"I read a lot about carbon sequestration in our forests. But in some places, like my hometown of Florida, conservationists also urge landowners to conduct prescribed burns on a regular basis. I know these prescribed burns are supposed to help a number of threatened species (e.g. red-cockaded woodpeckers and scrub jays), but don't the prescribed burns offset a large portion of the carbon storage provided by keeping forests intact?"
Blane Heumann and Mary Huffman:
Actually, prescribed burns like those you mention don't release a lot of carbon relative to what is stored in a forest stand. And the carbon that is released is recaptured in new plant growth fairly quickly.
In fact, a recent study of forests in the western United States1 concluded that controlled burning can be a strategy to help lower overall carbon dioxide emissions. Burning away the "fuel load" that has built up over time under controlled conditions means that the threat of unnaturally severe fires is lessened and overall carbon dioxide releases are reduced.
When weighing the need for controlled burning against the need to maintain or increase the amount of carbon stored in natural areas, one must consider the time frame and also the type of ecosystem.
All forests store carbon in trees, on the forest floor in the form of fallen leaves and dried grass (litter), in undergrowth such as small trees and shrubs, and even in soil. However, different forest types burn differently — a fire ecologist would say they have different fire regimes.
Many ecosystems, including longleaf pine forests that provide habitat for red-cockaded woodpeckers, are adapted to frequent, low-severity fires that burn litter and undergrowth while leaving the big trees alive and intact. In other words, only a small portion of the carbon stored in these systems goes up in smoke during a controlled burn.
Carbon stored both in large trees and in the soil — the majority of the carbon that is sequestered — is not typically released. (Many people don't know that carbon stored in forest soils can remain there for centuries.) In fact, low-intensity fires tend to stimulate regrowth of grasses and other species that quickly accumulate carbon in both leaves and roots after controlled burns.
What happens when fires are excluded from forests that are designed to burn? Consider that Florida longleaf pine stand. Without fire, plants grow freely and carbon accumulates. Some people mistakenly believe that we are being successful in abating climate change by excluding fire.
However, in this fire-adapted landscape, with frequent summer lightning storms, it's not a matter of whether the forest will burn, but rather when and how it will burn. Accumulated undergrowth adds to the potential for severe drought-year fires that can consume undergrowth, oxidize soil carbon, kill large trees and possibly threaten communities. It is these types of fires — those that hit forest habitats really hard — that represent the real culprit behind increasing levels of greenhouse gases in the atmosphere, in combination with the burning of fossil fuels.
Land managers and fire ecologists understand that fires are inevitable in many forests across the United States and so they attempt to influence how and when fire happens so that people and homes are protected and forests continue to provide a variety of benefits to both people and biodiversity.
It can be a delicate balancing act, and the science of carbon cycles and fire is in its infancy. But the bottom line is that, in the short term, maximizing carbon storage in forests can have negative implications for both forest health and longer-term carbon storage.
It's worth noting that fire is also important for maintaining healthy grasslands and many types of hardwood forests. Just as in southeastern and western U.S. pine forests, people managing these ecosystems need to understand and balance ecosystem-specific carbon sequestration issues with a host of other values and management objectives.
Blane Heumann is the Conservancy's director of fire management
Mary Huffman is the director of fire training for the Conservancy's North America region.