COS 91-3 - Forests in the global methane budget and the potential for “methane-smart” forestry

Background/Question/Methods

Although forests are globally important both as sources and sinks for CH₄, recommendations for “climate-smart” forest management have not considered forest CH₄ dynamics, and relevant data remains sparse. We have recently completed a series of studies on CH₄ flux and its components in continuous-cover managed forest in Ontario, making use of a portable gas analysis system that allows accurate real-time estimates of fluxes as low as 0.02 nmol m^-2 s^-1. Quantification of CH₄ fluxes from tree stems, branches, foliage, tree wounds, and coarse woody debris, and of harvest-related soil disturbance on soil CH₄ fluxes allows estimates of the whole-forest CH₄ budget under alternative management regimes.

Results/Conclusions

In upland sites, tree foliage consistently showed CH₄ uptake, though roughly balanced by CH₄ emissions from branches. Branch and foliage fluxes were ~10-fold the CH₄ emission from main stems. Coarse woody debris acted as weak CH₄ sink and tree wounds as a source comparable to main stems. Soil disturbance from logging machinery resulted in conversion of heavily trafficked sites from CH₄ sinks to CH₄ sources; the most heavily impacted sites showed emissions greater than regional wetlands, with peak emissions in areas where fine wood particles are incorporated into soil. Heavily impacted soils did not recover a CH₄ sink comparable to unimpacted soils within 20 years, which is the prevailing felling cycle length. The issue of “logging scars” in forest carbon budgets has been a recent focus of the Canadian eNGO community: our results suggest that reductions in CH₄ sinks or conversion of forests from CH₄ sinks to sources are pervasive. Management practices that could enhance net forest CH₄ uptake include soil protection, enhanced skid trail planning, and targeted restoration of CH₄ emission “hotspots” such as log landings. Preliminary estimates suggest that “methane-smart” forest management could be of similar importance globally to CH₄ emissions mitigation in the agricultural sector.