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

1. 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. 2. 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. 3. 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. 4. 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.