < .01). Using video collars, we identified (1) a pronounced tradeoff in summer foraging between lichens and shrubs and (2) the costs of insect harassment on eating. Understanding caribou foraging ecology is needed to plan for their long-term conservation across the circumpolar north, and video collars can provide a powerful approach across remote regions.