Soil Carbon Dynamics During Stand Rotation in Boreal Forests

Abstract

The utilisation of forest resources is frequently criticised for its adverse environmental impacts. In particular, the widespread use of rotation forestry practices, including clear-cutting, is viewed as a threat to ecosystems. Clear-cutting has short-term impacts that turn the forest into a net carbon (C) source, but such modifications to the ecosystem also have long-term effects. Any analyses of alternative management approaches should include at least one full rotation, which requires model extrapolations. In this study, we used data from a well-documented series of long-term forest monitoring sites in Finland (ICP Forests Level II), focusing on two recent clear-cut sites—one dominated by Scots pine and the other by Norway spruce. These data were utilised to constrain a soil C model and to extrapolate two future scenarios: rotation forestry and set-aside (unmanaged). We simulated these scenarios over a period exceeding one full rotation and compared the outcomes. Although the stand thinning events did not heavily affect the C balance of the studied sites, clear-cutting did. Each clear-cut event caused a negative soil organic carbon (SOC) balance for many decades. It took between 37 and 69 years (for Norway spruce and Scots pine, respectively) to reach a break-even point in which soil C losses compensated for C uptake. Emissions from coarse harvest residuals represented the greatest C source after clear-cutting, followed by fine roots and then foliage. When comparing such a scenario with a set-aside scenario, the soil C budget of the managed stand after clear-cut was negative until the first thinning or even until the following clear-cut, while the set-aside was always a C sink in soil. Thus, scenario analyses of forest C sequestration that disregard long-term soil C dynamics following management interventions may lead to biased conclusions. As a management regime, rotation forestry was relatively ineffective when evaluated considering C sequestration as an important ecosystem service.