Type Conversion - The Impact of Excessive Fire
Chaparral on its way to being type-converted to weedy, non-native grassland. This site is east of Alpine, California, off Interstate 8 in San Diego County. The far left shows a mature chaparral stand last burned during the 1970 Laguna Fire. The middle/into most of the background shows the fragile pyrogenic habitat that formed after the January 3, 2001 Viejas Fire. It's composed of a whole host of species including chamise, sugar bush, deerweed, and a number of other shrub species. In the right corner is a portion of the Viejas Fire area reburned in the 2003 Cedar Fire. The Cedar Fire reburn is filled primarily with non-native grasses. The majority of the resprouting shrubs have been killed and no obligate seeding species, such as manzanita, are present. The interval between the two fires was too short, causing the elimination of a healthy chaparral plant community. The photo was taken in 2004. The area was resurveyed in 2018/19. The Cedar Fire reburn area still had a significant grass component and sparse shrub growth.
A story of loss. Burned too often, chaparral converts to first, a less biodiverse habitat, then a non-native grass/weed invasion. Click to enlarge.
What is Type Conversion?
Type conversion is the ecological change from one type of plant community to another,
either through disturbance or some other environmental change.
When chaparral type converts, it typically changes from a native shrubland to a non-native grassland.
Please see a more detail description of chaparral type conversion here.
A common misconception is that chaparral is a "fire-dependent" plant community that supposedly needs to burn on a regular basis to remain healthy. It's much more complicated than that. Chaparral is not a simple, homogenous ecosystem. Each type of chaparral responds differently to fire depending on the species present, angle and direction of the slope on which it grows, local climatic conditions, and the frequency, intensity, and seasonality of the fire.
The one factor all types of chaparral have in common, however, is that they are all sensitive to fire intervals shorter than 30 years. A fire return interval ten years or less has been shown to guarantee ecological loss. Ten year is the minimal amount time it takes for a burned chaparral stand to mature enough to set enough seed in the soil to create a healthy, pyrogenic habitat after the next fire. As fire frequencies increase due to human-caused ignitions, the intervals between fires have been contracting, causing the complete elimination of chaparral in some areas and serious degradation in others. This is happening in both southern and northern California. As can be seen in the photo above, non-native grasses quickly invade frequently-burned areas, making it extremely difficult for healthy chaparral to grow. Look along canyons, fence rows, and steep hillsides (where the cattle can't go) and you will see the remnants of chaparral and California sage scrub habitat that was likely the dominant plant community in the area prior to human arrival. The older oaks remain, but they are surrounded by a carpet of alien grasses. Much of the summertime golden hills landscape on the central coast of California is actually a disturbed ecosystem dominated by invasive weeds.
Our four-page handout provides details and photos demonstrating Too Many Fires, No Chaparral. A complete discussion of the threats to the chaparral in Issue #28 of The Chaparralian.
A good summary of the literature concerning chaparral type conversion: Park, I.W. and G.D. Jenerette. 2019. Causes and feedbacks to widespread grass invasion into chaparral shrub dominated landscapes. Landscape Ecology 34: 459-471. Also see our Desert Fires page for details concerning type-conversion in desert ecosystems.
A good summary of the literature concerning chaparral type conversion: Park, I.W. and G.D. Jenerette. 2019. Causes and feedbacks to widespread grass invasion into chaparral shrub dominated landscapes. Landscape Ecology 34: 459-471. Also see our Desert Fires page for details concerning type-conversion in desert ecosystems.
The Science is Clear Chaparral is Threatened by High Fire Frequency
The scientific consensus is overwhelming – California native chaparral shrublands, one of the state’s primary sources of biodiversity and carbon sequestration, are being threatened by climate change and increased fire frequency. This consensus was clearly articulated by Michael O’Connell during his testimony to the Budget Subcommittee on October, 20, 2020 (at time stamp 1:54:10). Orange County Fire Chief Brian Fennessy also provided solutions to wildfire problem based on this consensus in his 2019 letter to Governor Newsom.
In addition, California state and federal policies and recommendations recognize the threat of fire to chaparral:
- California’s Climate Change Vulnerability Assessment of the state’s terrestrial vegetation predicts chaparral will likely disappear throughout much of southern California within the next century if current trends continue (Thorne et al. 2016).
- The United States Forest Service established a new leadership intent to protect chaparral in California because human-caused fires have increased fire frequency to the extent that chaparral can no longer survive and is being replaced with non-native annual grasses at an alarming rate (USFS 2011).
