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Chaparral is an evergreen shrubland or woodland vegetation type that occurs in areas characterized by hot, dry summer conditions, classified as a Mediterranean climate. Chaparral shrublands and woodlands are found on over three million ha (eight million ac) in California and in several locations in Oregon, Washington, and Arizona. Similar vegetation is found in Baja California (Mexico), the Mediterranean Basin, Central Chile, southwestern Australia, and southeastern South Africa. High-intensity wildfires that generally occur during the dry, hot season are associated with chaparral shrublands.
Chaparral occurs in areas with a Mediterranean-type climate that is characterized by mild, moist winters and hot, dry summers. Temperatures range from −3 ° to 18 C (26.6° to 64.4 °F) in the coolest month, precipitation pattern of <40 mm (1.6 in) of rain during summer, and summer temperatures above 22 °C (71.6 °F) (Chen and Chen 2019). In the United States, chaparral is mainly found throughout California, across varied terrain from mountain slopes and rocky hills to flat plains. Much of the densely populated southwestern third of California is dominated by chaparral (Keeley 2012).
Persistent drought and recurrent fire are the major drivers of chaparral species composition and structure (Fried et al. 2004). Chaparral soils are generally moisture-limited, thin and rocky, or sandy, and nutrient poor. Typical vegetation consists of dense shrubs and sparse herbaceous species that can tolerate the harsh environmental conditions and are adapted to the hot, dry summer and autumn seasons of the Mediterranean climate.
Strong easterly winds (often referred to as Foehn winds or Santa Ana winds in California) can occur between September and May and carry hotter, drier air into southern California. The harsh climatic conditions that often occur during the summer and fall in chaparral regions may be exacerbated by this influx of hotter, drier air and strong winds. These conditions are favorable to high-intensity wildfires that move rapidly through the highly flammable shrub crowns. Following fire in chaparral, very little vegetation remains on the site. However, the stand is able to begin regenerating quickly following fire, and within a decade, the vegetation resembles its pre-fire condition.
The defined types of chaparral found in California are dominated by evergreen shrubs but may have scattered small stature trees such as blue oak (Quercus douglasii) or mountain mahogany (Cercocarpus ledifolius). These types are described below (Fried et al. 2004).
Montane chaparral – Montane chaparral structure and composition vary by moisture, elevation, and other factors. Montane chaparral structure ranges from low, sprawling shrubs to 3 m (9.8 ft) tall shrubs depending on site conditions. It is typically dominated by whitethorn ceanothus (Ceanothus cordulatus), snowbrush ceanothus (Ceanothus velutinus), manzanita, huckleberry oak (Quercus vacciniifolia), mountain mahogany, California buckthorn (Frangula californica), or a combination of these species (Risser and Fry 1988).
Scrub oak chaparral – One or more scrub oak species dominate this chaparral type. Common oaks are scrub oak, Tucker oak (Quercus john-tuckeri), Palmer oak (Q. palmeri), interior live oak (Q. wislizeni), coast live oak (Q. agrifolia), Engelmann oak (Q. engelmannii), canyon live oak (Q. chrysolepis), and leather oak (Q. durata). This type tends to occur on chaparral sites that have more soil moisture than some of the other types.
Chaparral plants have adaptations that allow them to grow in harsh conditions of hot, dry summers, generally thin soils and limited water supply. Many chaparral species have thick, leathery evergreen leaves – or sclerophyllous leaves – that prevent wilting so the shrubs can continue to photosynthesize in very dry conditions (Parker et al. 2016).
Other adaptations help the plants reduce transpiration rates and water loss. These include small leaves with a waxy coating, recessed stomata on the underside of the leaves, or hairy leaves to trap water vapor. Adaptations to reduce sun exposure and leaf temperature include vertical orientation of leaves and light-colored leaves (Comstock and Mahall 1985). Furthermore, many chaparral shrubs have taproots or extensive root systems that help them reach water in the soil in order to survive the harsh climate.
Many of the chaparral plants have foliage with oils and waxy substances increasing flammability. Chamise is particularly flammable due to the oils and waxy substances along with its needle-like leaves (Barro and Conard 1991). Many chaparral shrub species also retain dead branches, which increases available crown fuel for combustion.
Another set of adaptations help chaparral plants regenerate following fire. Several chaparral species can resprout following fire from roots and burls. Others germinate following fire from seeds that have accumulated in the soil (Fried et al. 2004). Many perennial forbs regenerate from underground bulbs when stimulated by the increased nutrients available in the soils following fire.
Fire in Chaparral
Periodic stand-replacing shrub crown fires are common in chaparral. The combination of highly flammable vegetation and reoccurring strong Santa Ana winds create extreme fire behavior from late summer through early winter. Chaparral types are classified as having an intermediate fire return interval, which indicates that fires generally occur every 50 to 150 years on chaparral sites. (Conard and Weise 1998).
The historical fire regime for chaparral shrublands is a pattern of summer lightning fires with periodic large fires driven by Santa Ana winds (Keeley 2012). According to Keeley (2006), contemporary fire regimes are not qualitatively different from historical fire regimes. There is still a pattern of smaller summer fires and periodic stand-replacing fire events; however, fire frequency has increased due to ignitions by humans. Many species are eliminated from chaparral when fire frequency increases to more than one fire per decade (Keeley 2006). In areas where human ignitions have increased fire frequency, there has been substantial conversion of chaparral shrublands to grassland vegetation (Keeley 2012).
The primary fuel bed category that carries fire in the chaparral shrublands is the shrub crowns. The shrub crowns include leaves and stem wood. In general, shrub crown biomass ranges from 1.2 Mg/ha to 118 Mg/ha (0.54 to 52.64 tons/ac). The largest component is shrub stem biomass, and between 10 and 40% of that is standing dead wood (Ottmar et al. 2000). Leaf biomass contributes 3–25% of total shrub biomass (Bohlman et al. 2018; Regelbrugge and Conard 1997). Surface fuels contribute minimally to overall fuel loading in most chaparral types.
Fire in chaparral consumes most of the standing vegetation, and whatever litter, and other organic matter is present. However, burned shrub stems may remain standing after the fire and contribute to fuel loading in the regenerating stand. Chaparral begins regenerating quickly after fire, and within several years to a decade, the vegetation will look similar to its pre-fire condition. There are several modes of regeneration following fire including:
Obligate resprouters: These plants regenerate by resprouting from basal burls or underground root systems. These include toyon (Heteromeles arbutifolia), scrub oak, and some manzanita species.
Obligate seeders: The adult plants die in the fire, and seeds in the soil receive a fire cue that stimulates germination. These include ceanothus and some manzanita species. Germination cues include exposure to heat that can weaken the seed’s outer shell or chemicals from smoke, ash, or charcoal that can trigger germination (Keeley 1991; Keeley and Fotheringham 1998).
Facultative seeders: These regenerate post-fire by both seed germination and burl or root resprouting. The adult plant resprouts, and a fire cue enhances seed germination. Chamise is a facultative seeder.
Some species, termed fire followers, are not present in mature chaparral but appear soon after wildfires. Their seeds remain dormant in the soil until they are stimulated to germinate after fire. They may dominate post-fire sites for several years (Keeley 2012). These annual or short-lived perennials produce many seeds during this time and then die out as the shrubs regain dominance on the site. Their seeds will remain in the soil until the next fire when they will germinate and begin the cycle again.
Fires affect more than just the vegetation. Removal of leaf litter and changes in soils caused by fire can lead to higher potential for erosion (Barro and Conard 1991), especially on steep sites. Rain events following chaparral fire can initiate debris flows that can damage downhill homes or communities. Air quality and visibility are reduced by smoke from chaparral fires. Animal populations are also affected. Some species (e.g., some arthropods) are able to survive in the soil and others are able to fly or run away, but many don’t have a successful defense to fire (vanMagtem et al. 2015).
Chaparral occurs in the wildland-urban interface throughout much of its range (Conard and Weise 1998). In most cases, summer fires in chaparral can be controlled before they become destructive to property. But, high-intensity shrub crown fires are a major threat to homes and businesses (Keeley et al. 2009). The biggest challenge for chaparral managers is how to reduce the threat to communities and also manage natural resources appropriately.
Accidental or purposeful ignitions by humans are responsible for most chaparral fires (Moritz 1997; Conard and Weise 1998; Keeley et al. 1999). This has led to an increase in fire frequency in many chaparral areas. Short intervals between fires can lead to conversion from native shrubland to nonnative herbaceous vegetation, increased flammability, decreased slope stability, and loss of native biodiversity (Keeley 2005). Fire suppression has helped prevent these negative effects (Keeley 2001).
Following natural or accidental high-intensity fires, direct seeding with grasses is often used to protect slope stability. But the mixes often contain nonnative grasses that colonize quickly. This can promote type conversion to nonnative grassland vegetation.
Prescribed fire and mastication are often used to reduce fire risk in chaparral. There are some trade-offs associated with these potential fuel treatments. Prescribed fire maintains native diversity more successfully than mastication, but both prescribed fire and mastication can have negative effects on chaparral. Mastication can change the community structure to a shrub/grass mix, encourage nonnative plants and noxious weeds (Keeley et al. 2011), and have a higher probability of long-term erosion risk than prescribed fire. Prescribed fire also allows the introduction of nonnative plants (Keeley et al. 2011), but they tend to have lower invasive potential than plants that may be introduced following mastication. Prescribed fire treatments allow higher risk of catastrophic fire than mastication. (Wilkin et al. 2012)
Most large wildfires occur in late summer and fall seasons during Foehn or Santa Ana wind events. Under these severe conditions, fires can burn through the younger age classes. Therefore, fuel reduction treatments may not always be effective for suppressing high-intensity chaparral fires (Keeley et al. 2009).
Establishment of strategically placed dynamic fuel management zones in wildland areas to provide access and opportunities for fire control.
Intensive fire risk management zones (managed and developed cooperatively with local agencies and landowners) to protect values in the wildland-urban interface.
The intention of their plan is to provide a balance of community protection while maintaining chaparral biodiversity.
Chaparral shrublands occur in areas with a Mediterranean climate, which has hot, dry summer conditions. Fire is a major driver of chaparral species composition and structure, and common chaparral species have adapted to survive or regenerate following fires. The shrub crowns are highly flammable and are the primary carrier of fire in chaparral. Santa Ana winds can create extreme fire behavior in chaparral which endangers nearby towns and neighborhoods. Chaparral is managed to reduce fire hazard in populated areas, but some management strategies can be detrimental to chaparral vegetation. Therefore, managers attempt to place treatments strategically to reduce fire hazard in the wildland-urban interface but also maintain chaparral structure, composition, and function.
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