Coastal Sage Scrub Fire: History, Impact, and Management
ESPM C22AC Final Project
What is Coastal Sage Scrub?
Coastal sage scrub (CSS) is a highly diverse ecosystem that is characteristically dominated by short, aromatic, mostly drought-deciduous shrubs and interspersed with annual wildflowers ¹ . The coastal sage scrub ecosystem is found along the coast from northern Baja California to just above San Francisco.
The coastal sage scrub plant community is divided into two geographical subtypes: northern coastal scrub and southern coastal scrub. Northern coastal scrub extends from San Francisco into Southern Oregon. Southern coastal scrub extends from San Francisco to northern Baja California.
In large part due to invasion of exotic annuals, altered fire regimes, overgrazing, and urbanization, an estimated 70-90% of coastal sage scrub has been lost - making it one of the most threatened ecosystems in the United States.
History of Fire in Coastal Sage Scrub
The coastal areas of California have infrequent lightning ignited fires and thus a longer natural fire-return interval ² . However, coastal areas also had high density of Native Americans, who altered the natural fire frequency in coastal sage scrub through the use of cultural burning. Native Americans intentionally burned coastal sage scrub to cause type-conversion to exotic annual grassland. Short-lived grasses grow quickly during the wet season but dry out and leave behind combustible litter for most of the year ³ . Higher fine fuel loads can increase the spread and frequency of fire.
Type conversion: the ecological change from one type of plant community to another, either through disturbance or some other environmental change.
CSS before and after a type-conversion to exotic annual grassland
Native Americans caused localized type-conversion for various reasons: to enhance the growth and production of plants for materials and food, to open up dense shrublands for hunting game and facilitated travel, and to reduce the occurrence of hazardous creatures (e.g. rattlesnakes) that are harbored by shrubs. For example, some edible seed resources came from annuals that became briefly abundant after fire. Fire releases nutrients and reduces competition, encouraging robust plant growth. Additionally, fire and pruning encouraged the production of straight stems and flexible shoots, which was useful for basketry and making other items ⁴ . Type-conversion from dense woody shrubland to lower fuel volume grassland also reduced fire hazards near settlements during severe wind conditions. Additionally, type-conversion to grassland increased the annual flow from watersheds, which increased water resources for Native Americans ⁵ .
California Sagebrush
(Artemisia californica)
California Sagebrush is a dominant drought-deciduous shrub in CSS and particularly southern coastal scrub. Native Californians used this plant for medicine, ceremonies, and construction and to disguise their odor when hunting.
Coyote Brush
(Baccharis pilularis)
Coyote Brush is a perennial shrub that colonizes CSS after disturbances such as fire. Chumash Indians used Coyote Brush as a poison oak remedy. Branchlets of this plant were used to brush off small spines when collecting cacti.
Deerweed
(Acmispon glaber)
Deerweed is a keystone nitrogen-fixing shrub that replenishes nitrogen lost after a fire. Heat from fire stimulates seed germination, leading this plant to dominate an area the second year after a burn. The Chumash used the smoke from Deerweed to blacken materials for basketry designs. This plant was also used to thatch sweathouses.
Chia Sage
(Salvia columbariae)
Chia Sage is a low herbaceous plant found in CSS and one of the most widely utilized seed resources for Native Americans. This plant has strongly dormant seeds that are stimulated by smoke from fire.
Black Sage
(Salvia mellifera)
Black Sage is a dominant shrub in CSS and a major source of the ecosystem's aroma. Populations of Black Sage are decreasing due to increased fire frequency and nitrogen input, which favors competing non-native grasses. The Kumeyaay made tea from Black Sage stems and leaves and bathed in it as a medicinal treatment. The Luiseño harvested the plant's high-protein seeds for food.
Orange Bush Monkeyflower
(Diplacus aurantiacus)
Orange Bush Monkeyflower is a woody shrub that occurs in northern CSS. The Kumeyaay used monkeyflower to treat wounds and illnesses. Native Americans also used young stems and leaves for food.
Impact of Fire on Coastal Sage Scrub
Native Americans have used fire as a management tool in coastal sage scrub for thousands of years ⁵ . In recent times, fires in coastal sage scrub have been initiated by humans either intentionally or accidentally. While low intensity fires may burn off surface fuels but allow resprouting, repeated low intensity fires favor the proliferation of exotic annual grasses. The seeds of exotic plants have a high survivorship when fire intensity is low and a short return interval inhibits the regeneration of native species, allowing exotic species to take root and spread in their place.
CA State Highway 78 is located in a coastal sage scrub ecosystem that has been replaced by non-native weeds and annual grasses. This area burned in 2007 and again in 2018.
Exotic species can outcompete native species through multiple different mechanisms. Exotic plants decrease the soil's moisture and compete with native plants for light, water, and nutrients ¹ . Accumulation of exotic grass litter over multiple seasons burns readily and alters fire regimes by increasing fire frequency and decreasing the fire return interval. This initiates a feedback mechanism where fire favors the proliferation of exotic species which increases and maintains the frequency of fire beyond shrub tolerance. The relationship between native species, exotic species, and fire has resulted in the long-term persistence of grassland and a significant loss of coastal sage scrub throughout California.
Fire regime: the general pattern (frequency, area, seasonality, severity, and interactions) of fire in a particular ecosystem over a long period of time.
The historical fire return interval in coastal sage scrub is 40 years, but over the past century, has decreased to an estimated 20 years. Coastal sage scrub burns in stand-replacing fires, typically during the dry summer season. Fires can burn hundreds to thousands of acres in a single event ⁶ .
Fire also affects the composition of animal communities in coastal sage scrub. After a fire, there is often an increase in species that require an open-canopy habitat and a decrease in species that require closed-canopy habitat ¹ . Dense shrubs provide habitat for species such as the wrentit, California mouse, and western toad. Sparse shrubs in postfire, open canopies favor different species, such as the western fence lizard, horned lark, and kangaroo rat.
Wrentit, western toad, and California mouse found in closed-canopy coastal sage scrub
Western fence lizard, horned lark, and kangaroo rat found in open-canopy coastal sage scrub
Management, Conservation, and Restoration
On its own, fire suppression has potential to increase both the plant diversity and shortened fire interval of coastal sage scrub. However, this management strategy can result in greater fire hazards for homeowners around the wildland urban interface. Simulation models suggest that fire suppression should be spatially mixed with prescribed burning at fixed time intervals ⁷ . Frequent prescribed burning can be used in protective strips near the wildland urban interface while less frequent burning and fire suppression can be used in open land (i.e. park areas).
Fire suppression: the act of extinguishing a fire and preventing it from spreading
To conserve the remaining coastal sage scrub ecosystem, Habitat Conservation Plans (HCPs) are developed under the Endangered Species Act. HCPs have been implemented in several southern California counties and are being restored to native habitat ⁸ .
Because coastal sage scrub has been fragmented by urbanization, overgrazing, invasion by exotic annuals, nitrogen deposition, and altered fire regimes, it is one of the more challenging vegetation types to restore. Herbicides have potential to reduce competition from annual grasses and enhance shrub growth in years with high amounts of rainfall. Adding mulch and removing exotic grass litter has also been shown to increase native plant diversity by reducing competition for light and soil nutrients. Methods to control the seed bank of exotic species include: prescribed fire before and after dispersal, solarization, herbicides and litter removal, and mechanical treatments (i.e. mowing).
