Mechanisms underlying American marten (Martes americana) winter rest site selection across the Western Great Lakes Region

Abstract

Tree cavities are important microhabitats that many mammal species use as a secure place for resting and raising young. Cavities buffer ambient weather and moderate temperatures, which helps reduce the energetic costs of thermoregulation. The buffering capacity of tree cavities, however, varies with tree and cavity characteristics, making it difficult to estimate cavity-specific temperatures without the use of temperature loggers. Here, we introduce a dynamic model that incorporates fundamental processes of heat and mass transfer to predict the internal temperature of tree cavities as a function of cavity- specific physical characteristics and ambient temperature. To validate the model, we compared measured and modeled internal temperatures of 43 natural tree cavities used by American martens (Martes americana) or fishers (Pekania pennanti) and 27 artificial cavities designed for fishers. Cavities varied in physical and thermal characteristics, allowing us to assess the generalizability of the model. Predicted internal temperatures of natural and artificial cavities were similar to measured temperatures, with average RMSE values less than 1.77°C. Our results demonstrate that the model can accurately predict temperature within the tree cavities over time, has a tractable complexity that captures the main processes driving temperature within the tree cavity, is easily parameterized so it can be applied to many ecological questions, and is adaptable enough to be used in a range of conditions. Subnivean microsites are important rest sites for American martens (Martes americana) during the winter. With reduction in the spatial extent, temporal duration, and depth of snow cover from climate change, martens may be forced to use alternative microsites. In areas with shallow, transient snowpack, martens are using tree cavities. Whether tree cavities can provide the same energetic benefit, however, is not known. We evaluated the roles of ambient temperature, snowpack, tree cavity temperature, and subnivean temperature on marten winter rest site selection across the Western Great Lakes Region. Selection for tree cavities in lower peninsula Michigan, USA appears to be driven by shallow snow depths and warm ambient temps, by limited availability of subnivean microsites sites that would allow marten to benefit from the snowpack’s insulation when present, or both. Conversely, deep snow and cold ambient temperature conditions make subnivean sites ideal for resting in Minnesota, USA. When snow is present and ambient temperatures are cold, subnivean sites are warmer and more energetically favorable than tree cavities in both study areas. However, as ambient temperatures increase and the availability of deep snow decreases with climate change, tree cavities could become a warmer and more energetically favorable site. Alternatively, subterranean sites could be an important ground level microsite that mimics above ground subnivean sites, if available.