Forests have a complicated relationship with carbon and climate. They sequester huge quantities of carbon dioxide from the atmosphere, estimated at 10-20 percent of U.S. emissions, thus limiting its potential as greenhouse gas. In turn, forests are also impacted by changes in climate, which affects how much carbon they store.
Because forests managed in some capacity may hold less carbon than some of their “natural,” unmanaged counterparts and are harvested periodically, it’s logical to assume their carbon benefits are greatly diminished. However, the carbon forests hold at any point in time is only one of many factors affecting what the atmosphere sees, and those other factors tend to favor managed forests.
Time and space
Both temporal and spatial factors influence the carbon benefits of managed forests relative to those preserved with little intervention. In temporal terms, a “carbon debt” relative to the previous forest or land use may occur when a managed forest is established. The “payback time” upon which carbon in the newly-regenerating forest reaches that of the previous condition depends both on the size of the debt and the subsequent forest growth rate. In some cases, the new forest will not reach the carbon store of the previous condition. However, evidence suggests that the faster growing managed forest over multiple harvests and regeneration cycles eventually withdrawals more carbon from the atmosphere than that stored in the previous forest. Comprehensive assessments conclude the longer the time period examined the greater the carbon benefit of managed forest relative to its predecessor.
Spatial factors are also critical for assessing managed forest carbon benefits. Assessments are often based on an individual forest stand undergoing harvest and regeneration cycles. In reality, a forested landscape is typically comprised of multiple landowners and with varying management regimes and objectives. A study of European forests, for example, found that more intensive harvesting did not increase carbon debt when assessed at a large-scale, in contrast to findings from small-scale studies where individual stand dynamics have a greater impact.
In addition, surveys have found that 25 to 35 percent of managed forest landscapes are typically set aside or managed with reduced intensity along water bodies or in environmentally sensitive areas as dictated by forest certification requirements, best management practices, economic factors, and landowner preferences. These landscape-scale factors provide a more accurate depiction of the carbon implications of management.
Beyond the forest
Forest carbon assessments often focus solely on the forest itself, despite the fact that carbon continues to be stored in products following harvesting, where it may reside briefly (e.g., some paper products) or remain intact for many decades (structural lumber). Even greater carbon benefits are derived from biomass used to replace fossil fuels, either directly for bioenergy or when forest products replace more energy-intensive materials such as concrete and steel. Forest biomass is considered a renewable resource, assuming harvested forests are promptly regenerated and managed sustainably. By contrast, fossil fuels are only replenished over millions of years so replacing their emissions with renewables essentially amounts to a permanent reduction in carbon dioxide reaching the atmosphere. These post-harvest factors are the reason sustainably managed forests efficiently used to produce products and bioenergy have greater long-term carbon benefits than preserved or unmanaged forests.
Intensive management has carbon benefits
Management practices that increase forest productivity may cause initial net carbon emissions due to site disruption and fossil fuel use but have longer-term benefits by enhancing sequestration rates and the conversion of biomass to products and bioenergy. For example, a published global forest analysis found greater carbon benefits from active management compared to forest preservation over the course of 100 years. In the U.S., intensive practices have increased the productivity of southern pine by six times and midwestern poplar by eight times, compared to stands not actively managed.
Although greenhouse gases are emitted during forest harvesting and other practices, these emissions are small relative to other forest carbon components. Studies of southeastern U.S. loblolly pine, for example, showed emissions from management and harvesting practices comprise less than two percent of forest carbon stocks and emissions from the manufacturing and transport of fertilizers are less than five percent of the additional carbon sequestered in biomass as a result of fertilizer application.
Because faster-growing forests produce more biomass faster, they create flexibility to manage other lands less intensively or prioritize environmental values such as wildlife habitat as a primary objective.
Forest harvesting and soil carbon
It has traditionally been assumed that forest harvesting substantially reduces soil carbon due to reduced tree litter return to the soil, soil disturbance, and changes in soil temperature and moisture regimes that stimulate microbial decomposition of organic matter. Analyses of numerous forest harvesting studies around the world show mixed effects of harvesting on soil carbon, however, with a tendency for reductions in forest floor mass and associated carbon but often little to no impact on larger mineral soil carbon pools.
Although “biomass harvesting,” which involves the removal of residues traditionally left on site, tends to reduce soil carbon, responses vary widely as much of the carbon in decomposing residues is released as carbon dioxide to the atmosphere rather than adding to the soil pool. In addition, field surveys demonstrate that substantial quantities of residues typically remain on site following operational biomass harvests due to technical or economic limitations to gathering those materials. Residues removal are also typically used for bioenergy, thereby reducing carbon emissions when they substitute for fossil fuels.
Management, disturbance, and carbon loss
Forest management operations such as thinning reduce fuel loads and most often create a healthier forest condition more resilient to wildfire and pest and disease outbreaks that lead to substantial carbon losses. Active management has been shown to reduce both the frequency and severity of wildfires, which is the most significant disturbance agent and accounts for 62 percent of non-stocked forest area in the U.S. More extensive road systems in managed forest landscapes are a complicating factor; while roads can act as fire barriers and increase access to fire control vehicles, greater public access has been linked to more human-caused fires.
Economic incentives can limit forest conversion and associated carbon losses
Deforestation resulting from urbanization and other development is a major cause of forest loss in the U.S. In the U.S. South, urbanization has been projected to cause losses of up to 9.3 million hectares of forest land between 1997 and 2060. Unlike forest harvesting conducted as part of an ongoing management regime, deforestation and its impacts on forest carbon are most often permanent.
Markets for wood products and forest-derived bioenergy provide incentives for landowners to realize a return on their investment by keeping their forest land as forest. Globally, planted forests have been found to reduce harvesting of natural forests by 26 percent. Landowners and policymakers will continue to weigh environmental and economic tradeoffs with respect to the management, conversion, and preservation of forests.
These findings were described in an article entitled Conclusions and Caveats from studies of Managed Forest Carbon Budgets, recently published in Forest Ecology and Management. The Author of the paper is Eric D. Vance, recently retired from the National Council for Air and Stream Improvement, Inc. (NCASI).