USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
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Research Topics Fire Science

Carbon management in fire-prone forests

Sun rays break through the trees as fire technicians walk amongst areas of small patches of fire.
Stanislaus National Forest fire technicians tend to a prescribed on the Stanislaus-Tuolumne Experimental Forest. Prescribed fire often occurs after thinning treatments are performed. Such fire is part of the suite of treatments conducted in research to examine forest restoration techniques. U.S. Forest Service photo by Eric Knapp.

Forests store large amounts of carbon. As they grow, forests can become carbon sinks that offset anthropogenic (human-caused) emissions of carbon dioxide, a climate-warming greenhouse gas. Wildfires release carbon back to the atmosphere, and the amount of release increases with fire severity. Management treatments to reduce forest fuels can reduce fire severity and consequent carbon release in the event of a wildfire, but these treatments also reduce forest carbon stores in the near term. Researchers have considered the following questions:

  • Do young, fast-growing trees that are harvested for wood products provide greater long-term carbon storage than growing and retaining large, old trees?
  • What are the carbon costs and benefits of fuels reduction in fire-prone forests?

In California, recent state policy and political attention has been focused on the potential to mitigate the effects of climate change through forest management. The most ready means of increasing forest carbon stores is through establishing or reestablishing forests. Trees, particularly large, mature trees, can store large amounts of carbon for decades to centuries. Although developing countries often reduce their carbon stores as forestland is converted to other uses, forests in the United States have been a net carbon sink in the last century due to forest regrowth (particularly in the upper Midwest and New England) and, in some cases, fire suppression (see discussion below). For much of the U.S., where forestland cover is now relatively stable, research has focused on whether different management practices could stabilize or increase the amount of forest carbon storage (sequestration).

Carbon storage in old and young forests

In the past, some researchers have suggested that converting old forests to young, fast-growing plantations, whose harvested wood products could store carbon for several decades, would create a net increase in long-term carbon stocks. This approach was based on the idea that old forests are slow growing and thus  carbon sequestration slows down as  forests age. More recent research generally does not support this idea, as a global survey of old forests found that many continue to sequester carbon and have stocks that far exceed young, managed forests.

In addition, some research finds that large trees may contain even more carbon than currently estimated. This is because a tree’s carbon storage is estimated from its diameter and, unlike younger trees upon which most equations for estimating carbon storage are based, old trees may allocate most of their growth to the upper bole (trunk). If young forest trees could be efficiently harvested over several rotations (i.e. harvest cycles) and their carbon sequestered in wood products for centuries, they might match carbon stores in old forests dominated by large trees. However, this would be difficult with current wood use practices that discard much of the waste and the products have relatively short life cycles.

Carbon loss in forest operations and harvesting

Generally, the amount of carbon dioxide emitted from fossil fuel used in forest operations (i.e., diesel and gasoline) is quite small (often < 5 percent) compared with the carbon captured in the harvested forest biomass. Since the immediate carbon expense from machinery use is small, the greater problem is that the carbon in harvested forest biomass is not stored for long and often ends up, through decomposition, back in the atmosphere.

A recent global analysis of the longevity of harvested forest carbon found that after 30 years, in most countries (90 of 169), less than 5 percent of the carbon still remained in longer storage, such as wood products and landfills. Most temperate forest countries with longer-lived products, such as wood panels and lumber, had higher carbon storage rates, with Europe, Canada, and the U.S. averaging 36 percent of the forest carbon still stored after 30 years. This higher rate, however, is still far short of what large, long-lived trees would continue to accumulate and store over several decades to centuries.

Carbon dynamics and long-term balance in fire-prone forests

Recent research has proposed the idea of carbon carrying capacity. This concept may be particularly relevant to forest managers because it emphasizes carbon stability and the level of carbon storage that forests can maintain. In the absence of disturbance, a forest may accumulate more carbon as the density and size of trees increase. This additional biomass, however, makes the forest prone to disturbances, such as drought stress, pests, pathogens, and higher severity wildfire, which increase tree mortality. This mortality reduces carbon stocks as dead trees decompose and return  carbon to the atmosphere. Carbon carrying capacity, therefore, is lower than the maximum carbon storage potential of a forest, but represents the biomass that can be maintained given disturbance and mortality agents in the ecosystem. In frequent-fire forests such as Sierra Nevada mixed-conifer, the carbon carrying capacity is the amount that a forest can store and still be resilient (i.e., have low levels of mortality) to fire, drought, and bark beetle disturbances.

Management activities that alter the amount and longevity of sequestered forest carbon could lead to increased carbon storage and change the long-term carbon balance. Two studies that examined historical forest conditions in Sierra Nevada mixed-conifer forests suggest that this might be possible. Although historical forests were less dense due to frequent fire, they may have stored more carbon due to a greater number and size of large trees compared to today’s forests, which have fewer large trees, possibly due to increased mortality rates from increased stand density. Since carbon stores are calculated from total tree biomass (a three dimensional measure), they are much higher in a stand with a few large trees compared with a stand with many small trees, even if both stands have similar basal area (a two dimensional measure).

Other studies, however, have found higher carbon storage in modern fire-suppressed forests than in historical active-fire forests, suggesting that there may be considerable variability between different locations and levels of productivity. In general, forests managed so that growth and carbon accumulation are concentrated in large trees will also have longer, more secure carbon storage than stands where growth is concentrated in a high density of small trees prone to pest, pathogen, and fire mortality.