Effects of winter climate on growing season fluxes of CO2 from tree stems in a mixed hardwood forest: Implications for carbon storage

Background/Question/Methods

Forests of the northeastern U.S. typically have a continuous snowpack in winter, which insulates soils below it. However, winters with a late-developing or intermittent snowpack can result in colder soil temperatures and a greater frequency and depth of soil frost. Climate models project 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 ecosystem carbon fluxes and the ability of forests to mitigate climate change through carbon uptake and storage in biomass and soils. The objectives of this research were to quantify the effects of reduced winter snowpack and increased soil frost on rates of carbon dioxide (CO₂) efflux from tree stems. We are conducting a snow removal experiment in mixed stands of red oak (Quercus rubra) and red maple (Acer rubrum) trees at Harvard Forest in central Massachusetts. Snow was removed from three plots (13m x 13m) during the first four to five weeks of the 2010/2011 and 2011/2012 winters to mimic a later development of snowpack and to induce soil frost. 

Results/Conclusions

During the first year of this multi-year study snow removal increased the depth and duration of soil frost, increased the frequency of freeze-thaw cycles, and impeded soil warming in the spring compared to the reference plots. Aboveground losses of CO₂ from tree stems during the growing season were higher for red oak trees than red maple trees, but were not altered by a reduced snowpack and increased soil frost in the previous winter. Stem CO₂ efflux made an important contribution to ecosystem losses of CO₂ and was equivalent to 19-24% and 24-34% of belowground losses of CO₂ per unit surface area during the growing season for red maple trees and red oak trees, respectively. Results of this study to date suggest that tree stems play an important role in growing season ecosystem CO₂ fluxes and that these fluxes appear to be relatively insensitive to changes in winter climate that result in a reduced winter snowpack and increased soil.