Forest Soil Carbon and Climate Change

Issues

David D'Amore, Pacific Northwest Research Station
Evan Kane, Michigan Technological University

Any consideration of forest carbon storage must include soils.  In temperate forest ecosystems, the amount of carbon stored in soils is often greater than the amount stored aboveground in living and dead plant biomass.  Although the relative amounts of organic carbon in plant and soil components vary by climate and region belowground carbon is a substantial carbon pool.  The total amount of carbon stored in aboveground forest biomass (living and dead) varies far less across diverse forest types, with an average aboveground stock for US forests being 55 Mg carbon ha-1 (1). In contrast, belowground carbon stocks show more variation, even when we limit our consideration to the top meter of soil.

Figure: The total stock of soil organic carbon varies for forest types in the U.S., with high stocks occurring where a forest type is common, or, in areas where there is a lot of soil carbon per unit area.  It may be a challenge to increase soil carbon storage in extensive forest types that have a relatively high carbon density per unit area, especially if carbon is protected by cold temperatures or saturated conditions  (for example, red bars depicted above).  On the other hand, extensive forest types presently occupied by forest type groups with low carbon density represent opportunities for increasing soil carbon through management activities (e.g., green bars).  Data from Johnson and Kern, 2003 (2).

This variability in belowground carbon is an important consideration for evaluating forest management options that maintain and promote soil organic carbon, rather than allowing it to be lost to the atmosphere.  Here, we focus discussion of changes in soil carbon in terms of inputs and outputs.  The primary management controls we have available include such input factors as stand type, productivity, and rotation age, and such output terms as controls on decomposition and carbon export; these latter factors are affected by management activities related to changes in stand structure and microclimate, wildfire and changes in site drainage.

Understanding Forest Soil Carbon

The amount of carbon stored in soils depends on a variety of factors, including carbon inputs rom vegetation, carbon losses from decomposition and biodegredation, soil physiochemical characteristics, and climate variables like temperature and precipitation.  Forest contributions to the soil organic carbon pool include leaf litter, coarse woody debris, roots, root exudates, and dissolved organics leached from the litter layer. Whether these inputs are stabilized in the mineral soil matrix (sequestered) or biodegraded and returned to the atmosphere as carbon dioxide depend on complex, fine-scale interactions involving soil minerals, plants and soil organisms, and organic components (where most of the nutrients are stored), and these interactions are all influenced by larger-scale factors like climate and stand management.

Perhaps counter-intuitively, large aboveground carbon stocks do not necessarily result in large belowground carbon stocks. This is because high levels of forest productivity require pools of nutrients that become available through the decomposition of organic matter, and decomposition also releases carbon to the atmosphere.  On the other hand, conditions which impede decomposition are often related to the accumulation of soil carbon, such as a lack of oxygen (high water table) and cold temperatures.  While aboveground plant productivity is largely governed by nutrient availability and drainage conditions, belowground carbon stocks are heavily influenced by soil mineralogical and chemical characteristics. Soils with higher clay content are generally associated with larger soil carbon stocks because they offer a greater amount of surface area for carbon to “stick” to. In addition, the stickiness of the mineral matrix is related to the type of clay that forms from the parent material and soil conditions. For example, the carbon-rich forest soils (Andisols) of the Northwest are formed from volcanic parent material which yields soils with very high surface area. These mixed conifer-hardwood systems have some of the largest soil carbon stocks in the country. In contrast, the mixed hardwood soils of the Northeast and northern Midwest are formed on sandy substrates which have low surface area and consequently smaller soil carbon stocks which are very sensitive to disturbance.

Conditions that limit biodegradation also increase carbon stored in soils. Inundation, freezing temperatures, and highly acidic soils are all associated with accumulation of organic matter, either in the litter layer or the mineral soil through association with clay minerals like iron and aluminum. In general, increasing forest productivity means increasing inputs to soil carbon stocks, however increasing plant inputs can also stimulate biodegradation of soil carbon in some systems. Therefore, management decisions must consider both the vegetation and soil characteristics. 

Forest harvest and wildfire can accelerate rates of carbon loss from soils. Harvest removes the amount of vegetation that can be added to soils while wildfire removes organic carbon from both vegetation and soil. Both disturbances can accelerate erosion or leaching losses from soils and lead to a loss of carbon from the terrestrial realm and a transfer to “downstream” ecosystems. Harvest practices that do not appropriately consider equipment suitability for a given site (e.g., slope, soil texture, or seasons of operability) can cause significant losses of soil organic matter (3,4).  Moreover, harvesting can have indirect effects on carbon storage by changing microclimate conditions, such as changes in soil temperature and moisture.  At the same time, it is worth mentioning that analyses of harvesting and fire on soil carbon stores show that the effects of these events are typically small when considered in the broader context of landscape-scale sources in variation, such as with changes in soil carbon across different vegetative communities, soil types, and physiographic provinces (5,6). 

Likely Changes 

There are a few ways in which soil carbon may change in response to climate change. Changes may occur through changes in the relationship between biomass production and decomposition, or alternatively, in response to shifts in vegetation communities that are driven by climate change.

