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.
Rothermel, R.C. “How to predict the spread and intensity of forest and range fires.” U.S. Forest Service. Gen. Tech. Rep. INT-143. Intermountain Forest and Range Experiment Station, Ogden, UT (1983). https://doi.org/10.2737/INT-GTR-143; Anderson, H.E. “Aids to determining fuel models for estimating fire behavior.” U.S. Forest Service. Gen. Tech. Rep. GTR-INT-122. Intermountain Forest and Range Experiment Station, Ogden, UT (1982). https://doi.org/10.2737/INT-GTR-122; Agee, J.K. and C.N. Skinner. “Basic principles of forest fuel reduction treatments.” Forest Ecology and Management (2005) 211(1-2): 83–96. https://doi.org/10.1016/j.foreco.2005.01.034; Robert E. Keane. “Wildland Fuel Fundamentals and Applications.” Springer (2015). https://doi.org/10.1007/978-3-319-09015-3.
Kitzberger, T. et al. “Decreases in fire spread probability with forest age promotes alternative community states, reduced resilience to climate variability and large fire regime shifts.” Ecosystems (2012) 15: 97–112. https://doi.org/10.1007/s10021-011-9494-y; Agee, J.K. “Fire Ecology of Pacific Northwest Forests.” Island Press (1993) 121-124.
Agee, J.K. “Fire Ecology of Pacific Northwest Forests.” Island Press (1993) 121-124; Brown, P.M. et al. “Identifying old trees to inform ecological restoration in montane forests of the central Rocky Mountains, USA.” Tree Ring Research (2019) 75(1): 34–48. https://doi.org/10.3959/1536-1098-75.1.34.
Thompson, J.R. and T.A. Spies. ”Vegetation and weather explain variation in crown damage within a large mixed-severity wildfire.“ Forest Ecology and Management (2009) 258(7): 1684-1694. https://doi.org/10.1016/j.foreco.2009.07.031; Odion, D.C. et al. “Patterns of fire severity and forest conditions in the western Klamath Mountains, California.“ Conservation Biology (2004) 18(4): 927-936. https://doi.org/10.1111/j.1523-1739.2004.00493.x.
Stevens, J.T. “Fire resistance trait data for 29 western North American conifer species.” U.S. Geological Survey data release (2020). https://doi.org/10.5066/P97F5P7L.
Habeck, R.J. “Sequoiadendron giganteum.” In: “Fire Effects Information System.” USDA Forest Service Rocky Mountain Research Station, Fire Sciences Laboratory (1992). https://www.fs.usda.gov/database/feis/plants/tree/seqgig/all.html.
Zouhar, K. “Abies concolor.” In: “Fire Effects Information System.” USDA Forest Service Rocky Mountain Research Station, Fire Sciences Laboratory (2001). https://www.fs.usda.gov/database/feis/plants/tree/abicon/all.html.
Moris, J.V. et al. “Using a trait-based approach to asses fire resistance in forest landscapes of the Inland Northwest, USA.” Landscape Ecology (2022) 37, 2149–2164. https://doi.org/10.1007/s10980-022-01478-w.
Hanson, C.T. and M.P. North. “Post-fire survival and flushing in three Sierra Nevada conifers with high initial crown scorch.” International Journal of Wildland Fire (2009) 18(7): 857–864. https://doi.org/10.1071/wf08129.
Countryman, C.M. “Old-growth conversion also converts the fire climate.” USDA Forest Service Fire Control Notes (1956) 17(4): 15–19. https://www.fs.usda.gov/sites/default/files/fire-management-today/FSPubs-FMT-79%283%29.pdf (last accessed June 30, 2023); Kitzberger, T. et al. “Decreases in fire spread probability with forest age promotes alternative community states, reduced resilience to climate variability and large fire regime shifts.” Ecosystems (2012) 15: 97–112. https://doi.org/10.1007/s10021-011-9494-y; Frey, S.J.K. et al. “Spatial models reveal the microclimatic buffering capacity of old-growth forests.” Science Advances (2016) 2(4): e1501392. https://doi.org/10.1126/sciadv/1501392; Agee, J.K. “Fire Ecology of Pacific Northwest Forests.” Island Press (1993) 121-124.
Agee, J.K. and C.N. Skinner. “Basic principles of forest fuel reduction treatments.” Forest Ecology and Management (2005) 211: 83–96. https://doi.org/10.1016/j.foreco.2005.01.034.
Lesmeister, D.B. et al. ”Older forests used by northern spotted owls functioned as fire refugia during large wildfires, 1987-2017.“ Research Square (2021) https://doi.org/10.21203/rs.3.rs-280175/v1; Zald, H.S.J. and C.J. Dunn. ”Severe fire weather and intensive forest management increase fire severity in a multi-ownership landscape.“ Ecological Applications (2018) 28(4): 1068-1080. https://doi.org/10.1002/eap.1710; Bradley, C.M. et al. ”Does increased forest protection correspond to higher fire severity in frequent-fire forests of the western United States?“ Ecosphere (2016) 7(10): e01492. https://doi.org/10.1002/ecs2.1492.
Rothermel, R.C. “How to predict the spread and intensity of forest and range fires.” U.S. Forest Service. Gen. Tech. Rep. INT-143. Intermountain Forest and Range Experiment Station, Ogden, UT (1983). https://doi.org/10.2737/INT-GTR-143; Anderson, H.E. “Aids to determining fuel models for estimating fire behavior.” U.S. Forest Service. Gen. Tech. Rep. GTR-INT-122. Intermountain Forest and Range Experiment Station, Ogden, UT (1982). https://doi.org/10.2737/INT-GTR-122; Agee, J.K. and C.N. Skinner. “Basic principles of forest fuel reduction treatments.” Forest Ecology and Management (2005) 211(1-2): 83–96. https://doi.org/10.1016/j.foreco.2005.01.034; Robert E. Keane. “Wildland Fuel Fundamentals and Applications.” Springer (2015). https://doi.org/10.1007/978-3-319-09015-3.
Reilly, M.J. et al. “Cascadia Burning: The historic, but not historically unprecedented, 2020 wildfires in the Pacific Northwest, USA.” Ecosphere (2022) 13(6): e4070. https://doi.org/10.1002/ecs2.4070; Dillon, G.K. et al. “Both topography and climate affected forest and woodland burn severity in two regions of the western US, 1984 to 2006.” Ecosphere (2011) 2(12): art130. https://doi.org/10.1890/ES11-00271.1; Abatzoglou, J.T. et al. “Human-related ignitions concurrent with high winds promote large wildfires across the USA.” International Journal of Wildland Fire (2018) 27(6): 377-386. https://doi.org/10.1071/WF17149.
Evers, C. et al. “Extreme winds alter influence of fuels and topography on megafire burn severity in seasonal temperate rainforests under record fuel aridity.” Fire (2022) 5(2): 41. https://doi.org/10.3390/fire5020041; Lesmeister, D.B. et al. “Older forests used by northern spotted owls functioned as fire refugia during large wildfires, 1987-2017.” Research Square (2021) preprint https://doi.org/10.21203/rs.3.rs-280175/v1; Zald, H.S.J. and C.J. Dunn. “Severe fire weather and intensive forest management increase fire severity in a multi-ownership landscape.” Ecological Applications (2018) 28(4): 1068-1080. https://doi.org/10.1002/eap.1710; Uzoh, F.C.C., and C. N. Skinner. “Effects of Creating Two Forest Structures and Using Prescribed Fire on Coarse Woody Debris in Northeastern California, USA.” Fire Ecology (2009) 5, 1–13. https://doi.org/10.4996/fireecology.0502001.
Comments