Myth #3: Large chaparral wildfires are unusual and preventable.
Large chaparral fires have occurred prior to 2003 and will continue to occur. Southern California has one of the worst 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 County (the 2003 Cedar fire burned 273,246 acres). During the same time period another fire in San Diego County burned more than 60,000 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.
"Santa Ana. Sept. 25. - The fire which has been burning for the past two days still continues in the canyons. The burned and burning district now extends over one hundred miles from north to south, and is 10 to 18 miles in width. Over $100,000 worth of pasturage and timber has been destroyed." Los Angeles Times, September 27, 1889.
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 (wooden fences, wood piles, ornamental vegetation, and native vegetation) directly around structures.
Myth #4: Chaparral is adapted to fire.
It is best to think of each type of chaparral as adapted to a particular fire regime 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 all the important variables involved such as fire frequency, season of burn, intensity (level of heat), or severity (amount of living material consumed). Too much fire (increased frequency) will destroy a chaparral system. Fire during the cool, moist season can seriously damage the seed bank. See our Fire & Nature, Threats and Fire & Science pages for more details.
Myth #5: Chaparral plant species are "oozing combustible resins."*
This type of comment is frequently seen in articles about fires in chaparral, giving the impression that the reason chaparral shrubs are so flammable is primarily due to the presence of flammable chemicals, waxes, or oils in their leaves and stems. While it is true some chaparral plant species (and many other drought-tolerant plants including pines) have chemicals within their tissues than can be flammable, 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 fuels 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 too much moisture in the plants. If "oozing combustible resins"* were the main cause of flammability, chaparral would burn easily during any season.
See the paper below for additional details.
See Alessio, G.A., et. al 2008. Influence of water and terpenes on flammability in some dominant Mediterranean species. International Journal of Wildland Fire 17: 274-286.
*The hyperbolic "oozing" quote came from the July 2008 issue of National Geographic.
Myth #6: Hot chaparral fires "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 and the residence time for any heat on the surface is relatively short. In fact, hot fires actually serve an important purpose in shrubland ecosystems by destroying the seeds of invasive species. One sign of a healthy, chaparral ecosystem that is recovering from a fire are large areas of blackened ground (punctuated with resprouting shrubs and tiny shrub seedlings) remaining long after the first rainy season. Hot fires are a natural part of the chaparral. Contrary to popular opinion, we don't need to rush in and cover the ground with mulch or seed. A healthy ecosystem will recover quite well without our help.
For more information see:
Keeley, J.E., T. Brennan, and A.H. Pfaff. 2008. Fire severity and ecosystem responses following crown fires in California shrublands. Ecological Applications 18: 1530-1546.
A related issue to fire severity impacts has to do with "hydrophobic soils." Supposedly fire makes the surface of soils in chaparral water repellant because of all the "waxy" chemicals that burned off the plants' leaves (see Myth #5 above). It is then concluded that unless we "do something", rains will roll off the burn slopes like they were covered in wax paper, causing massive floods. Well, it has never been adequately demonstrated that this indeed occurs or that hydrophobic soil conditions are particularly important in post-fire chaparral environments. In fact, whatever "hydrophobic" condition occurs after a fire usually disappears after the first light rain.
Myth #7: Chemical inhibition in the chaparral (allelopathy).
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 phenomenon in science texts as well as along chaparral nature trails. 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 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. 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." You may purchase a copy of the actual reprint on our Membership Page. If you are interested in obtaining an electronic copy, please request one by writing us an EMAIL.
|
