Seedling responses to decreased snow depend on canopy composition and small‐mammal herbivore presence

Abstract

Winter is becoming warmer and shorter across the northern hemisphere, and reductions in snow depth can decrease tree seedling survival by exposing seedlings to harmful microclimates. Similarly, herbivory by small mammals can also limit the survival and distribution of woody plants, but it is unclear whether winter climate change will alter small-mammal herbivory. Although small-scale experiments show that snow removal can either increase or decrease both soil temperatures and herbivory, we currently lack snow-removal experiments replicated across large spatial scales that are needed to understand the effect of reduced snow. To examine how winter herbivory and snow conditions influence seedling dynamics, we transplanted Acer saccharum and Tsuga canadensis seedlings across a 180 km latitudinal gradient in northern Wisconsin, where snow depth varied seven-fold among sites. Seedlings were transplanted into one of two herbivory treatments (small-mammal exclosure, small-mammal access) and one of two late-winter snow removal treatments (snow removed, snow unmanipulated). Snow removal increased soil freeze-thaw frequency and cumulative growing degree-days (GDD), but the magnitude of these effects depended on forest canopy composition. Acer saccharum survival decreased where snow was removed, but only at sites without conifers. Excluding small mammals increased A. saccharum survival at sites where the smallmammal herbivore Myodes gapperi was present. Excluding small mammals also increased T. canadensis survival in plots with < 5 cm snow. Because variation in canopy composition and M. gapperi presence were important predictors of seedling survival across the snow-depth gradient, these results reveal complexity in the ability to accurately predict patterns of winter seedling survival over large spatial scales. Global change scenarios that project future patterns of seedling recruitment may benefit from explicitly considering interactions between snow conditions and small-mammal winter herbivory.

Snow depth and woody debris drive variation in small‐mammal winter seed removal

“…This supports the hypothesis that shallow snow may favor smallmammal foraging by increasing the apparency of small food items (Anderson, 1986;Guiden et al 2019). Our results were likely driven by undocumented shifts in consumer activity during late winter (i.e., after our snow removal treatment), as there was no…”

“…Starting in December 2017, we measured snow depth in each plot approximately every three weeks. Within each woody-debris treatment, half of the plots were selected to have snow removed starting February 2018, and half were left unmanipulated as a control (Guiden et al 2019).…”

“…The loss of the subnivium can alter the strength of many important species interactions, including predation (Sonerud, 1986; Kausrud et al 2008) and herbivory (Fisichelli et al 2012; Martin & Maron, 2012; Sanders‐DeMott et al 2018). However, given the complex effect of subnivium in reducing both metabolic costs and predation risk (Pauli et al 2013), the direction and strength of these effects often depends strongly on local context (Maron et al 2014; Guiden et al 2019). Experiments that manipulate the availability of subnivium across gradients in climate or predation risk will therefore improve our ability to predict the effect of winter climate change on species interactions, and corresponding climate‐mediated shifts in abundance and distribution (Brown & Vellend, 2014; Guiden et al 2019).…”

“…Temperate forests are experiencing disproportionately large and fast decreases in snowfall due to rising winter temperatures (Kreyling, 2010; Drescher & Thomas, 2013; Zhu et al 2017). However, little is known about how decreased snowfall will alter interactions between plants and consumers, such as small mammals (Guiden et al 2019). Many small‐mammal species remain active over winter (Wolff & Durr, 1986; Guiden & Orrock, 2020), and their winter foraging can have a strong effect on plant survival and diversity in the growing season (Howe & Brown, 2000; Korslund & Steen, 2006; Guiden et al 2019).…”

“…However, little is known about how decreased snowfall will alter interactions between plants and consumers, such as small mammals (Guiden et al 2019). Many small‐mammal species remain active over winter (Wolff & Durr, 1986; Guiden & Orrock, 2020), and their winter foraging can have a strong effect on plant survival and diversity in the growing season (Howe & Brown, 2000; Korslund & Steen, 2006; Guiden et al 2019). Seeds represent an important food source for many temperate forest small mammals (Lobo, 2014), particularly in winter when other resources are scarce (Banks & Dickman, 2000; Howe & Brown, 2000).…”