Myth #1: Chaparral needs fire to "renew" and clean out "decadent" or "over-grown" vegetation.
Myth #2: Past fire suppression has built up "unnatural" levels of "fuel," i.e. habitat.
Myth #3: Large chaparral wildfires are unusual and preventable.
Myth #4: Chaparral is adapted to fire and "needs" to burn frequently.
Myth #5: Chaparral plant species are "oozing combustible resins."
Myth #6: Hot chaparral fires "sterilize" the soil.
Myth #7: Chemicals suppress seed germination under the chaparral canopy (allelopathy).
FACT #1: Old-growth chaparral is a beautiful, healthy ecosystem. It does NOT need fire to "renew" or clean out "built-up" or "over-grown" vegetation.
As an ecosystem, old-growth chaparral does not "need" fire to remain healthy. The terms "over-grown, "decadent," and "senescent" are value judgments that may be applicable to our managed backyards and urban parks, but not wild plant communities. Old-growth chaparral continues to be a productive ecosystem, growing fresh, new growth in its upper canopy every year. In fact, some chaparral plants require the leaf litter and shade provided by older chaparral stands for their seeds to successfully germinate. Instead of becoming "trashy" or unproductive, as some claim, old-growth chaparral (in excess of 60 years) is actually just beginning a new cycle of life. To find out more about old-growth chaparral is, please go to our Old-Growth page.
FACT #2: Past fire suppression efforts have protected the chaparral. It has NOT led to an "unnatural" accumulation of vegetation.
The California Statewide Fire History Database clearly shows acreage burned per decade over the past 100 years in Southern California has remained relatively constant, despite fire suppression efforts by firefighters (see graph below). Unlike some forests, fire has NOT been excluded from the chaparral. In fact, fire frequency in Southern California has been increasing dramatically over the past century (and the increase is heading north). If it had not been for fire suppression, much of the chaparral we see today, especially at lower elevations, would have been converted to weedlands.
Each bar represents the area burned per decade (1900 to 1990) with a 10-year running annual average line during the 20th century for nine California counties. Top row: Monterey, San Luis Obispo, Santa Barbara. Middle row: Ventura, Los Angeles, San Bernardino. Bottom row: Riverside, Orange, San Diego. Horizontal axis in years, vertical axis area burned in hectares. One hectare equals 2.47 acres. Adapted from Keeley and Fotheringham (2003) as found in Fire, Chaparral, and Survival in Southern California (Halsey 2005).
FACT #3: Large, infrequent chaparral wildfires are natural and inevitable in California.
Large, high-intensity chaparral fires have occurred prior to recent times and will continue to occur. Southern California has one of the most fire-prone climates on earth. For example, more than 300,000 acres burned in the Santiago Canyon Fire during the last week of September, 1889 in Orange and San Diego Counties (the 2003 Cedar fire burned 273,246 acres). As with modern fire storms, there were numerous other wildfires across Southern California that week. However, the fires didn't inflict much damage on the human community because few people lived in the backcountry back then. Now, with so many homes up against the wilderness, fires can become catastrophic. The newspaper quote below from 1889 offers a perspective similar to how the 2003 and 2007 fires were viewed:
"During the past three or four days destructive fires have been raging in San Bernardino, Orange, and San Diego...It is a year of disaster, wide-spread destruction of life and property - and well, a year of horrors."
- The Daily Courier, San Bernardino
The best ways to prevent loss of life and property are to retrofit existing structures to make them more fire safe, plan communities so they are not built in high fire risk areas, and maintain proper fuel management (which includes wooden fences, wood piles, ornamental vegetation, and native vegetation) directly around structures. Please see our Protecting Your Home page for more information.
FACT #4: Chaparral has adapted to particular fire patterns, NOT to fire per se; today, fire occurs too often in chaparral.
It is best to think of each type of chaparral as adapted to a particular fire regime (pattern) rather than just "fire." This is an important distinction because when people say chaparral is adapted to or needs fire there is no reference to the important variables involved including fire frequency, season of burn, intensity (level of heat), or severity (amount of living material consumed). Too much fire (increased frequency) will eliminate a chaparral ecosystem. Fire during the cool, moist season can seriously damage the seed bank due to steam produced in the soil during the fire. For more information, please visit our check out the extensive fire ecology research on our Too Much Fire page.
FACT #5: Chaparral plant species have adapted to survive drought and disturbances like fire; they are not "born to burn."
Misconceptions are frequently seen in articles about fire in chaparral, giving the impression that chaparral shrubs will explode in flame with the slightest spark. One reason given is the the presence of chemicals, waxes, or oils in their leaves and stems. While it is true that some chaparral plant species (and many other drought-tolerant plants including pines) have aromatic chemicals within their tissues, this does not mean that's why they ignite or burn so hot when they do. Chaparral plants ignite and burn hot when the environmental conditions are right, namely high temperature, low humidity, and low fuel moistures (the amount of moisture in the plant). In addition, the leaves and stems on many chaparral shrubs are quite small, creating perfect burning conditions - a lot of surface area and space for oxygen. Chaparral shrubs burn so easily because they provide fine, dry material during drought conditions. While plant chemicals are certainly involved in the burning process, they do not really make a significant contribution to the flammability of chaparral in general. This also explains why it is usually quite difficult to get chaparral to burn in the spring. There's just too much moisture in the plants. If "oozing combustible resins"* were the main cause of flammability, chaparral would burn easily during any season.
*The dramatic "oozing" quote came from the July 2008 issue of National Geographic. Being "born to burn" was a comment made by a member of the San Diego Board of Supervisors when justifying the need to conduct large scale prescribed burn programs in the backcountry.
See the paper below for additional details on fire and chaparral shrubs:
FACT #6: Infrequent, hot, intense wildfires are a normal part of chaparral ecosystems. Such fires do NOT "sterilize" the soil.
