The Role of Fire in California's Oak Woodlands
Fire is a natural part of Californiaís oak woodland ecosystem. It has also served as an important management tool since Native Americans first inhabited these areas. Fire plays a role in the development of oak woodland stand structure, oak regeneration processes, the development of habitat for wildlife, nutrient cycling, and economic uses of oak woodlands for domestic livestock. The ecological effects of fire will vary depending on the frequency of fires, the size of patches that occur from fire-induced mortality of mature trees. Adjacent vegetation types, such as chaparral and montane forests, influence fire effects in oak woodlands.
At this time, there is relatively little information on oak woodlands and fire. This article provides a brief discussion of the state of our knowledge and suggests how fire might be incorporated into future management activities in oak woodlands. The recent increase in the acreage of stand destroying fires in oak woodlands resulting from decades of attempting to exclude fire from our wildland areas, points to our need to develop strategies in which fire is included in management activities in order to sustain the economic and ecological values of our oak woodlands.
Because of the long period of human habitation of oak woodlands, it is extremely difficult to separate the ìnaturalî role of fire from the human use of fire as a management tool. Fires caused by lightning have played a role in shaping oak woodlands, Lightning-caused fires originate from major storms coming northward from Mexico. It is speculated that decades may pass between major lightning-caused fire events in oak woodlands (Griffin, 1977). Oak woodlands are extremely well adapted to hot summer fires (Mooney, 1977). Mature oak s can survive regular low intensity ground fires, and most woodland oak species have the capacity for young seedlings and saplings to resprout after being top-killed by fire.
Native Americans made frequent use of fire in their stewardship of oak woodlands. There are numerous accounts of burning by native Americans in woodlands to enhance habitat for game species, to improve access for hunting and gathering acorns, and to maintain plant materials in an appropriate growth form for crafts (Jepson, 1910; Cooper, 1922). However, it is almost impossible to document the frequency, intensity, and extent of burning by Native Americans from existing fire ecology studies.
The first European settlers in oak woodlands continued with the burning practices utilized by Native Americans. Prescribed burning in oak woodlands and chaparral lands was a common management strategy to keep stands open for livestock production and to encourage forage production. Surveys indicated that oak woodland burning on an interval of 8 to 15 years between ignitions was the common management strategy used by ranchers (Sampson, 1944). Local prescribed burning associations were set up in various locations around the state, where neighbors came together annually to help conduct burns in the highest priority areas.
The use of burning as a management tool to mimic the effects of nature ceased on the stateís conifer forest lands in the early part of the century. However, ranchers continued the extensive use of prescribed burning until the 1950ís. At that time, the use of fire in oak woodlands declined, driven by negative urban attitudes towards fire promoted by ìSmokey the Bearî campaigns, increasing housing density in rural areas of the state, concerns about liability from escaped prescribed fires, and air quality concerns. Fire suppression became the standard management strategy on oak woodlands, as it had become decades earlier on conifer lands.
One of the few detailed studies of this trend in changing fire frequency was carried out by McClaren and Bartolome (1989) in Central Sierra oak woodlands. Figure 1 shows the fire history in one sample area at the University of Californiaís Sierra Foothill Research and extension Center. This shows that for this one area, fire frequency in these foothill oak forests was around 25 years prior to settlement by Europeans in the mid-1800ís. After settlement by Europeans, the use of fire as a management tool can be observed, with a fire frequency of about every 7 years.
Effects of Fire on Oak Woodland Sustainability
Studies have shown that higher fire frequencies in the past may have created conditions more conducive for oak regeneration. McClaren and Bartolome (1989) compared oak stand age structure with fire history, and showed that oak recruitment was associated with fire events. Most oak recruitment in their Central Sierra study area occurred during periods of high fire frequency in the 1880's to 1940's. Oak recruitment has been rare since fire suppression.
The factors leading to enhanced oak regeneration from higher fire frequencies are not entirely clear. Allen-Diaz and Bartolome (1992) looked at the blue oak seedling establishment and mortality with the treatments of grazing and prescribed burning. Neither of these treatments significantly affected oak seedling density nor the probability of mortality when compared to unburned and ungrazed areas. This suggests that seedling establishment is compatible with grazing and fire, but does not explain the positive role of fire in the recruitment of seedlings into saplings. Lawson (1992) studied the effect of prescribed fire on coast live oak and Engelmann oak in southern California and found higher seedling mortality in areas of prescribed fire.
Perhaps the importance of fire on oak regeneration is explained by the enhanced postfire oak sprout growth documented by both Bartolome and McClaren (1989) and Lawson (1992). Bartolome and McClaren concluded that in areas of moderate grazing with fire intervals of around 7 years, seedlings taking up to 18 to 20 years to exceed the livestock browse line (around 5 feet) would survive to become saplings and persist in the stand. In heavily grazed areas, only those trees that exceeded the browse line in 10 to 13 years would be recruited. Other factors affecting oak regeneration which would be influenced by the timing of fire events include: the seedbed for acorns; the competition for moisture from herbaceous species; and the habitat for wildlife species that feed on acorns and seedlings.
