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.
The major goal of this study was to investigate the hydrology and water chemistry response of a forest ecosystem at the Hubbard Brook Experimental Forest, NH to winter climate change effects of decreases in snowpack accumulation and increases in soil freezing. There were two major components of this work. The first was to examine long-term trends in soil water and stream chemistry to examine if air pollution controls which have limited sulfur dioxide and nitrogen oxide emissions were diminishing the effects of acid deposition on waters draining forests in New Hampshire, particularly associated during the highly acidic period of spring snowmelt. The second was an experimental study utilizing natural climate gradients associated with spatial variations in elevation and aspect and temporal variations in meteorological conditions to evaluate the effects of snowpack depth and soil freezing on soil solution and stream chemistry. Our long-term measurements have shown that there have been decreases in soil water and stream concentrations of sulfate and nitrate, the two major components of acid deposition. These changes have coincided with measureable recovery in the acidification of soil water and streams associated with decreases in acid deposition. More importantly, rates of recovery of streams during snowmelt, the most acidic condition during the year, are comparable to year-around trends. This finding suggests that streams are recovering from acid deposition during the most acidic time of year (spring snowmelt) at a rate that is similar to the overall rate of ecosystem recovery. Our experimental ecosystem climatic gradient study showed marked spatial and temporal variation in snowpack accumulation and soil freezing across the Hubbard Brook Valley. This variation in winter climate conditions revealed that soil process and snowmelt are strongly influenced by the extent of snowpack accumulation and soil freezing. In a future warmer world, we would anticipate that changing climate will result in diminished snowpack and greater soil freezing, which will alter forest ecosystem process and structure. Winter climate change is an important and understudied component of global change. This study also included professional training of undergraduate and graduate students at Syracuse University; presentations at technical and public meetings on climate change effects; briefings and outreach activities to inform natural resource managers; and interactions with schoolyard science education programs.
