Pacific Southwest Research Station

Research Topics Fire Science

Fire in chaparral ecosystems

Smoke billows up from the valley below during the Rim Fire. Inciweb photo.

Chaparral is the shrub-dominated, evergreen vegetation common at middle elevations in much of California. It occupies about 3.4 million hectares (8.5 million acres) from the mountains of southern California through the Coast Ranges, Sierra Nevada foothills, and into the southern Cascades and Klamath Mountains.

More than 100 evergreen shrub species have been reported in chaparral; most of these shrubs have round or elliptical leaves. Only a few species, notably chamise—the most widespread of the chaparral shrubs, have needle-like leaves which can enhance its flammability. When chaparral burns, fire spreads through the shrub canopy so a chaparral fire is appropriately called a crown fire.

Montane chaparral tends to be less flammable than lower-elevation southern California chaparral because of the absence of chamise, its shorter stature, and cooler climatic conditions, but after a long summer dry period all such vegetation can support high intensity fire that often consumes the aboveground plant parts.

Chaparral vegetation is well adapted to fire and regenerates readily after fire, either through sprouting from stem bases (lignotubers) or from soil-stored seed. Although mature chaparral consists mainly of shrubs, herbaceous plants are the dominant vegetation during the first few years after fire. Many of these "fire-followers" are annuals, the seeds of which have lain dormant in the soil since shortly after the last fire. Germination is stimulated by heat or by chemicals in smoke or charred wood.

Fire probably occurred once to three times a century in chaparral environments (known as a fire return interval) or even longer in some places during pre-settlement times. In chaparral environments of southern California, fires now occur more frequently and most are human-caused, because urban areas with plentiful sources of ignition are in close proximity to wildlands.

Repeated fires at short intervals (fewer than 10 years) that kill young plants before they produce seed can reduce populations of "fire-following" shrub species. In addition, non-native grasses often colonize chaparral stands recovering from fire and persist until shrubs fill in and close the canopy; however, if fire occurs during this grass phase, the reduced fire intensity can allow grass seeds to survive and perpetuate a cycle of more frequent fire and reduced shrub cover. Steep slopes where chaparral ecosystems have converted to grasses and other herbaceous plants are more prone to soil slippage and slope failure during high-intensity rainstorms, likely due to decay of deep shrub roots. Re-establishment of chaparral shrubs after grass conversion is difficult and a topic of active research.

High-elevation chaparral

Montane chaparral occurs within a forested matrix in mountain areas of California, often on shallow soils, exposed slopes, or where high-severity fire has occurred. Its presence complicates fire management because under certain conditions the shrubs burn and either damage forest trees or serve as ladder fuels which can change a surface fire to either torching trees or a spreading crown fire in conifers.

Various mechanical treatments such as crushing and mastication are being studied to determine their effectiveness as a fuel treatment while understanding the ecological impacts. The fundamental fire behavior research described above will also help us understand how a fire might spread in these complex fuel beds which are composed of both live shrubs and trees with dead fuels such as conifer litter and woody fuels.

During long fire-free intervals, conifers may replace the shrub community, while repeat high-severity fire will maintain chaparral species. Shrub regeneration occurs by sprouting, often within weeks after the fire has passed, and by fire stimulated seed germination. Sprouting shrubs may be an important source of nutrition for burned area herbivores, even functioning as an attractant for some species such as deer.

Predicting fire behavior in chaparral

Tools used to predict fire behavior in chaparral and other fuel beds made up of primarily live vegetation are based on the Rothermel fire spread model. This model was created using information from dead woody fuels and was adapted to be used in the live fuel beds found in chaparral and other western fuels such as sagebrush and pinyon-juniper as well as the palmetto-gallberry fuel type of the southeastern U.S. While the tools may work reasonably well under extreme conditions, their application to moderate and marginal conditions when a fire may or may not spread is less certain.

