Reduced Winter Snowpack and Greater Soil Frost Reduce Live Root Biomass and Stimulate Radial Growth and Stem Respiration of Red Maple (Acer rubrum) Trees in a Mixed-Hardwood Forest

Northeastern U.S. forests are currently net carbon (C) sinks, but rates of Closs from these ecosystems may be altered by the projected reduction in snowpack and increased soil freezing over the next century. Soil freezing damages fine roots, which may reduce radial tree growth and stem respiration. We conducted a snow removal experiment at Harvard Forest, MA to quantify effects of a reduced winter snowpack and increased soil freezing on root biomass, stem radial growth and respiration in a mixed-hardwood forest. The proportion of live fine root biomass during spring (late-April) declined with increasing soil frost severity (P = 0.05). Basal area increment index was positively correlated with soil frost severity for Acer rubrum, but not Quercus rubra. Rates of stem respiration in the growing season correlated positively with soil frost duration in the previous winter, (R-LMM(m)(2) = 0.15 and 0.24 for Q. rubra and A. rubrum, respectively). Losses of C from stem respiration were comparable to or greater than C storage from radial growth of Q. rubra and A. rubrum, respectively. Overall, our findings suggest that in mixed-hardwood forests (1) soil freezing has adverse effects on spring live root biomass, but at least in the short-term could stimulate aboveground processes such as stem respiration and radial growth for A. rubrum more than Q. rubra, (2) stemrespiration is an important ecosystemC flux and (3) the increasing abundance of A. rubrum relative to Q. rubra may have important implications for C storage in tree stem biomass.