The impacts of a reduced winter snowpack on soil respiration in a mixed-deciduous northern forest

Background/Question/Methods

In northern forests, soil frost is a natural event that occurs when there is insufficient snowpack accumulation to insulate soils in winter. Northern forests typically have a continuous snowpack for much of the winter. However, winters with a late-developing or intermittent snowpack occasionally occur and may result in colder soil temperatures and a greater frequency and severity of soil frost. Such variability in winter climate can affect rates of soil respiration and may contribute to interannual variability in ecosystem carbon exchange. Climate models predict a reduction in snowpack depth and duration by the end of the 21^st century in the northeastern U.S. which may have important implications for belowground carbon fluxes. We are conducting a snow removal experiment to quantify the impacts of a reduced winter snowpack on soil respiration. Six 13m x 13m plots were established in a mixed-deciduous forest at Harvard Forest in central Massachusetts. Snow was removed from three plots in early-winter and soil respiration quantified in each plot. As a companion to this field experiment, we conducted a laboratory experiment to compare rates of soil respiration in response to severe soil frost (-15 ºC), mild soil frost (-0.5 ºC), and no soil frost (+5 ºC) conditions.

Results/Conclusions

Results from the laboratory experiment indicate that rates of soil respiration decline with decreasing soil temperature and increasing soil frost severity. Results from the first winter of the snow removal field experiment indicate that while shallow soil frost (<5 cm depth) was present prior to snowpack development, the presence of a snowpack in the reference plots facilitated thawing of the frozen soils beneath it. In contrast, soil frost depth in the snow removal plots increased throughout the winter. A reduced snowpack also increased the frequency of freeze-thaw events in the top 5 cm of the soil. Early soil respiration data from the field experiment indicate that heterotrophic soil respiration comprises most of total soil respiration. If rates of soil respiration in response to soil frost in the field experiment follow the patterns observed in the laboratory experiment then these results suggest that soil frost can be an important factor explaining interannual variability in ecosystem carbon exchange. Furthermore, if predicted changes in winter climate lead to an increase in the frequency and severity of soil frost, rates of soil respiration may decrease resulting in greater carbon storage in mixed-deciduous forests of the northeastern U.S.