Both vegetation growth and decomposition processes will respond to changes in temperature. Decomposition, which releases mineral nutrients through weathering and produces carbon dioxide as organic matter breaks down, will rise along with temperature increases as long as factors like oxygen, moisture, and vegetation inputs are not limiting.  Since both primary production and decomposition may increase with temperature, changes in soil carbon will likely depend on how individual forests respond to factors other than temperature in different soil types.  For example, changes in precipitation patterns are predicted to create exaggerated periods of both drought and extreme precipitation (7), leading to either increased erosion of stabilized mineral soil carbon, or decreased decomposition linked to low oxygen availability in wet soils. 

Table:  Forest management considerations for soil organic carbon, by soil order (broad taxonomic unit of soil).  Data from Eswaran et al., 2000 (8). 

Climate changes are likely to favor different forest communities across the landscape (9), which has consequences for the quantity and intrinsic properties of carbon inputs to the soil.  Although it is not possible to predict the future species composition of forests, current observational studies can lend some insight into the changing nature of stand carbon inputs with climate.  Shifts in forest types, growth rates, and soil decomposition have been observed across varying landscape positions and within climate gradients, and some studies have observed specific climate conditions which optimize biomass and soil carbon sequestration within a forest type (10,11).

Options for Management

Forest management for increased carbon storage is not a new concept:  there is a well-developed science on optimizing tree biomass and approximately half of biomass is carbon.  However, managing forests to optimize biomass accumulation and carbon sequestration is difficult because there is still a great deal of uncertainty about how to increase soil organic carbon storage in managed stands.

In practice, there are two aspects of managing forests for soil carbon storage, including reducing carbon losses and increasing inputs from net primary production.  The main source of uncertainty regarding management options for soil carbon stem from the difficulty in quantifying the large variability in soil carbon stocks in a spatially extensive way.  Moreover, there are few studies where soil carbon stocks have been measured over time, both prior to harvesting and repeatedly after several rotations (13). Studies to date do suggest that managed forests are effective at maintaining carbon stocks and show little long-term effects on soil carbon stocks with harvesting (5,14,15).  In fact, some studies have suggested that site preparation can effectively mix surface litter into the mineral soil and marginally increase soil carbon near the surface (16); however, this is not the normal condition after harvesting in most forest types (5) and integrating mixing of surface organics with mineral soil could also enhance decomposition.  It is important to note that existing studies focus mainly on soils which have lower carbon storage potential (Spodosols, Alfisols, Inceptisols, and Ultisols).

Forest management affects many soil processes. For example, compaction from harvesting equipment can reduce total pore space, which may be beneficial for plant production in poorly-structured soils (17).  Soils with reactive mineralogy, such as with clay or argillic horizons, may be more able to stabilize carbon inputs where management increases primary production, but these soils would also be more susceptible to increased compaction during harvest. Carbon losses can be reduced by avoiding harvest practices that lead to erosion or exposure of the forest floor. It is likely that the forest types and their associated soils which have high carbon densities as a result of climate protection mechanisms (e.g., cold temperature or water saturation) are much more likely to lose carbon if disturbed, which may occur as a result of predicted changes in climate.  By considering mechanisms of soil carbon stabilization, distribution and density of soil organic carbon, and increased harvest rotations in management, forest managers can help to manage soil carbon.

Mechanism of Soil Carbon Stabilization

The vulnerability of soil carbon to changes in climate or a change in management activities is dependent on the mechanisms responsible for maintaining a positive carbon balance in a forest soil.  Climate factors that decrease soil decomposition, such as cool temperatures or saturated conditions, are nearly impossible to control in managed forests.  As such, managing for increased soil carbon in forests where carbon is currently protected by cool temperatures or frozen soil may be a losing battle; cool moist forests are at a greater risk of losing soil carbon in a warmer world with more variable precipitation or under conditions where harvesting is intensified.  Increased growth rates will increase the fresh litter pool, but litter is likely to also be turned over faster as conditions improve for growth. There may be a net gain in the aboveground stock of carbon in live biomass, but it may come at the expense of the soil. On the other hand, carbon in more reactive soils such as Ultisols or Andisols is stabilized by physico-chemical mechanisms as opposed to cool temperatures. These soils may be more responsive to management activities which increase carbon inputs to the soil, such as fertilization or afforestation. We note here that there is huge variability within soil Order, so a more specific inquiry as to a soil’s “reactivity” would be available at the Family level of classification, which includes a soil’s particle size category and a descriptor of mineralogy (these details are available through the USDA Soil Survey [direct link]. 

Distribution and Density of Soil Carbon by Forest Type

Soil carbon stocks may be more vulnerable to loss from harvesting or disturbance in forest types where more carbon is held in the surface organic matter (“O” horizons, or “duff”) when compared to forests where carbon is harbored deeper in the soil profile or stabilized with mineral complexes.  Forest types with higher levels of soil carbon near the surface also generally have higher carbon densities and usually accumulate carbon because of cool and/or wet conditions, which would be likely to change when disturbed. Even small changes in climate or soil carbon stabilization in forests with high carbon densities can result in relatively large declines in carbon storage.  For example, Alaskan forests and eastern aspen-birch forests have large soil carbon stores because they have high carbon densities in surface organic horizons, which are vulnerable to increased mineralization through disturbances such as altered drainage or wildfire.  On the other hand, oak-hickory forests contain large stocks of  soil carbon where the distribution of soil carbon is not concentrated in the surface horizons, which probably makes it less vulnerable to climate change. Because oak-hickory forests are widely distributed, this forest type may represent an opportunity for increased carbon storage through management activities, which promote plant growth.

Conclusions