The Chaparral Biome
|Biology - Biomes|
The chaparral is a biome little known even by people living in it. Physically, it is a dense, impenetrable thicket of shrubs and trees. Chaparral is a Spanish word meaning "a thicket of low, shrubby, evergreen oaks" as found in Spain. It is possible, but far from certain, that the word comes from "chaps" that Spanish cowboys wore as leg protection while riding through the stiff-branched shrubs.
The term chaparral is now applied more generally to dense shrublands that are climax biological communities of a specific biome, known appropriately as the chaparral biome. A biome is a distinctive ecosystem determined by world climatic regions and is thus a portion of the biosphere, or living world. Some examples of typical biomes are the desert, the northern conifer forest, and the chaparral.
World climate, which determines the distribution of the various biomes, is a product of three main factors: available moisture, radiation, and temperature. Various combinations of these three factors determine the climate of a particular region and the type of biome present there. The climate necessary for the chaparral biome is found on the north and south sides of the two subtropical high-pressure belts around the world, at 30 to 35 degrees latitude.
These are regions of dry, warm air that result in large areas of subtropical desert. To the polar sides of these belts are the temperate zones of cool, wet climates controlled by polar air currents. Lands bordering both sides of the Mediterranean Sea have a climate marked by mild, wet winters and hot, dry summers. So characteristic are these conditions that wherever they are found, ecologists term the climate Mediterranean. Such Mediterranean climates are found at five places around the world: the Mediterranean region, western California, central Chile, southern South Africa, and western and southern Australia.
In California, two types of storms travel across the state from November through April. The first are periodic Pacific storms that originate in the northern temperate and polar zones. The second are subtropical frontal storms that are few in numbers but are slow-moving, soaking storms. Together, these storms wet the chaparral region all winter. In summer, the Pacific storms slide to the north, blocked by a stationary high-pressure cell, and drought persists from May to November. Radiation levels are high both winter and summer as a result of the lack of clouds, except when there are storms.
Temperatures are mild to near freezing during the winter and often above 100 degrees Fahrenheit, with 1 percent humidity, during the summer drought. In terms of biological activity, the season begins with the first fall rains. At this time most seeds germinate immediately. The seedlings grow vigorously through the mild winter and flower by midspring. Next season's seed matures by late spring or early summer when the drought and high temperatures begin. Annual and perennial plants are termed drought evaders because their seeds ripen before the dry season and remain dormant during it.
The shrubs and trees do most of their annual growth at the start of the fall rains in October and November. Many are in flower as early as December and January. By flowering this early, there is adequate moisture available for seed maturation before the drought begins. The animals are also adapted to this wet-dry season cycle.
The Colombian black tail deer mates in late summer, and fawns are born in early spring. Most of the larger animals migrate out of the chaparral biome during the hot, dry summer and fall, to the cooler, moist habitats in adjacent conifer forest biomes. Many plants are dormant during the summer, a season of low activity in the chaparral. A biome, by definition, is characterised by a particular type of climax plant and animal community that appears much the same over large areas.
The chaparral biome looks similar in different parts of the world, but the assemblage of plants and animals varies from area to area due to factors not controlled by world climate. These include changes brought about by geological time, shifting of continents, and so forth. Each of the five world locations for chaparral has their own local names for the shrub climax community. Examples are maquis, garrique, tomillares around the Mediterranean; macchia and fynbush in South Africa; melle scrub in Australia; and chaparral in California.
The term chaparral biome applies to all of these areas, although some ecologists prefer to use the name Mediterranean chaparral biome. In the following discussion we will use the California chaparral biome as an example. This biome covers some seven million acres and is governed by the same general principles as chaparral everywhere. The plants that dominate the chaparral community are sclerophyllous shrubs.
The term sclerophyllous refers to leaves that are small, stiff, and evergreen. Such leaves can withstand the long summer drought, although the plants characteristically shed half or more of their total number of leaves in early summer as a water-saving device before the hot, dry weather begins. Some shrubs and trees lose all of their leaves by midsummer to evade water loss, and they become totally dormant. At climax stage, the ground surface beneath the shrubs contains few plants. This is a result of the intense shading that reduces the amount of light for growth and the presence of chemical factors we will discuss later.
Surprisingly, more than nine hundred species of plants occur in California chaparral, making it far richer in species number than the desert, alpine, or the two conifer biomes of the region. Chaparral occurs as massive stands covering thousands of acres as one unit, or it may occur as patches in a mixture of grasslands, chaparral, and oak woodlands. In the patch community situation, local climate, soil types and depths, and soil water availability determine the community type present. Over time, these community types remain constant. Even with intense cattle grazing and regular forest fires, the community patches shown here have the same boundaries as a hundred years ago, as determined by old photographs.
Chaparral shrub species have two root systems to utilise different sources of water: a lateral, near-surface system to catch light, low-penetration rains; and a deep central taproot system which provides moisture throughout the dry season. At the end of the dry summer season, ground water is usually exhausted in the chaparral biome. This explains why the oak woodland community or other tree communities are absent as these require more ground water than the chaparral does.
The various chaparral community types are named by the dominant plants present. Depending upon local non-climatic conditions, different chaparral community types occur with differing dominant shrub and herb combinations. At least four general chaparral community types are recognized throughout the chaparral biome, based on height of shrubs, other plants present, and overall arrangement of plants.
Type one: the garrique of the Mediterranean and coastal scrub chaparral of California are open communities of small, evergreen sclerophyllous shrubs, two to five feet tall, and perennial herbs such as Douglas Iris. Shrubs of sage and coyote bush are most common. During the winter season, both large and small hawk species are seen in abundance patrolling a few feet above the chaparral, hunting for small rodents and rabbits. The hawks migrate to other biome types in the summer.
Type two: the maquis of the Mediterranean, shown here with the shrub scotchbroom as a dominant. This species has invaded California and now covers hundreds of thousands of acres just as it does in the Mediterranean. In California, the chamise and manzanita communities represent the maquis type of dense, evergreen sclerophyllous shrubs, three to ten feet tall, with undershrubs, perennial herbs, and bulb plants. This type of community is the most common and widespread, being found in all five of the chaparral areas of the world. Seventy percent of the California chaparral, five million acres, is of the chamise shrub community. At climax state, the chamise shrub covers 80 to 90 percent of the land surface with no ground-level, green-leafed herbs. At climax, chamise shrublands are 50 percent dead material. One understory herb, the parasite chaparral broomrape, derives its energy by attaching itself to chamise roots. A few other shrub species are present in low number.
Columbia black tail deer browse heavily on young chamise shoot tips as a preferred food. The greatest number of deer per acre of all the community types in California, from grassland to mountain forest, are found in the chamise chaparral type. The manzanita chaparral community is much less common. Because of its stiff, intricate branching, it is much harder to penetrate by large mammals such as deer. Deer generally avoid the manzanita community as they rarely eat manzanita shoot-tips, and it provides poor shelter during hot weather. Large rodents in the manzanita community attract coyotes, fox, and bobcats as predators. The Pacific rattlesnake occurs in large numbers in the manzanita community and utilise the large two to three-pound ground squirrels as a prime food source. The ground squirrel is found in such abundance that rattlesnakes of five to six feet are frequent.
Type three: sclerophyllous forest of low, open evergreen trees, such as the oak forests of the Mediterranean and California. In Australia, low species of eucalyptus form Type Three chaparral. The dwarf oak chaparral of California tends to be more open to light and therefore contains many other shrub species, plus a rich herb flora.
Type four: the mixed woodland chaparral of California, at the upper edge of the chaparral climatic biome, limits the lower woodland forest mixed with all of the various chaparral shrub species, and forms a complex ecosystem with numerous ecological niches for animals and plants.
All the different chaparral communities, including the mixed woodland chaparral, are patrolled mornings and evenings by the turkey vulture, looking for dead animal remains for its food source. In California mountains at intermediate to high elevations, mountain chaparral is a successional community and is not a part of the chaparral biome. Mountain chaparral occurs after the climax forest is removed by fire or logging. Since mountain chaparral is in the climatic range of the northern conifer forest biome, it becomes a succession stage, rather than climax, and is replaced by climax conifer forest.
A special feature of the chaparral biome is that intense fires regularly remove the climax community to bare ground. Climax status is achieved rapidly in the chaparral biome, usually in ten years or less, and it is relatively short-lived, fifty to one hundred years, as fire regularly reduces the chaparral back to naked soil.
