Effects of winter climate change and soil frost on forest nutrient retention and productivity in northeastern U.S. forest ecosystems

Background/Question/Methods
Variation in winter climate within northeastern forests influences ecosystem processes by affecting the physical properties of soils and the integrity of fine root systems. However, because most ecosystem-scale research focuses on the growing season, our understanding of how factors such as soil freezing affects carbon (C) and nitrogen (N) dynamics is very limited. Soil freezing is primarily controlled by meteorological variables that determine the annual accumulation of insulating snow cover (e.g., temperature, precipitation). Therefore, changes in climate may alter soil freezing dynamics. Climate projections for the northeastern U.S. indicate that air temperatures will increase by 2.1-5.3 °C in the next century. While precipitation is more uncertain, projections indicate that winter precipitation will increase by 12-30% with a greater proportion falling as rain rather than snow. These changes in climate are expected to decrease snow pack depth and duration. Thus, warmer air temperatures in the future may increase the annual depth and duration of soil frost.
Within northeastern U.S. forest ecosystems, soil freezing can lead to increased forest N losses via NO₃^- leaching to nearby streams and gaseous losses as N₂O to the atmosphere. Among dominant woody plants, soil freezing can also lead to increases in fine root mortality. However, impacts of soil freezing on plant nutrient uptake, sequestration and forest productivity are less well understood. Although there is growing awareness that winter is a more significant period for N cycling than was previously perceived, the effects of soil freezing on plant nutrient uptake and forest productivity remain unknown.
In this study, we sought to determine the impacts of a later onset of snow and soil frost on forest nutrient retention and productivity by conducting a snow-removal experiment at the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. Following snow manipulations, we examined plant nutrient uptake, trace gas fluxes from soils and productivity in mixed stands of sugar maple (Acer saccharum)-American beech (Fagus grandifolia) and red spruce (Picea rubens)-Balsam fir (Abies balsamea).

Results/Conclusions
The results of the first year of our multi-year study suggest that the impacts of winter climate change on forest ecosystems affects hardwood forests more than conifer forests. We found a greater impact of changes in snow cover and soil frost in the mixed sugar maple/beech stands compared to the spruce/fir stands. Our results also illustrate the importance of examining the impacts of changes in winter climate on plant nutrient uptake, forest productivity and plant-microbial interactions.