Can fire "sterilize" the soil and if it can, does it really matter? A hot fire can certainly kill biological life on the soil surface. But unless there are burning logs that maintain continuous amounts of heat over long periods of time, the amount of "soil sterilization" is relatively minor. Since trees and logs are not part of the chaparral ecosystem, chaparral fires are generally quite fast - the residence time for any heat on the surface is relatively short. In fact, hot (high-intensity) fires actually serve an important purpose in the chaparral by destroying the seeds of invasive species. One sign of a healthy, chaparral ecosystem that is emerging after fire are large areas of blackened ground (punctuated with resprouting shrubs and tiny shrub seedlings) remaining long after the first rainy season. Hot, high-intensity, infrequent fires are a natural part of the chaparral ecosystem. Contrary to popular opinion, we don't need to rush in and "do something," post fire such as covering the ground with mulch or seed (or replanting). A healthy ecosystem will do quite well post fire without our help.
A related issue to fire severity impacts has to do with "hydrophobic soils." It is often suggested that fire creates some kind of water repellant layer under the soil like a sheet of wax paper because of the "waxy" chemicals that burned off the plants' leaves (see #5 above). It is then concluded that unless we "do something", rains will cause massive erosion and debris flows as the top soil surface washes away. However, hydrophobic soil conditions are not particularly important in post-fire chaparral environments. In fact, whatever "hydrophobic" condition occurs post fire usually disappears after the first light rain. See page 193-194 in High Severity Fire in the Chaparral.
FACT #7: Seeds of most chaparral plant species are naturally dormant. Chemicals from chaparral shrubs do NOT suppress seed germination or growth of plants beneath the chaparral canopy.
Chemical inhibition, or allelopathy, suggests plants are capable of suppressing the growth or germination of neighboring competitors. Although an intriguing idea, actual chemical inhibition in Nature has been notoriously difficult to prove. “To my knowledge,” wrote plant ecologist J. H. Connell in 1990, “no published field study has demonstrated direct interference by allelopathy in soil…while excluding the possibility of other indirect interactions with resources, natural enemies, or other competitors.”
This lack of scientific verification, however, has not prevented the concept from being presented as a well-understood and accepted phenomenon in science texts. Dramatic explanations are seductive, especially if they provide interesting answers to intriguing problems. If repeated often enough, they become dogma and influence thinking for decades.
C.H. Muller, an accomplished botanist from the University of California, Santa Barbara, suggested allelopathy explained the lack of plant growth under the canopy of mature chaparral stands in Southern California (Muller, et.al. 1968). According to his hypothesis, chemicals washed off the leaves of chamise and manzanita shrubs, suppressing the germination of seeds in the ground below. When the chaparral burned, flames denatured the toxic substances releasing the seeds from inhibition. This resulted in the remarkable number of shrub seedlings and wildflowers emerging in post-fire environments. The problem with this explanation is that the soil chemicals suspected of suppressing growth actually increase after a fire. In addition, the dormancy found in chaparral plant seeds is innate, not caused by some outside, environmental factor. The seeds are dormant before they hit the ground. Chaparral seed dormancy evolved because poor growing conditions under mature shrubs selected for seed traits postponing germination until those conditions improved. Under xeric (dry) conditions, germinating under a shady canopy with hungry herbivores scurrying around is not a recipe for success. Fire quickly removes those problems and sets the stage for chaparral renewal. The post-fire seedling response in chaparral can be easily explained without invoking the notion of chemical inhibition.
Muller also suggested allelopathy was the cause for bare zones often found around purple sage (Salvia leucophylla) and California sagebrush (Artemisia californica) (Muller, et.al. 1964) in the coastal sage scrub community. Later investigations revealed these bare zones are primarily the work of herbivores (Bartholomew 1971), not volatile substances from the plants themselves. To little furry rodents like the California mouse (Peromyscus californicus) and the pacific kangaroo rat (Dipodomys agilis) the world is a dangerous place. Cover is critical to their survival since they are on the dietary preference list of local carnivores like coyotes, snakes, and hawks. Consequently, they have a tendency to remain under shrubbery with only occasional, quick forays into surrounding grassland to nibble on available seeds or new growth. They will stray only as far as they can quickly leap back to safety. Bare zones, therefore, can be viewed as “calculated-risk terrain” where rodents have a fair chance of grabbing food without getting caught. Bare zones are bare because herbivores exploit the space to grab available snacks.
Do volatile compounds in certain coastal sage scrub plants ever play a role significant enough to make a difference in naturally occurring vegetation patterns? “As far as I know, the question of why grasses grow within bare zones during wet years, despite animal activity, has never been adequately addressed,” Bob Muller said when reflecting upon his father’s work. “Why don’t animals always eliminate seedlings, regardless of the level of moisture?
The explanation favored by C.H. Muller provides a reasonable hypothesis for this phenomenon; heavy rains leach toxins from the soil, removing inhibitory chemicals and permitting seedling success. However, without further investigation the question remains unresolved.
“The critical issue,” John Harper (1975), a prominent plant population biologist from England explained, “is to determine whether such toxicity plays a role in the interactions between plants in the field. Demonstrating this has proved extraordinarily difficult – it is logically impossible to prove that it doesn’t happen and perhaps nearly impossible to prove absolutely that it does.”
-From "Fire, Chaparral, and Survival in Southern California." See the Book Excerpts page for more details.
The full paper dealing with allelopathy has been published in the Journal of the Torrey Botanical Society 131(4), 2004, pp. 343-367, "In search of allelopathy: an eco-historical view of the investigation of chemical inhibition in California coastal sage scrub and chamise chaparral." If you are interested in obtaining an electronic copy, please request one by writing us an EMAIL.