Biological Sciences, Santa Barbara City College

Biology 100: Concepts of Biology


Photo, D Stockton
"Controlled" burn at Cachuma Lake

Fire in the Chaparral

Wildfires in California can occur at any time but they are more prevalent during the dry season between the end of summer and the start of the winter rains. All catastrophic fires occur during the extreme Santa Ana wind conditions. 

Chaparral fire intensity depends on the mixture of plants in the chaparral and age of the individual plants. The ratio of dead to live fuel is much greater in old than in young chaparral and varies from species to species. However, the tendency to ignite does not generally increase with age.  Fire plays a key ecological role in the development and perpetuation of chaparral communities.


Chaparral shrubs are very flammable due to the resinous foliage, woody stems, accumulated litter, and standing dead branches. Flammability of chaparral species increases over time through deposition of flammable leaf litter impregnated with volatile oil (oils in the leaves help make the plant drought resistant).

Chaparral shrubs and herbaceous perennials have four different survival strategies to respond to a fire.  These strategies are:
1.  Obligate resprouters.  These plants survive fires by resprouting only e.g. Toyon and Scrub Oak.
2.  Obligate seeders.  The adult plants die in the fire.  Seeds receive a fire cue and then germinate e.g. Ceanothus.
3.  Fire followers.  These annual plants require some fire cue for germination..
4.  Facultative seeders:  The adult plant resprounts and a fire cue enhances seed germination.
The soil insulates against heat and, in a fire, protects the thick, woody, underground basal burls of Chamise (Adenostoma fasciculatum), and Manzanita (Arctostaphylos). These burls contain many dormant buds that sprout after a fire so the shrubs regenerate quickly. Sprouting species have a competitive advantage over non-sprouting species that only become reestablished from seeds.

Poison oak (Toxicodendron diversilobum) survives a fire by having an extensive underground root system.

Some plants survive fire by producing seeds with tough seed coats to insulate the embryos against killing temperatures. Other species require fire-associated temperatures to crack their seed coats allowing water penetration and subsequent germination of the seed. The fire has not caused germination, but has weakened the seed coat to allow hydration of the embryo. In all strategies of shrub establishment from seed, adequate soil moisture is necessary.

Fire followers
Some herbaceous species only germinate following a fire. They are called "fire annuals" or "fire followers". Their seeds are retained in the soil and only germinate following a fire. Fire poppy, Papaver californicum, is an example. Fire poppies (pictured on right) germinate profusely immediately following a fire. They may remain on the site for several years, but disappear as the chaparral species return. The seeds remain viable and will germinate when a fire next comes through the area.

 


poppies after the Painted Cave fire, S. Williams, photo
Fire-dependent annuals and perennials provide the vegetative cover that helps to reduce the heavy erosion possible on steep mountain slopes after the protective cover has been burnt off.

Reseeding after a fire

After a fire, shade is removed that would otherwise hinder seedling success. Fire also removes or reduces the allelopathic germination inhibitors found in the both decaying organic material and the soil. Germination can also be induced by chemicals from combustion products.  Smoke is an important chemical stimulant and smoke induced germination has recently been reported for Emmenanthe penduliflora, a California chaparral annual.  Click here for more information about smoke induced germiaation.

There are two categories of mechanisms by which fire produced chemicals stimulate germination:  changes in the seed coat and external layers which make the seed water permeable (heat shock germination) and chemicals that induce enzymes or growth regulators.  Rhamnaceae (Buckthorn family, including Ceanothus, Coffeeberry, Redberry) utilize heat-shock germination. 

Fagaceae (oaks) produce acorns.  These are dependant on moisture and must germinate the year they fall to the ground. 

 


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