Harvesting Southern Pines for Bioenergy: Potential Impacts on Soil

11.25. loblolly pine stand herbicides by JimMiller Bugwood
Loblolly pine stand treated with herbicide to control other vegetation. Photo by Jim Miller, courtesy of Bugwood.org.

Soils are the foundation of the forested ecosystem, producing timber and clean water while supporting biodiversity and storing carbon. A new study led by U.S. Forest Service Southern Research Station (SRS) scientist D. Andrew Scott examines how harvesting for bioenergy affects soil ecosystem services in loblolly pine plantations.

Many southern pine stands are being harvested more frequently and more intensively to produce bioenergy. However, the potential impacts on soil are unclear, and there are few management guidelines to ensure that soils are able to continue providing ecosystem services.

Scott, a research soil scientist with SRS Southern Pine Ecology and Management unit, and his colleagues measured stand volume, soil carbon storage, and woody plant biodiversity on 13 sites across Louisiana, Mississippi, North Carolina, and Texas.

The sites are part of an international Long-Term Soil Productivity Study, a network of experiments that evaluate the effect of soil characteristics on plant growth. The paper was recently published in the Soil Science Society of America Journal.

Fallen dead trees and branches, stumps, and trees usually too small to be harvested for timber can all be useful for producing bioenergy. To determine whether removing large amounts of organic material affected soil productivity, the scientists used a range of harvest intensities in the study sites – from harvesting only the stems of mature trees to removing all organic matter above the mineral layer.

“In some sites that were intensively harvested and already nutrient-poor, there were temporary reductions in growth,” says Scott. “But overall, it seems harvesting had limited and site-specific effects on timber volume production.” Scott and his colleagues also measured soil carbon storage and woody plant diversity, and found them relatively unaffected by biomass harvesting.

Harvesting also packs soil particles more closely together, causing compaction. “When we began the study, we suspected that soil compaction would limit tree growth,” says Scott. “We were surprised to find that stand volume was highest in the sites with more soil compaction.”

The researchers also studied the effects of herbicide use. Many landowners and managers use herbicides to keep grasses, shrubs, and unwanted trees from competing with pine, and although controlling unwelcome plants isn’t necessarily a part of harvesting, it could affect forest soil ecosystem services of wildlife habitat and stand volume.

The scientists found that herbicide vegetation control had the biggest impacts on pine growth. On some sites, there were major increases in tree growth. For example, in the North Carolina sites, there was a 46 percent increase in the average stand volume on sites where competing vegetation was controlled.

Forest soils provide a host of ecosystem services, and although intensive harvesting may cause short-term decreases in timber production, there were only minor changes to soil ecosystem services after harvests.

“Biomass harvesting appears to have had very minor effects on these stands,” says Scott. “Our study suggests that soils in southern pine stands will continue providing ecosystem services even where biomass harvesting occurs.”

Read the full text of the article.

For more information, email Andy Scott at andyscott@fs.fed.us.

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