- The California Board of Forestry’s Vegetation Treatment Program (2020) states that, “coastal sage scrub and chaparral, are experiencing fires too frequently, resulting in changes to their ecology.”
- The California State Legislature amended the Public Resource Code (PRC 4483) to mandate additional consideration for chaparral and coastal sage scrub plant communities that are being increasingly threatened by fire frequency.
Please see our 2020 letter to the California State Assembly addressing this issue.
The 2019 Cave Fire - type conversion in process
After the 2019 Cave Fire above Santa Barbara, Santa Barbara County fire officials claimed that the fire mostly burned in the 1990 Paint Fire area, where vegetation was "old and dense." This is the standard description of most fires in chaparral.
In truth, 50% of the Cave Fire's footprint overlapped an area that last burned just 10 years ago in the 2009 Jesusita Fire. This observation is important because it demonstrates what the science has shown us for years: wind-driven fires will move quickly through even young vegetation.
Ten years is not enough time for chaparral shrub species to grow to maturity and produce enough seed (the natural fire return interval for chaparral is 30 - 150 years or more). Such a short fire return interval will compromise the re-burned habitat and will likely lead to type conversion to non-native grasses and weeds which pose a greater fire danger.
Fortunately, much of the northern part of the fire area had remained unburned for at least a few decades. The various native plant species in these areas will explode with life over the next several years.
Ten years is not enough time for chaparral shrub species to grow to maturity and produce enough seed (the natural fire return interval for chaparral is 30 - 150 years or more). Such a short fire return interval will compromise the re-burned habitat and will likely lead to type conversion to non-native grasses and weeds which pose a greater fire danger.
Fortunately, much of the northern part of the fire area had remained unburned for at least a few decades. The various native plant species in these areas will explode with life over the next several years.
Half of the 2019 Cave Fire re-burned an area that had been burned ten years before, risking type-conversion of the chaparral habitat there.
A Sample of Type Converted Areas in California
North side of Highway 52 in San Diego County next to between Lakeside and Mission Trails Regional Park. What was once a pristine stand of chamise chaparral is now being type-converted to non-native, weedy grassland. The 2003 Cedar fire probably sealed much of the area's fate.
State Highway 78 between Escondido and Ramona. Nearly the entire California sage scrub and chaparral ecosystem that once existed here has been eliminated and replaced by non-native weeds. The Witch Creek Fire swept through this area in 2007. It was burned again in 2018.
State Highway 60 in Riverside County. All that is left of the rich sage scrub and chaparral ecosystems that once decorated these hills are isolated clumps of sugarbush surrounded by a sea of invasive weeds. Fires often burn here every year because annual weeds create highly flammable "fine" fuels.
Foothills of the San Bernardino National Forest. This landscape often burns every year. It was likely sage scrub habitat prior to its type conversion. Only non-native weeds and grasses are present now.
Misconceptions abound. This image accompanied a 2019 article in the Sierra Club's national magazine that was originally captioned, "Prescribed fire is used to prevent shrubs from encroaching on California's coastal grasslands." We wrote the magazine and tried to help the editor understand that the photo actually showed an area that was once covered by native shrubland, but had been type converted to non-native weeds by overgrazing and other land abuse (the impact of old livestock trails can even be seen along the weedy hill slopes). The editor modified the caption to read, "Prescribed fire is used on California's coastal grasslands." The fact that this is not a coastal grassland, but rather a destroyed native landscape with obvious cow trails remained unrecognized. The pervasive misunderstandings about type conversion, fire, native habitat are difficult to correct.
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The Tejon Pass along Interstate Highway 5. There are so many fires in the Tejon Pass that the chaparral in many places is only represented by a few solitary shrubs.
Impact of grazing near Oak Flat, Tuolume County. Livestock are a significant force in type converting shrublands to grasslands.
They not only eat the native plants, but also disturb the soil with their hooves.
This is the reason John Muir called sheep "hoofed locusts." Photo: Ron Wolf.
State Hwy 330 going into the San Bernardino National Forest. Weeds and grasses push their way up the mountain after repeated fires.
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Trabuco Ranger District of the Cleveland National Forest. This is a fuel break along the ridgeline of the Santa Ana Mountains. All native vegetation has been eliminated by crushing and repeated fire. The effectiveness of this type of habitat elimination in terms of preventing the spread of wildfires is highly questionable because ridgelines are natural fire breaks themselves. However, the natural resource damage is unquestionably significant.
Resources on Type Conversion and Invasive Weeds
Effects of invasive alien plants on fire regimes. Brooks, M.L., C.M. D’Antonio, D.M. Richardson, J.M. DiTomaso, J.B. Grace, R.J. Hobbs, J.E. Keeley, M. Pellant, D. Pyke. 2004. Bioscience 54:677-688.
