Predicting Soil Frost and its Response to Climate Change in Northeastern U.S. Forests

Depth and duration of seasonal snow cover has important effects on temperate forest ecosystems. In the northeastern U.S., recent predictions are that climate warming over the coming century will cause an increase in soil freezing as soils lose the insulation of continuous wintertime snow cover. These studies have also linked soil freezing to elevated nitrate export from soils and streams. In the present study, we used a physically based energy and water exchange model, SHAW (Simultaneous Heat and Water), to predict soil frost and snowpack dynamics at three forested sites in New England: Hubbard Brook (NH), Harvard Forest (MA), and Howland Forest (ME). Results indicate an inverse relationship across all three sites between the depth and duration of the snowpack and soil frost. Simulations were conducted for all three sites with historical weather data for the past 20-40 years, and for future projections (2000-2100) using two different IPCC climate scenarios (A1fi and BI) derived from statistically downscaled GCM simulations. Under both scenarios and at all three sites, SHAW predicted that both the amount of soil frost and the number of extreme soil freezing events will decrease during the 2000-2100 period. In addition, there was no relationship between predicted soil frost, 1966-2000, and observed stream nitrate concentration at Hubbard Brook. These results run counter to existing theories regarding both the impacts of soil frost and the changes that are expected to occur into the future. There was, however, a positive correlation between predicted soil frost and growing season CO2 uptake at Harvard Forest over the 1992-2002 period. This suggests that soil freezing does play an important role in forest biogeochemistry, albeit a different role than that which has been discussed in the literature