Soil carbon dynamics after forest harvest: an ecosystem paradigm reconsidered

In one of the most influential studies in the recent history of forest ecology, W. W. Covington (1981) described a pattern in organic matter storage in the forest floors of northern hardwood stands as a func-tion of date of harvest. We review the history of the use and misuse of Covington’s curve, describe the studies that tested and failed to support early inter-pretations of the curve, and provide some alternate interpretations. The curve suggested that forest floor organic matter declines by 50 % within 20 years after harvest, and this decline was attributed to accelerated decomposition and changes in litter inputs after harvest. Subsequent studies showed that decomposition rates of surface litter generally decrease after clear-cutting, but accelerated decom-position remains possible in the Oe and Oa hori-zons. Changes in litter inputs are still difficult to evaluate, because the rate at which woody debris enters the forest floor is unknown. Although Cov-ington attempted to minimize variation due to me-chanical disturbance during logging, a reasonable alternative explanation for low organic matter in the forest floor of young stands is that surface ma-terial is mixed into mineral soil during harvesting operations. The pattern of forest floor organic mat-ter in stands of different ages may be partly due to changes over time in logging technology and the intensity of biomass removal, in addition to succes-sional effects. It is important to distinguish between mechanisms that release carbon to the atmosphere and those that transfer it to the mineral soil before making inferences about nutrient cycling and car-bon sequestration. Key words: forest floor; Covington; decomposi-tion; forest harvest; chronosequence; soil carbon model; soil organic matter; paradigm