Fire frequency impacts non-sprouting chaparral shrubs in the Santa Monica Mountains of southern California. Jacobsen A.L., S.D. Davis, and S.L. Fabritius. 2004. In Ecology, Conservation and Management of Mediterranean Climate Ecosystems. Eds. M. Arianoutsou and V.P. Papanastasis. Millpress, Rotterdam, Netherlands.
Fire management impacts on invasive plants in the western United States. Keeley, J.E. 2006. Conservation Biology 20: 375-384.
Alien plant dynamics following fire in Mediterranean-Climate California Shrublands. Keeley, J.E., M. Keeley, C.J. Fotheringham. 2005 Ecological Applications 15: 2109-2125.
Fuel breaks affect nonnative species abundance in California plant communities. Merriam, K.E, J.E. Keeley, J.L Beyers. 2006. Ecological Applications 16: 515-527.
Chaparral Fuel Modification: What Do We Know - and Need to Know? J.E. Keeley. 2005.
Fire and invasive plants on California landscapes. Keeley, J.E., J.F. Franklin, C. D’Antonio. 2011. In D. McKenzie et al. (eds.), The Landscape Ecology of Fire, Ecological Studies 213. Springer Science + Business Media B.V.
Papers relating to the importance of long fire return intervals in chaparral.
Fire suppression impacts on postfire recovery of Sierra Nevada chaparral shrublands. Keeley, J.E., A.H. Pfaff, and H.D. Safford. 2005. International Journal of Wildland Fire 14: 255-265.
Are long fire-free periods needed to maintain the endangered, fire-recruiting shrub Arctostaphylos morroensis (Ericiaceae)? Odion, D., and C. Tyler. 2002. Conservation Ecology6: 4.
Fire, soil heating, and the formation of vegetation patterns in chaparral. Odion, D.C., and F.W. Davis. 2000. Ecological Monographs 70: 149-169.
Quote from Odion and Davis. 2000. "At the interface between human development and chaparral vegetation, desirable management from biological and slope stability perspectives argues not for relatively short rotation hazard reduction burning, but for improving characteristics of the built environment (defensible space, structures, landscaping, safe evacuation means, etc.), in efforts to reduce the perils of fire posed by living near chaparral."
Fire frequency impacts non-sprouting chaparral shrubs in the Santa Monica Mountains of southern California. Jacobsen A.L., S.D. Davis, and S.L. Fabritius. 2004. In Ecology, Conservation and Management of Mediterranean Climate Ecosystems. Eds. M. Arianoutsou and V.P. Papanastasis. Millpress, Rotterdam, Netherlands.
Fire management impacts on invasive plants in the western United States. Keeley, J.E. 2006. Conservation Biology 20: 375-384.
Alien plant dynamics following fire in Mediterranean-Climate California Shrublands. Keeley, J.E., M. Keeley, C.J. Fotheringham. 2005 Ecological Applications 15: 2109-2125.
Fuel breaks affect nonnative species abundance in California plant communities. Merriam, K.E, J.E. Keeley, J.L Beyers. 2006. Ecological Applications 16: 515-527.
Chaparral Fuel Modification: What Do We Know - and Need to Know? J.E. Keeley. 2005.
Fire and invasive plants on California landscapes. Keeley, J.E., J.F. Franklin, C. D’Antonio. 2011. In D. McKenzie et al. (eds.), The Landscape Ecology of Fire, Ecological Studies 213. Springer Science + Business Media B.V.
Papers relating to the importance of long fire return intervals in chaparral.
Fire suppression impacts on postfire recovery of Sierra Nevada chaparral shrublands. Keeley, J.E., A.H. Pfaff, and H.D. Safford. 2005. International Journal of Wildland Fire 14: 255-265.
Are long fire-free periods needed to maintain the endangered, fire-recruiting shrub Arctostaphylos morroensis (Ericiaceae)? Odion, D., and C. Tyler. 2002. Conservation Ecology6: 4.
Fire, soil heating, and the formation of vegetation patterns in chaparral. Odion, D.C., and F.W. Davis. 2000. Ecological Monographs 70: 149-169.
Quote from Odion and Davis. 2000. "At the interface between human development and chaparral vegetation, desirable management from biological and slope stability perspectives argues not for relatively short rotation hazard reduction burning, but for improving characteristics of the built environment (defensible space, structures, landscaping, safe evacuation means, etc.), in efforts to reduce the perils of fire posed by living near chaparral."