[EXCLUSIVE] Forest Service Studies Soil Impacts of Bioenergy Logging
– by Josh Schlossberg, The Biomass Monitor
A recent study from the U.S. Forest Service’s (USFS) Rocky Mountain Research Station investigates the potential impacts on forest productivity from logging for biomass energy. While the study focuses primarily on the Northern Rockies region—where only a handful of small combined heat and power and biomass heating facilities operate—many of the findings may be applied to western forests.
The study, Impact of Biomass Harvesting on Forest Soil Productivity in the Northern Rocky Mountains, by Woongsoon Jang and Christopher Keyes from the University of Montana, and Deborah Page-Dumroese with the Rocky Mountain Research Station in Moscow, Idaho, assesses one of the main environmental concerns surrounding an expansion of bioenergy, the impact on forest soil productivity.
USFS defines forest productivity as the “integration of all environmental factors encompassing soil productivity, climate, topography, geology, vegetation, and the history of natural disturbances and anthropogenic interventions.” Ultimately, the question is whether logging for bioenergy may impair future forest growth.
Logging for bioenergy involves removing more organic matter from the forest than conventional logging for lumber alone. The practice of whole-tree logging extracts not just merchantable tree trunks for lumber, but also treetops, branches, and other logging byproducts, and has a “substantial impact on live vegetation,” according to study authors.
Though whole tree logging is not typically employed in the western U.S. forests, the authors predict that forests will “likely be managed more intensively in the future,” in part for biomass energy.
Organic matter—woody debris, leaf litter, humus, and duff—plays a “critical role” in forest productivity, and can be found on the forest floor, and in all layers of the soil, including deep mineral soil. Organic matter supports the cycling of soil carbon and sequestration, the availability of Nitrogen, moisture levels, and biodiversity.
Foresters are concerned that whole-tree logging may affect the cycling of nutrients and water, and compact soil.
Trees and understory vegetation make use of mobile nutrients and transform them into stationary nutrients, then release them back into the forest when dropping leaves or branches, or following death. Root systems access other essential nutrients, mostly from organic matter.
Nutrient removals during logging, especially whole-tree logging for biomass, can be “substantial,” according to the study, as tops and branches contain the highest nutrient content in the tree.
“Most western forests are considered to be [Nitrogen] deficient” and logging for bioenergy may exacerbate this situation, the study cautions. Calcium, too, is lacking in the soil pool in forests across the U.S., leaving forests “vulnerable.” Studies have also shown declines in Phosphorus and Potassium after whole tree logging.
In northeastern forests, USFS has found that whole tree logging removed two to eleven times more nutrients than conventional logging. In upland mixed oak forest in Tennessee and coniferous forests in Maine, whole tree logging resulted in three times more nutrient removal than conventional logging.
Study authors cite a 1980 study conducted in the Rocky Mountain region that estimated the amount of nutrients removed from bioenergy logging would “likely be recovered” within seventy to one hundred years. Noting that this recovery is dependent on “nutrient pools” in soil and on the forest floor, the authors recommend that branches, leaves, and needles should be left onsite on soils that are “nutrient limited.”
Canopy cover is an important aspect of forest productivity. The presence or absence of canopy determines the amount of light reaching the forest floor, which can change tree and undergrowth plant species composition and alter microclimates.
Greater sun exposure means higher soil temperatures. Such heating can “significantly” affect soil productivity, as soil temperature affects functions such as root growth, decomposition, and the mineralization of Nitrogen.
Further, a lack of forest canopy decreases transpiration (release of water by trees) and rainfall interception, which can increase soil moisture, speeding decomposition. Unless nutrients are absorbed by vegetation, they may leach into soil and/or groundwater.
The study discusses the role of water in forest productivity, noting that aboveground vegetation can determine soil moisture content. Soil water is “crucial for seedling survival, establishment, and growth,” and necessary for forests that experience summer droughts.
High levels of organic matter can increase porosity of soil, allowing it to hold more water. While removal of organic matter may increase soil temperatures, resulting in early season growth, a lack of soil water content can impede summer growth.
