In the northeastern U.S., climate is changing more rapidly in winter than in summer. The impacts of winter climate change on ecosystems are greatly complicated by effects on snow depth and soil freezing. Snow is important as an insulator of the soil, and many northern hardwood forest soils normally remain unfrozen during the winter. A lack of snow can result in soil freezing, which is a significant disturbance to forest ecosystems, potentially killing tree roots and microorganisms and disrupting nutrient cycling processes leading to losses of nutrients to water and air. These losses can decrease the productivity of the forest ecosystem and lead to air and water pollution. In this project investigators will use a landscape-scale approach to evaluate three aspects of the effects of changes in snow depth on soil freezing and the cycling of carbon and nitrogen in the northern hardwood forest at the Hubbard Brook Long Term Ecological Research site (HBR) in New Hampshire. First, the research will address uncertainty about the occurrence of colder soils in a warmer world, and whether this pattern will increase losses of nitrogen, an important plant nutrient and source of water and air pollution, from the northern hardwood forest at HBR. Uncertainty about the extent and effects of soil freezing in a warmer world is rooted in limitations in our understanding of where two "tipping points" occur across a forest landscape experiencing climate change; where snowpacks are too shallow to insulate the soil, and where air temperatures are too warm to freeze the soil. This uncertainty will be addressed by measuring and modeling snow depth, and soil climate across the entire ~3000 ha Hubbard Brook Experimental Forest (HBEF), which encompasses the range of climate variability that has been predicted for the northeastern U.S. over the next 50-100 years. Measurements of soil temperature, moisture and frost, and nitrogen losses to water and air will be made at 20 experimental field sites that experience a broad range of long-term snowpack regimes to explore critical uncertainties surrounding soil freezing events and their effects on nitrogen cycling. Second, the proposed research will address how winter climate change affects microbial and soil invertebrate processes and resultant changes in carbon flow during winter, testing the hypothesis that carbon flow is the key integrative regulator of winter microbial activity. To test this hypothesis, the movement of isotopically labeled carbon and nitrogen will be traced from sugar maple detritus into and through the soil ecosystem in six intensive study sites that represent the full range of variation in winter climate at HBEF. Third, the proposed research will address if soil freezing alters hydrologic controls of ecosystem nitrogen processing at snowmelt and subsequent export of nitrogen to receiving waters. Snowmelt dynamics will be tested by adding isotopically labeled nitrogen to the snowpack and tracing its movement into and through the soil ecosystem. Effects of soil freezing on export to receiving waters will be tested by analyzing nitrogen export and solutes that serve as natural tracers of hydrologic flowpaths on small watersheds that differ in winter climate/soil freezing.
The project will include education and policy activities that explore the effects of winter climate change in the Northeast U.S. on recreation, timber harvesting, biomass energy production, and other ecosystem services. The project will connect ecosystem researchers with relevant stakeholders and interest groups, including loggers and foresters, ski-area operators, maple sugar producers, recreational snowmobile users, conservation-oriented NGOs, citizen scientists, and public and private land managers through a "Science Links" program. There will also be two pilot educational initiatives: a winter field course for undergraduates and creation of a teaching guide for middle and high school teachers. Finally, the research will be well integrated into ongoing HBR research on modeling the effects of climate change, fostering extension of current climate change modeling studies to depict the hydrologic and biogeochemical response to soil freezing under future climate change scenarios.
Understanding how atmospheric deposition of pollutants and climate change are affecting forest ecosystems is important for evaluating impacts on ecosystem functions and services. A key element in the functioning of forest ecosystems is nitrogen. Depending on its concentration and availability, it can serve as a limiting nutrient or a pollutant to forest ecosystems. Our study has evaluated that the role of atmospheric nitrogen inputs during the winter with respect to the retention and/or loss of this element using an experimental approach in the field. The winter period is critical for understanding the factors that affect nitrogen since for forests with substantial periods of snow cover the snowmelt period is the time when the vast majority of nitrogen is leached from the forest soil and enters surface and ground waters. We used stable isotopes of N of an inorganic form of N (ammonium) to trace the transformations and fluxes of nitrogen chemical forms. The use of stable isotopes of elements is an environmentally safe technique to follow elemental transformations and fluxes. It was once thought that during the winter that little biogeochemical activity occurs. We also evaluated the influence of the type of forest floor as a function of watershed elevation and hence vegetation type in affecting the dynamics of nitrogen. Our results show that forest floor characteristics affect the amount of inorganic N generated. During the winter the added ammonium was rapidly converted to a more mobile form of inorganic nitrogen (nitrate). This nitrate was available for transport to surface and ground waters during snow melt. The chemistry of these waters can affect potability and other necessary ecosystem and human services.
