guest commentary

Montana Fires and Soil: Thinking About the Next Generation

By Tom DeLuca, Guest Writer, 8-18-08

	Pan of Little Bull timber sale, taken in 2002 during a public field tour of the Bitterroot National Forest's Burned Area Recovery plan. Photo by Matthew Koehler of the WildWest Institute.

News coverage of wildland fires—whether the 20th anniversary of the fires in Yellowstone National Park or coverage of recent fires in Montana—is back on the front pages. While many of the anniversary stories have focused on the regeneration of lodgepole pine, aspens, and wildlife habitat, few reports have focused on the role of these fires in the long-term condition of the soils cradling the trees and habitat.

Every farmer and gardener understands the importance of keeping soils productive. But in the case of forest soils, few consider what is necessary to keep these soils healthy and productive. Interestingly, dead and decaying trees in a forest environment play a crucial role in the health of Montana’s forest soils. And while no gardener would ever imagine burying trees in the soil, forest soils are quite different than garden soils and deserve special consideration.

Given that lodgepole pine forests, such as those in Yellowstone, are commonly subject to stand replacing fires every 100 to 300 years, one might wonder if there is a role for dead, charred trees in the soil environment.  And people often ask whether there is any reason to leave the dead and decaying wood in the forest.

The answer to this question is a resounding “yes.” The reason lies partly in the importance of organic matter to soil productivity and development. In short, the single most important factor determining soil productivity is organic matter. Organic residues in soil represent a source of nutrients, water holding capacity, improved soil structure, aeration and infiltration, and a source of energy for soil microbes.

Forest soils function on a bit of a boom and bust cycle. Trees grow for tens to hundreds of years slowly accumulating woody biomass while the trees and shrubs contribute a relatively small quantity of carbon to the soil annually. Then every 100 to 300 years the soil receives a major influx of carbon in the form of trees killed by insects, windthrow, or fire. This addition of woody biomass is a significant part of what keeps the soil working. In a nutshell, the death and decay of today’s trees fuels the growth of future forests.

In contrast to forest soils, farm soils receive a more constant supply of carbon from fine root turnover and annual decomposition of plants. Most of this carbon is converted to carbon dioxide during decomposition, but some is stored as the rich, black substance known as humus. This slow accumulation of humus creates the soft, dark prairie soils of the Great Plains so famous for their productivity.

Downed trees and associated woody roots in forests decompose slowly providing soil microorganisms with the energy required to form soil and to liberate nutrients for future forest communities.  Large decaying stems create organic rich patches that hold water, enhance nitrogen fixation, and create microhabitats. Most of the decaying wood is converted to carbon dioxide, but again, a portion remains in the soil as humus.  The humus improves the soils physical condition and creates a ‘slow release’ source of energy for soil microbes. The charcoal associated with charred stems of downed and fire killed trees also contributes to the long-term health of the soil.  Charcoal, which has extreme longevity in soil (lasts for thousands of years), provides long-term carbon storage and it functions much like humus improving aeration, water and nutrient holding capacity.

Is there an adverse impact associated with the removal of dead and scorched trees after a fire or insect infestation? The answer is yes. Removal of dead trees reflects a loss of one rotation of coarse woody material, the loss of nutrients held in the dead trees, and, in the case of fire, the loss of charcoal which reflects long-term carbon storage. Further, timber harvest often exposes mineral soil and compacts surface soils which inhibits infiltration and seedling growth and enhances soil erosion potential.

Does that mean we should not harvest trees in order to protect soil? The answer is no. Timber is an important resource which we should strive to source bioregionally and one that must be managed sustainably.  To achieve sustainable timber management requires that we understand how soils function under natural conditions so that management systems can be designed that emulate nature - much as a good farmer or gardener would do.

Timber management that minimizes impacts to ecological diversity, incorporates fire (where appropriate), minimizes soil disturbance, and provides a healthy influx of large logs to soils on near natural cycles will be required to prevent long-term soil degradation and sustain forest productivity.

Thomas H. DeLuca, Ph.D. is a senior forest ecologist with The Wilderness Society and formally a professor of forest soils at the University of Montana where he remains an adjunct professor. Contact him at tom_deluca@tws.org.

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