Many ecosystems around the world experience wildland fire as part of a natural cycle. Fire wholly or partially consumes live and dead vegetation such as grasses, shrubs, trees, downed woody material, litter, and duff. Depending on the weather, fuel dryness, and the vegetation type and structure, fire can vary from low to high severity. High-severity fires cause high levels of vegetation mortality, while low-severity fires cause low levels of vegetation mortality. Rapidly or over a period of years, vegetation regrows after the fire. Many times, the vegetation is of the same species, but given the pressures of climate change (e.g. hotter and drier conditions in many places), the same vegetation may fail to regrow. A species may be replaced by a similar but different species. Forested areas may shift to rangeland areas. Also, fire severity has increased following a century of timber harvest and fire suppression in the US, which has caused denser vegetation assemblages in many places that are composed of smaller trees (smaller trees are often less resistant to fire) and vegetation that’s closer to the ground and can pass fire from the surface up into the crowns of trees (this vegetation is called “ladder fuels”). Many areas exist in a state of fire deficit, meaning they have not had enough fire to remain resilient.
For millennia, Native Americans used fire to manage ecosystems. This practice was discouraged and sometimes criminalized by Euro-American settler colonialists. Removal of Native American burning from many ecosystems has contributed to fuel buildup and shifts in forest structure.
There are also places on the landscape where vegetation is close to the way it existed historically, prior to fire suppression, due to recent fires or recent fires with severity within the range of historic variability. For example, in much of the Southeast, prescribed fire is frequently used to maintain ecosystems such as the longleaf pine.
Given these widespread changes in forest structure in many parts of the US, it can be desirable to take management actions to change fire occurrence or fire severity. We can regard the fire cycle as having three stages: pre-fire, during the fire, and post-fire. Management actions to reduce fire occurrence or fire severity take place largely during the pre-fire phases. Actions can be chosen based on the forest type and structure (for example, a dense forest with low-hanging branches and undergrowth might indicate mechanical thinning followed by prescribed fire). In an area that has been recently burned at low severity, for example, no actions might be needed.
