Turning Trees into Bioenergy: What are the Effects on Soil?

A crowded loblolly pine plantation in Arkansas is thinned, providing small branches and other woody biomass that can be used to generate electricity. Photo by Andy Scott, U.S. Forest Service.
A crowded loblolly pine plantation in Arkansas is thinned, providing small branches and other woody biomass that can be used to generate electricity. Photo by Andy Scott, U.S. Forest Service.

Timber has been harvested for hundreds of years, but current bioenergy operations use more parts of a tree than ever before; small branches that used to be considered non-merchantable are now often harvested instead of left rotting on the forest floor.

“The increased use of small branches and formerly non-merchantable wood has been linked to changes in energy cycles, short and long-term hydrology, and changes in a number of soil properties,” says Andy Scott, a research soil scientist at the U.S. Forest Service Southern Research Station.

Scott and his co-author Deborah Page-Dumroese, a soil scientist at the Forest Service Rocky Mountain Research Station, recently wrote an overview of the issues around bioenergy and site productivity. Their study was published in the journal BioEnergy Research as part of a special issue on the USDA Regional Biomass Research Centers.

The first coordinated, nationwide research on soil changes after timber harvesting dates back to 1990, when a site on the Palustris Experimental Forest near Alexandria, Louisiana became part of the Long-Term Soil Productivity (LTSP) experiment. Started in 1989, the LTSP has grown into a network of more than 100 field sites in the U.S. and Canada. Although the LTSP was not designed specifically to test the impacts of harvesting biomass, it evaluates soil organic matter and compaction – two issues that are intrinsically linked to biomass harvesting.

In traditional logging operations, only the tree trunks are harvested, and huge piles of branches and other organic matter are left behind to rot and return their nutrients to the forest floor. However, on some of the LTSP sites that Scott and his colleagues reviewed, all aboveground organic matter and nutrients had been removed.

“Most plant nutrients are in the leaves and branches,” says Scott. “Harvesting a whole tree, rather than just the trunk, removes three times as many nutrients from a site.” Losing the nutrients could affect soil’s fertility, as well as its ability to store carbon – and ultimately its ability to produce new trees.

However, in a testament to the resilience of forests, removing all aboveground organic matter and nutrients from LTSP sites did not cause widespread losses in tree productivity. After 10 to 15 years, only the LTSP sites that were extremely infertile to begin with were affected. “However, 15 years is still quite young for managed forests,” says Scott. “Nutrient deficiencies could still occur as the trees mature.”

Harvesting for biomass also increases the amount of traffic on a site. Trucks, tractors, excavators, and other logging machines lumber through a stand, disturbing soil as they go. With each pass, their tires pack soil particles together,  sometimes tightly enough to limit the movement of air, water, and nutrients.

Compaction can reduce microbial activity and root growth, although trees are typically less affected by compaction than shrubs, flowers, grasses, and other vegetation competing to survive. In the LTSP sites, soil compaction had mixed effects on productivity. In heavy clay soils it hindered tree growth, while in sandy soils it actually increased productivity.

Scott and his colleagues also considered the use of products like biochar. Biochar looks like charcoal, and is created through a similar process, but it has a long tradition of use as a soil additive. Indigenous people in the Amazon used it to improve soil for millennia, and scientists have found it benefits soil in gardens and farms. It also stores carbon for long periods of time in a highly stable form. Biochar has not been thoroughly studied in forests, but it could potentially improve soil health and productivity. It could even be produced as a by-product of producing energy with the small branches and other woody residues.

“Biomass harvesting is no panacea,” says Scott. “However, in forests with an overabundance of small-diameter trees, it can help reduce wildfire severity and increase forest health. And with sensible precautions, biomass harvesting will not diminish the soil’s ability to support plant growth.”

Read the full text of the article.

Read more about the role of the Palustris and three other U.S. experimental forests in the LTSP network.

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

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