CO2 and Methane Fluxes in a Northern Hardwood Forest: Surprising Patterns and Clues for Climate Mitigation Management

Abstract:

Northern temperate forests are expected to show: (1) strong carbon sink strength, particularly in regrowing forests managed for timber production; (2) positive responses of productivity to moderate increases in temperature, in particular to spring warming; (3) strong N limitation such that N additions through fertilization or deposition result in increased productivity and C uptake; (4) a low but uniform rate of methane uptake due to methanotroph activity in the soil. Data is synthesized here from eddy covariance measures of CO₂ and methane flux in conjunction with large plot measurements and stand-level experimental studies to address each of these predictions – with strong evidence found AGAINST each of them. Eddy covariance measurements from 2010-2012 in a selection-managed forest measured 13-16 years following a partial stand harvest indicate a net CO₂ source of from ~1-4 Mg C ha^-1 y^-1, in large part due to consistently high respiration rates. By far the largest carbon loss occurred in 2010, a year in which spring leaf flush by the dominant tree species Acer saccharum and Fagus grandifolia corresponded with seasonally exceptional temperatures of 31-33°C, and a corresponding decline in leaf-level photosynthesis and leaf area index of the dominant (but not other) tree species. Analyses of spatial variation in forest productivity and direct experimental additions of N and P suggest that the forest is not N limited, but rather co-limited by P and cations (especially Ca and Mg). In contrast to the strong observed CO₂ source, the study site is a strong sink for methane, sufficiently strong that predicted climate forcing effects due to methane uptake more than offset those due to CO₂ losses. Chamber studies indicate that methane uptake is not uniform, but rather that local depressions and lower slopes can show high methane emissions consistent with hypoxia effects. Current studies are examining methods of enhancing forest climate mitigation through management to address non-N nutrient limitations through use of pyrolyzed forestry mill waste material (“biochar”) as a soil amendment, and by silvicultural methods to enhance local tree species diversity, including species with more southern distributions (“passive silvicultural assisted migration”).