Retaining forest biomass by reducing logging damage

Global concern over rising atmospheric concentrations of carbon dioxide is stimulating development and implementation of policies aimed at reducing net greenhouse gas emissions by enhancing carbon sinks. One option for reducing net emissions is to lessen damage to residual forests during selective logging, thereby retaining additional carbon in biomass. A pilot carbon offset project was initiated in Sabah, Malaysia, in 1992 in which a power company provided funds to a timber concessionaire to implement guidelines aimed at reducing logging damage; in doing so, the utility gained potential credit towards future emissions reduction requirements. To quantify the carbon retained due to this effort we compared dipterocarp, forests logged according to reduced-impact logging guidelines with forests logged by conventional methods in terms of the above- and below-ground biomass both before and after logging. Prior to logging, the forest stored approximately 400 Mg biomass ha(-1), 17 percent of which was belowground. High volumes of timber were removed from both of the logging areas (mean CNV = 154, Rn = 104 m(3)ha(-1)). Ferry-one percent of the unharvested trees <60 M DBH were severely damaged (uprooted and crushed) from logging in conventional logging areas in contrast to 15 percent in reduced-impact logging areas. Approximately 18 and 12 percent, respectively, of the remaining residual trees in conventional and reduced-impact logging areas suffered less severe damage (e.g., crown or bark damage). Mortality rates of the less severely damaged trees in all DBH classes were higher during the first year in conventional logging areas than in reduced-impact logging areas. One yr post harvest, conventional and reduced-impact logging areas contained biomass equivalent to about 44 percent and 67 percent of pre-logging levels, respectively. Approximately 62 percent of the difference in carbon retention was due to fewer trees killed in the reduced-impact logging areas; the remaining 38 percent was due to a lower mass of branches, stumps and waste wood from felled trees in reduced-impact logging areas. Mortality of damaged trees in both areas may contribute to net decreases in biomass for several years after logging. More and larger trees remained undamaged where reduced-impact logging was practiced, hence future biomass increment and yields of marketable timber are expected to be greater in the reduced-impact logging areas than in conventional logging areas.