The rate of forest fire spread is typically dictated not by the total quantity of fuels, but by the quantity of highly flammable small fuels found in the foliage and branches of smaller trees and shrubs. As individual trees grow and progress through the mature stage of development, they acquire adaptive properties to survive fires far better than younger trees and do not typically contribute significantly to the rate of fire spread, due in part to their higher moisture content. As such, older, larger trees tend not to play a significant role in determining fire behavior and should not be the focus of efforts to reduce wildfire hazards.
Tree resistance to wildfire increases over many decades. Key adaptations include increasing bark thickness, shedding lower branches, increasing height, and developing more open crowns. Together, adaptations like these make it difficult for fire to ignite tree boles or climb into flammable canopies, in larger, older trees, particularly in fire-adapted forest types. When ignited, fires tend to spread more slowly though the patchy and heterogeneous fuels of mature forests compared to the continuous and homogeneous fuels typically found in young tree farms. Among western trees, fire resistance is generally more developed in mature pines, certain cedars, Douglas-fir and western larch. Additionally, mature giant Sequoia are one of the most fire-resistant trees on the planet, capable of surviving flame lengths that climb into their crowns. Even white, grand, and other true fir species – considered fire intolerants – often survive fires if they are mature and have developed thick bark and higher crowns. Some fir species targeted in federal fuel reduction efforts are in fact fire resistant. Moreover, some conifers may appear dead after intense fire, only to flush new needles and new shoots the following spring.
At the stand level, mature and old-growth forests often function as fire-resilient refugia. Stands dominated by older trees tend to have increased shade and humidity, cooler temperatures, and reduced wind speeds that retain moisture and enhance stand-level fire resistance. Due to higher levels of moisture associated with older forests, they require much more fire energy and exposure to ignite and typically, there is proportionately less biomass available to burn. This trend has been observed repeatedly in U.S. forests, especially in the Pacific Northwest.
As a result of these and other adaptations, older trees and the stands dominated by them generally have a relatively small effect on overall fire behavior across landscapes. Older trees can contribute to burn duration, burning for longer periods if they ignite. However, mature trees rarely contribute significantly to the rate of fire spread. Mature trees, as a group, are more resistant to fire than their younger counterparts. And weather—not older trees—often plays a determinative role in fire intensity (which is a measure of heat energy produced by a fire and refers to the effect of fire on vegetation, soils, etc.), particularly during extreme events. Even large logs from downed mature trees—which are in deficit in many federal forests—often do not play the major controlling role in fire intensity.
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