“One of the most adverse impacts” on soil productivity from logging is compaction from machinery. Compaction “increases soil bulk density, hampering air movement and water permeability.”
Compacted soil means less presence of microbes, insects, and small soil-dwelling mammals. It can also hamper fine root development in trees and understory plants.
The study determines that “the negative impact of compaction on forest productivity is substantial.”
“Forest floor displacement” can also be a byproduct of whole-tree logging, which can leave mineral soil “exposed to erosion, rainfall impact, and localized nutrient removals.”
Wildfire and Insects
The study discusses the link between western forests, wildfire, insects, and bioenergy. Some logging practices are meant to mimic wildfire, however authors point out that one major difference is what “remains in the understory and belowground layers.”
When it comes to tree mortality, “biomass harvesting can have a similar impact as stand-replacing wildfires.”
Study authors compare fuel reduction logging to low severity fire in regards to the amount of understory removed. However, they caution that fuel reduction logging, such as repeated thinnings, can result in “insufficient nutrient cycling for healthy and productive forests.”
USFS asserts that biomass logging has “less severe impacts” when compared to some instances of high severity wildfire, since logging can “conserve the understory vegetation, forest floor, and soil layer.”
Authors note that after a forest fire, much of the carbon remains on site in the form of organic matter, while logging typically removes these materials and the carbon. However, the study cites experiments that have found “no loss of soil [Carbon] with increasing biomass removal intensity.”
USFS found that insect or pathogen outbreaks (such as fungus) result in greater stand complexity than logging for biomass.
Further, bioenergy logging “may have a stronger negative impact on productivity than insect or pathogen outbreaks and wind.”
The study notes potential future impacts of climate change on forests. If logging practices harm soil or decrease soil nutrients, climate change may “exacerbate the negative impacts on forest productivity.”
However, increased CO2 levels may result in an increase in forest growth, accompanied by a boost of nutrients.
Climate-induced drought can also decrease forest productivity.
The ultimate question regarding bioenergy logging and soil is whether the “amount of nutrient influx through precipitation and fixation can balance the nutrient requirement for vegetation growth throughout the rotation.” In other words, since bioenergy logging will remove nutrients, will natural processes replenish them in order to restore balance, or will it result in a deficit?
The study concludes that long term impacts of logging for bioenergy will “likely cause few impacts on within-stand nutrient cycling if the forest floor and mineral soil are protected.”
Authors cite the North American Long-Term Soil Productivity study, which didn’t show “any significant impacts of intensive biomass removal on vegetation production after 10 years.”
Matthew Koehler, director of the WildWest Institute in Missoula, is skeptical of the study’s findings. He said there is “plenty of research proving that industrial logging of forests causes long-term, native impacts to soils, especially in parts of the country…where soil productivity is marginal even under ideal circumstances.”
He’s not convinced that “biomass logging, which extracts even more organic matter from forest ecosystems over conventional logging methods,” will have few soil impacts.
“Even the researchers own findings claim that the supposed ‘few impacts’ will only be achieved if—and that’s a big if—the forest floor and mineral soil are protected,” he said.
European experiments cited in the Forest Service study back Koehler’s concerns, finding that “differences in stand production between whole-tree harvesting and conventional harvesting are more commonly detected.”
The Forest Service acknowledges there are “still numerous gaps in our understanding of intensive biomass utilization for bioenergy production, nutrient cycling, stand productivity, and soil quality.”
Woody biomass in the Northern Rockies is in its fledgling state, so concerns about a large expansion may not yet be warranted, according to Martin Twer, BioEnergy Associate Specialist with Montana State University Extension Forestry, University of Montana College of Forestry and Conservation, in Missoula Montana.
The low price of natural gas and oil is a major obstacle to the development of biomass energy in Montana. “Without much of an explicit dollar value on carbon, pollution, loss of productivity, effects on communities, demand-side management,” said Twer, “it can be challenging to make a general and economically viable case for alternative/renewable energy sources to substitute in that market segment.”
Despite the challenges, Twer certainly sees biomass as having a “larger role to play in Montana’s energy portfolio than is currently the case.”