Because the Rothermel model did not explicitly model the physics (heat transfer) and chemistry of ignition and spread, it does not perform well in live fuels. Recent research has demonstrated that live fuels are not simply "really wet" dead wood, but have characteristics which are not currently well-understood or modelled that are important to fire spread. The relative importance of fuel, weather, and fire characteristics is complex and changes over both spatial and temporal scales.

Improving our understanding of the physics and chemistry of fire in chaparral and other live fuels is an active area of research by the Forest Service and several university cooperators. Experiments, measurements and modeling occur from the scale of a single leaf to the scale of a mountain range. Improved ability to predict fire behavior is necessary to improve 1) firefighter safety, 2) use of prescribed burning to manage hazardous fuels, and 3) protection of homes in the wildland-urban interface.

Publications and references:

• Albini, F.A.; Anderson, E.B. 1982. Predicting fire behavior in U.S. Mediterranean ecosystems. In: Conrad, C.E.; Oechel, W.C., eds. Proceedings of the symposium on dynamics and management of Mediterranean-type ecosystems. Gen. Tech Rep. PSW-58. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station: 483-489.
• Barro, S.C.; Conard, S.G. 1991. Fire effects on California chaparral systems: an overview. Environment International. 17: 135-149.
• Chen, S.; Fujioka, F.; Benoit, J.W.; Huang, J. 2010. Meso-scale spectral model simulation over San Jacinto Mountain Range. In: Proceedings of the Joint 2010 CWB Weather Analysis and Forecasting and COAA 5th International Ocean-Atmosphere Conference. Taipei, Taiwan: Central Weather Bureau. 4 p.
• Coen, J.L.; Riggan, P.J. 2014. Simulation and thermal imaging of the 2006 Esperanza Wildfire in southern California: application of a coupled weather–wildland fire model. International Journal of Wildland Fire. 23(6): 755–770.
• Cohen, J.D. 1986. Estimating fire behavior with FIRECAST: user’s manual. Gen. Tech. Rep. PSW-90. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 11 p.
• Engel, M.D.; Williams, K.; McDonald, C.J.; Beyers, J.L. 2013. The feasibility of chaparral restoration on type-converted slopes. In: Narog, M.G., tech. coord. Proceedings of the Chaparral Restoration Workshop. Arcadia, CA.
• Keeley, J.E. 1991. Seed germination and life history syndromes in the California chaparral. The Botanical Review. 57: 81-116.
• Keeley, J.E. 2000. Chaparral. In: Barbour, M.G.; Billings, W.D., eds. North American Terrestrial Vegetation. New York, NY: Cambridge University Press: 203–253. Chapter 6.
• Keeley, J.E.; Fotheringham, C.J. 2001. Historic fire regime in southern California shrublands. Conservation Biology. 15: 1561-1567.
• Keeley, J.E.; Keeley, M.B.; Fotheringham, C.J. 2005. Alien plant dynamics following fire in Mediterranean-climate California shrublands. Ecological Applications. 15: 2109-2125.
• Minnich, R.A. 2001. An integrated model of two fire regimes. Conservation Biology. 15: 1549-1553.
• Quinn, R.D. 1990. Habitat preferences and distribution of mammals in California chaparral. RP-PSW-202. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 11 p.
• Rice, R.M.; Foggin, G.T., III. 1971. Effect of high intensity storms on soil slippage in mountainous watersheds in southern California. Water Resources Research. 7: 1485-1496.
• Rothermel, R.C.; Philpot, C.W. 1973. Predicting changes in chaparral flammability. Journal of Forestry. 71(10): 640–643.
• Skinner, C.N.; Taylor, A.H. 2006. Southern Cascades bioregion. In: Sugihara, N.G.; van Wagtendonk, J.W.; Shaffer, K.E.; Fites-Kaufman, J.; Thode, A.E., eds. Fire in California's ecosystems. Berkeley, CA: University of California Press: 195-224.
• Weise, D.R.; Zhou, X.; Sun, L.; Mahalingam, S. 2005. Fire spread in chaparral—'go or no-go?'. International Journal of Wildland Fire. 14(1): 99–106.