Fire Science

Fire Ecology

Fire plays a critical ecological role in many plant and animal communities around the world. Fire regimes (the combination of fire frequency, behavior, and spatial patterns) vary through space and time depending on the plant communities (aka fuels) and other factors such as weather, climate, topography, and human activities present in an area. Many plants and animals exhibit a variety of adaptations that allow them to survive – and often even thrive – in the presence of fire. For example, ponderosa pine has thick bark and high canopies that protect growing tissues and needles from lower-intensity surface fires. Others are able to come back rapidly after fire, such as aspen that sprout from root stock buried underground and protected from fire. In many ecosystems, plants and animals depend on fire to maintain or promote healthy ecosystems, and without fire are significantly altered into new, less fire-tolerant conditions.

Humans have removed fire from many areas where it historically was common, especially those dominated by ponderosa pine and other fire-dependent species across the western US. This has led to denser forests containing smaller and younger trees, and resulted in uncharacteristically severe stand-replacing fire behavior in many recent wildfires. This has also resulted in a loss of resilience in many of these forests, as they do not recover quickly after the widespread loss of seed-producing trees. Recognition of these changes has led land managers to begin to reintroduce fire back into fire-dependent ecosystems. Carefully managed prescribed fires increase the health of fire-dependent forests by reducing tree densities and creating landscape mosaics, restoring wildlife habitats, and recovering soil properties and understory vegetation diversity. Uncontrolled wildfire remains as a destructive force on the landscape, threatening lives, property, and the natural environment, and fire suppression must still remain a dominant form of fire management in most areas. However, land managers and the public must recognize that the question is not if our western landscapes will ever burn again, but when. Use of prescribed fires and other tools such as managed wildfires (where natural ignitions are allowed to burn under very specific weather conditions in wilderness and other remote areas) must be allowed in order to remove excess fuels, to reduce the risks of fires impacting human communities and infrastructure, to protect fire fighters and the public, and to restore fire to its natural role in our ecosystems.

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Tree Ring Research

A fire-scarred ponderosa pine from the Black Hills in South Dakota. Each of the black arrows marks a date when a low-intensity surface fire killed a portion of the growing tissue (the cambium) of the tree, but did not kill the tree itself. If fire kills the cambium all around the circumference, it will kill the tree by girdling. Ponderosa pine has thick bark (as do many fire-adapted trees) that normally protects the cambium, but occasionally fire is hot enough to burn through the bark. And once scarred, the bark is thinner on the newly growing wood and subsequent fires are able to leave their marks on the tree in future years. By looking at the history of fires, we can then understand the fire regime of these ecosystems much better. (photo by Peter M. Brown)

A Short History of Fire in Ponderosa Pine Ecosystems of the Western US

In the Past Forest: Forests were mostly open and largely consisted of multi-aged, multi-sized trees, with a great deal of diversity in their spatial patterns. A pulse of small trees would come in but most would be killed in the next fire that came along. Low-intensity surface fires occurred on average ever 5 to 20 or 30 years in these forests, burning mainly through grass and herbaceous plants and needle litter at the base of the big trees. Occasionally young seedlings or saplings would live through their first fires and become part of the overstory forest.

In Current Forests: The first impact that occurred in many areas was selective harvest of many of the larger trees. Ponderosa pine makes an excellent timber tree, tall and straight-growing. The second was putting out all the low-intensity surface fires through both livestock grazing (which removed the grass and herbaceous fuels) and, beginning early in the 20th century, active fire suppression. This had the result that now all the regenerating trees were able to grow into denser stands, with ladder fuels. This change in forest structure fuels much more intense and destructive fires when an ignition occurs under extreme weather conditions.

In Future Forests: In many areas, land managers are trying to first restore stand structures by thinning most of the smaller trees and recreating spatial diversity in spatial patterns. However, the biggest need is then to restore fire, which also recreates wildlife habitat, understory diversity, and soil properties. The ultimate goal is the last panel, to restore the open forest, dominated by multi-sized and –aged trees with large ranges of diversity in all ecosystem components and processes. Prescribed fire is an integral tool in this entire process to get us to the future forests of Colorado.