Snow depth, soil temperature and plant–herbivore interactions mediate plant response to climate change

Abstract

Northern forest ecosystems are projected to experience warmer growing seasons, as well as winters with reduced snowpack depth and duration. Reduced snowpack will expose soils to cold winter air and lead to increased frequency of freeze‐thaw cycles. The interactions between warmer soils in the growing season and colder soils in winter may have important implications for the phenology, productivity and nutrient content of forest plants. We conducted an experiment at Hubbard Brook Experimental Forest, NH, USA, to examine the effects of growing season warming, reduced depth and duration of winter snowpack, as well as increased frequency of soil freeze‐thaw cycles on sugar maple (Acer saccharum) and red maple (Acer rubrum) saplings. We examined the direct effects of soil temperatures on plant root health, timing of leaf‐out, foliar nitrogen, rates of photosynthesis and growth, as well as the indirect effects of snowpack reduction on herbivory on plant stems. A smaller winter snowpack and increased frequency of soil freeze‐thaw cycles in winter led to increased root damage and delayed leaf‐out for maple saplings. Snowpack reduction decreased rates of stem herbivory in winter, indicating that alleviation of above‐ground stem damage in winters with reduced snowpack may offset the root damage incurred from successive soil freeze‐thaw cycles in winters with low snowpack. Synthesis. By examining the response of two dominant tree species to simulated climate change in both the growing season and winter, we find that plant responses are mediated through a combination of changes in soil temperature and plant–herbivore interactions that differentially affect above‐ and below‐ground plant components. These results highlight the feedbacks between trophic levels that shape forest function and demonstrate the need for considering climate change across seasons in global change experiments to determine how forest function may change in the future.

Declines in northern forest tree growth following snowpack decline and soil freezing

“…By contrast, Reinmann and Templer () found that reduced winter snowpack and increased soil freezing has either no effect or a small stimulatory effect on tree growth in a red oak ( Quercus rubra )‐red maple ( A. rubrum ) forest in Massachusetts, U.S.A.; however, that study was conducted in a region that does not typically experience a persistent insulating snowpack and the study involved only 1 year of reduced snowpack and increased soil freezing (we did not observe a significant decline in tree growth until after the second year of snow removal in this current study). Additionally, findings from a sapling experiment indicate that roots of sugar maple trees are more sensitive to soil freezing than those of red maple trees (Sanders‐DeMott, McNellis, Jabouri, & Templer, ). The different findings between these studies could also point to distinct acclimation responses of trees to soil freezing based on the typical snow and soil freezing environments where they grow.…”

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

“…seedlings were more likely to be discovered by small-mammal herbivores where snow was shallow). However, even after escaping herbivory as seedlings, trees may face M. gapperi herbivory as saplings that could delay phenology or decrease survival (Sanders-DeMott et al 2018). However, the small spatial extent of our snow manipulation (12 × 1.5 m plots) may not alter small-mammal behavior in the same way that projected regional declines in snowfall could alter behavior in the future.…”

“…These effects are important to understand because warming winter temperatures are decreasing snow depth and snow cover across the northern hemisphere (Dye 2002, Henry 2008, Brown and Robinson 2011, possibly exposing plants to damaging freeze-thaw events that increase mortality or delay phenology (Tierney et al 2001, Inouye 2008. Small mammals, such as the southern red-backed vole Myodes gapperi, can be important consumers of seeds or seedlings in temperate ecosystems during winter (Haken and Batzli 1996, Ostfeld et al 1997, Howe and Brown 2000, Manson et al 2001, Korslund and Steen 2006, Sullivan and Sullivan 2008, and snow cover may facilitate herbivory by small mammals by providing a refuge from predation or extreme temperatures (Kausrud et al 2008, Pauli et al 2013, Sultaire et al 2016, Sanders-DeMott et al 2018. Small mammals, such as the southern red-backed vole Myodes gapperi, can be important consumers of seeds or seedlings in temperate ecosystems during winter (Haken and Batzli 1996, Ostfeld et al 1997, Howe and Brown 2000, Manson et al 2001, Korslund and Steen 2006, Sullivan and Sullivan 2008, and snow cover may facilitate herbivory by small mammals by providing a refuge from predation or extreme temperatures (Kausrud et al 2008, Pauli et al 2013, Sultaire et al 2016, Sanders-DeMott et al 2018.…”

Northern forest winters have lost cold, snowy conditions that are important for ecosystems and human communities

“…, Sanders‐DeMott et al. ). Because snow cover provides subnivean refuge, the reductions we observed in Snow Covered Days may also translate into limits on small mammal abundance as well as enhanced competition among predators, for example allowing red fox ( Vulpes vulpes L.) to outcompete Arctic fox ( Vulpes lagopus L.; Penczykowski et al.…”

“…At the same time, reduced frequency of Snow Covered Days might be detrimental to smaller mammals who gain access to browse higher up on shrubs when snowpacks are deep (Nordengren et al 2003). Further, shifts in wildlife browsing activity that result from changes in snow depth could have cascading effects on regeneration of forest plants in the understory (Christenson et al 2014, Sanders-DeMott et al 2018b). Because snow cover provides subnivean refuge, the reductions we observed in Snow Covered Days may also translate into limits on small mammal abundance as well as enhanced competition among predators, for example allowing red fox (Vulpes vulpes L.) to outcompete Arctic fox (Vulpes lagopus L.; Penczykowski et al 2017).…”