Snow depth manipulation and its influence on soil frost and water dynamics in a northern hardwood forest

Climate change will likely result in warmer winter temperatures leading to less snowfall in temperate forests. These changes may lead to increases in soil freezing because of lack of an insulating snow cover and changes in soil water dynamics during the important snowmelt period. In this study, we manipulated snow depth by removing snow for two winters, simulating the late development of the snowpack as may occur with global warming, to explore the relationships between snow depth, soil freezing, soil moisture, and infiltration. We established four sites, each with two paired plots, at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire, U.S.A. and instrumented all eight plots with soil and snow thermistors, frost tubes, soil moisture probes, and soil lysimeters. For two winters, we removed snow from the designated treatment plots until February. Snow in the reference plots was undisturbed. The treatment winters (1997/1998 and 1998/1999) were relatively mild, with temperatures above the seasonal norm and snow depths below average. Results show the treated plots accumulated significantly less snow and had more extensive soil frost than reference plots. Snow depth was a strong regulator of soil temperature and frost depth at all sites. Soil moisture measured by time domain reflectometry probes and leaching volumes collected in lysimeters were lower in the treatment plots in March and April compared to the rest of the year. The ratio of leachate volumes collected in the treatment plots to that in the reference plots decreased as the snow ablation seasons progressed. Our data show that even mild winters with low snowfall, simulated by snow removal, will result in increased soil freezing in the forests at the HBEF. Our results suggest that a climate shift toward less snowfall or a shorter duration of snow on the ground will produce increases in soil freezing in northern hardwood forests. Increases in soil freezing will have implications for changes in soil biogeochemical processes.