Fire also has a major affect on the structure and composition of oak woodland stands. Lawson (1993) showed the differential effects of fire on coast live oak and Engelmann oak. Coast live oak had a higher mortality than Engelmann oak following fire. Although coast live oak had higher height growth in unburned areas, Engelmann oak had a higher height growth following fire. The thicker bark of Engelmann oak provided more protection for the Engelmann oak sprouts. The study concluded that concerns about the decline of Engelmann oak habitats in Southern California might be mitigated by reintroduction of fire to encourage Engelmann oak in these mixed stands. These kinds of relationships need to be developed for all the major woodland types in the state so that future management strategies can be designed. Huntsinger and Bartolome (1992) present a state-transition model which provides a good framework for evaluating the effect of fire on oak woodland composition.
Developing Prescribed Burning Plans
Catastrophic oak woodland losses in the recent Southern California wildfires and throughout the state, have shown the devastating effect of attempting to remove fire from the oak woodland ecosystem. The probability of large-scale losses to fire due to fuel accumulations within woodlands, as well as in adjacent chaparral and conifer forests, increases each year. Stagnant oak stands with little sapling recruitment, is also at least partly a result of the unnatural removal of fire. Landowners and managers concerned with sustainability of oak woodland economic and ecological values need to find ways in which fire can be reintroduced into the system. Shown below are a series of steps that one should take in the design of a prescribed burning program.
- Set management objectives: An important preliminary step in any management activity is to determine the objectives for an area. It may be to enhance the economic value of a ranching enterprise, improve wildlife habitat, maintain water quality, reduce the probability of catastrophic fire losses, or a combination of all of these. All the stakeholders in a project area need to decide upon these objectives.
- Develop partnerships: Areas in the state where prescribed burning activities are being successfully implemented are those where partnerships in the community have been maintained and developed. Groups of landowners, adjacent public land managers, and representatives of resource management agencies should all be brought together. The Coordinated Resource Management Planning (CRMP) process is a useful mechanism to bring people together. Experience and expertise can be shared, and resources can be pooled to reduce costs. Assistance programs such as the Vegetation Management Program, the Forest Stewardship Program, and cost-share programs administered by the Agricultural Stabilization and Conservation Service, should be evaluated and utilized whenever feasible.
- Develop an assessment of current vegetation and fuel conditions: A survey of the project area should be carried out to prioritize areas for treatment. Existing vegetation cover types should be mapped out, and the fuel loads of each assessed. Important ecological zones such as riparian areas and locations of sensitive species, as well as areas prone to erosion should be identified. Houses, roads, and other improvements should also be located. Geographic Information Systems (GIS) and remote sensing technology, coupled with ground surveys, can help in this task.
- Design the burn program: Given this background information, a plan for annual burning activities can be designed. This should consider the size of the burn unit, the time of year for burning, weather conditions necessary to obtain the appropriate fire intensity, and the location of firebreaks and equipment.
- Conduct the burn: Once all of the preceding steps have been accomplished, the actual burn can be carried out. Make sure all participants and equipment are lined up well in advance, and all background work has been completed. Build enough contingency into planning so that the project can respond to weather variability that will certainly affect when the burn takes place.
- Monitor the results: Information on the effects of different prescribed burning strategies on oak woodland systems is lacking. A system of evaluating the impacts of the burn is needed, and whether the objectives for the area are met, needs to be put into place. This provides feedback for future years and helps to refine future management decisions.
Allen-Diaz, B.H. and J.W. Bartolome. 1992. Survival of Quercus douglasii (Fagaceae) seedlings under the influence of fire and grazing. Madrono 39(1):47-53.
Cooper, W.S. 122. The broad-scherophyll vegetation of California. Carnegie Inst. Wash. Pub. 319. 124 pp.
Griffin, J.R. 1977. Oak woodland. pp. 382-415 in M.G. Barbour and J. Major (eds.), Terrestrial vegetation of California. John Wiley, NY.
Huntsinger, L. and J.W. Bartolome. 1992. Ecological dynamics of Quercus dominated woodlands in California and southern Spain: a state-transition model. Vegetatio 99-100:29-305.
Jepson, W.L. 1910. The silva of California. Univ. of Calif. Mem., Vol. 2. 480 pp.
Lawson, D.M. 1993. The effects of fire on stand structure of mixed Quercus agrifolia and Q. engelmannii woodlands. unpublished MS Thesis, San Diego State University. 122 pp.
McClaren, M.P. and J.W. Bartolome. 1989. Fire-related recruitment in stagnant Quercus douglasii populations. Canadian Journal of Forest Research 19: 580-585.
Mooney, H.A. (ed.). 1977. Convergent evolution in Chile and California Mediterranean climate ecosystems. Dowden, Hutchinson and Ross, Inc., Stroudsberg, Pennsylvania.
Sampson, A.W. 1944. Plant succession on burned chaparral lands in northern California. California Agr. Exp. Stn. Bull. No. 685.
prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc