Chaparral

Chaparral

Chaparral is an evergreen shrub vegetation that dominates the rocky slopes of southern and central California. It forms a nearly continuous cover of closely spaced shrubs 6 to 12 feet (2 to 4 meters) tall, with intertwining branches that make the vegetation nearly impenetrable to humans. Herbaceous vegetation (grasses and wildflowers) is generally lacking, except after fires, which are frequent throughout the range. Because of complex patterns of topographic , soil, and climatic variations, chaparral may form a mosaic pattern in which patches of oak woodland, grassland, or coniferous forest appear in sharp juxtaposition . Fire frequency and soil are major factors that determine these patterns. Chaparral is replaced by grassland on frequently burned sites, especially along the more arid borders at low elevations (where shrub recovery is more precarious due to drought) and on deeper clay soils and alluvial plains , and by oak woodland on more moist slopes (where fires are less frequent and often less intense).

California chaparral is distributed in a region of Mediterranean climate, which has cool (40°F), wet winters and hot (95°F), dry summers. Rainfall is 10 to 20 inches (25 to 100 centimeters) annually, two-thirds of which falls November to April in storms of several days duration.

Plants of the Chapparal

The most widely distributed chaparral shrub is chamise (Adenostoma fasciculatum ), an adapted shrub with short needlelike leaves, which is distributed from Baja California in the south to Oregon in the north. Buckbrush (Ceanothus spp.) and manzanitas (Arctostaphylos ) are large genera (about seventy species each) and often form pure stands commonly referred to as manzanita chaparral or ceanothus chaparral. Some species are highly restricted in distribution, whereas others are nearly as widespread as chamise. Most species in these two genera are endemic to the California chaparral and have suites of characters reflecting a long association with fire. For example, many species of Ceanothus and Arctostaphylos have woody tubers at their base that sprout new stems after fire. All species in these two genera produce deeply dormant seeds that accumulate in the soil and require fire for germination.

At the lowest elevations throughout much of its range, chaparral is commonly replaced by a smaller and highly aromatic vegetation known as soft chaparral or coastal sage. This vegetation differs from chaparral by being summer-deciduous; this loss of leaves during drought confers a greater ability to tolerate the drier conditions at low elevations. The dominant shrubs are only 3 to 6 feet (1 to 2 meters) tall and include California sagebrush (Artemisia californica ), black sage (Salvia mellifera ), California buckwheat (Eriogonum fasciculatum ), deerweed (Lotus scoparius ), and monkeyflower (Mimulus aurantiacus ). These smaller shrubs grow rapidly and have well-developed wind dispersal of seeds so they often colonize disturbed sites.

The Californian Mediterranean climate is conducive to massive wild-fires. Mild, wet winters contribute to a prolonged growing season, which, coupled with moderately fertile soils, result in dense stands of contiguous fuels. Long summer droughts produce highly flammable fuels that are readily ignited by lightning from occasional thunderstorms but more commonly as the result of human carelessness. On average fire frequency for any one area is about every two to three decades, but this may be more frequent than in the past. Throughout much of its range, chaparral forms a continuous cover over great distances, and as a result, huge wildfires that cover tens of thousands of acres are common, particularly during Santa Ana wind conditions. These dry winds from the east occur every autumn and often exceed sixty miles per hour. Some scientists have suggested that massive wildfires are an artifact due to modern-day fire suppression, which causes an unnaturally heavy accumulation of plant fuel. Others dispute this conclusion, pointing to evidence that shows this vegetation has always experienced large, high-intensity fires.

The Role of Fire

Although shrubs dominate chaparral, the community comprises a rich diversity of growth forms, many of which are conspicuous only after fire. In addition to evergreen shrubs and trees, there are semi-deciduous subshrubs, slightly ligneous (hardened) suffrutescents , woody and herbaceous vines, and a rich variety of herbaceous perennials and annuals. A large number of these species arise from dormant seeds deposited into the soil decades earlier, following an earlier fire. Dormancy is broken in some seeds by heat but in many other species smoke from the fire triggers germination. In the first spring following fire, there is an abundant growth of herbaceous plants, which are relatively short-lived and are replaced by shrubs within the first five years. The postfire herbaceous flora often is dominated by annual species that live for less than one year, and species diversity is typically greatest in this first year after fire. Recovery of shrub biomass is from basal resprouts and seedling recruitment from a dormant soil-stored seed bank.

The striking contrast between the diminished herb growth under mature chaparral and the flush of herbs after fire is thought to be caused by allelopathic (chemical) suppression of germination by the overstory shrubs. Many of the smaller shrubs, such as sage (Salvia spp.) or sagebrush (Artemisia spp.), release volatile, aromatic compounds , and it has been suggested that these compounds inhibit the growth of competing grasses and wildflowers. This theory holds that fire destroys these toxins , and that this occurs throughout the shrub land and in a zone at the border between shrub lands and grasslands, forming a meter-wide strip known as the bare zone. However, experiments in which animals have been excluded from the bare zone demonstrate that the lack of herbaceous plants in and around mature shrub lands is as much due to animal predation as it is to chemical inhibition. In addition, it appears that the vast majority of species that germinate after fire do so more because their dormant seeds are stimulated to germinate by fire.

Resource agencies often respond to wildfires with emergency revegetation programs, which drop grass seed on newly burned sites with the expectation that this will reduce soil erosion and eliminate the threat of mud-slides and flooding. The rationale for this management is that burned sites have greatly increased surface flow of rainwater and thus high soil erosion. Emergency seeding is considered essential on sites following exceptionally intense fires because of the anticipated negative effects. Throughout the state of California the seed of choice has been the nonnative annual rye-grass (Lolium multiflorum ). However, there is abundant evidence that this practice fails to substantially reduce threats of mudslides and flooding and competitively displaces the native flora.

Some scientists have suggested that chaparral shrub lands become senescent if they are free of fire for more than a few decades. Detailed studies, however, find that these shrub land ecosystems can retain productive vegetation for a century or more, and in fact some shrubs require long periods without fire for successful seedling recruitment.

see also Allelopathy; Biome; Ecology, Fire.

Jon E. Keeley

Bibliography

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