Many different fire-adaptive mechanisms are present in the chaparral biome to offset the frequent fires. At the time of burning, temperatures reach twelve hundred degrees Fahrenheit. This results in total combustion of all organic matter, except for a few large limbs. The heat intensity is of short duration because the quantity of fuel per unit area is low, and therefore the fire moves through rapidly as a wall of heat and flame. Due to the short, intense heat level, only a few inches of soil serve to insulate burrowing rodents, reptiles, and plant bulbs from harm.
Plants of the chaparral communities show three types of adaptations for rapid re-colonisation after a fire. First, many of the plants form seed at an early age. Even shrubs and trees may produce seed after only a year or two of growth. Second, they may have special seed-germination adaptations to fire. These include long-lived seed that remain dormant for fifty to one hundred years or more. Such seeds remain buried in the soil until the land is cleared of shade and chemical factors by fire. Some seeds require heat to break the thick seed coat before germination can occur and are thus heat dependent. Nurserymen who grow chaparral species now know that to obtain germination they must place a layer of leaf litter over certain seeds and burn it.
Another seed-germination adaptation is the fact that many seeds are inhibited from germination by chemicals given off from the leaves and other plant parts. When fire burns these compounds from the soil, the seeds germinate at once.
The third type of adaptation to rapid re-colonisation after fire involves the sprouting ability of tubers and burls. Some chaparral plants, such as the manroot, have underground tubers with a large store of water and nutrient energy. In one known case, a manroot tuber placed on a concrete sidewalk, with no contact with soil or water, continued to put forth its fifty-foot vines for three years. Many of the dominant climax shrubs, such as chamise shown here, have woody, underground stem burls that usually re-sprout immediately, often within a week of a fire. Some manzanita species also have burls that re-sprout after fire.
Bulb dormancy (a summer drought-avoidance mechanism) is also used by many chaparral bulb species as a means of survival when the dense, dark understory of chaparral at climax does not allow enough light for growth. During the growing season following a fire, flowering bulb plants are common, even though no leaves or other clues revealed their presence in years prior to the fire. Bulb plants of the chaparral biome require three to five years from seed and thus could only come from mature, long-dormant bulbs.
The term climax community was used earlier without full explanation. It is a concept where a specific plot of land, if given sufficient time, will develop into a final, long-lasting community, one that will not be replaced by another community. Should the climax community ecosystem be destroyed, the change from bare land, through a series of temporary communities to a final long-lasting climax community, is the process termed succession. Succession after a fire is rapid.
First, there is a herb phase of one to three years. The first year, the largest number of herb species appear in large masses. Some herbs specifically require fire as part of their life cycle. These are called pyrophyte endemics. The fire poppy only appears and flowers the first year after a fire and then may disappear for fifty or more years until the next fire. The shrub phase of succession starts the first year after a fire from burl sprouts and seedlings, and by the fifth year are tall enough to shade out the shorter herbs and approach a climax community.
Inhibition of seed germination between fires has been related to chemicals produced by some of the chaparral shrubs. This is termed allelopathy. For example, chamise shrub leaves have a water-soluble compound that washes to the soil and stops germination of numerous herbs. This, in part, explains the bare soil appearance beneath mature chamise.
When chamise dies or is cut away, an immediate wave of herbs appear the next season as if a fire had swept through. Fire also removes this chemical compound. Manzanita shrubs in all parts, leaves, stems, roots, and old leaf litter, contain chemicals, which inhibit seed germination of itself and other plant species. Even upon removal of manzanita stems and leaves, the chemical compounds in the soil continue to inhibit seed germination for some years. Here, fire is important in burning the compound out of the soil so that seed germination can take place. In chaparral communities, there is a narrow zone of bare earth between the chaparral and the adjacent grassland communities.
When first observed, it was thought to be due to chemical allelopathy. Tests showed that grass and herb seedlings treated with extracts of the adjacent allelopathic shrubs would not grow. However, an animal ecologist disputed this and performed an experiment by placing wire cages over portions of bare zone. He showed that the herbs and grass would grow if protected and that small herbivores, primarily small rodents and rabbits, use chaparral as protective cover, from predatory hawks.
The small animals venture only a short, safe distance into the grassland to graze and gather seed. It is the constant grazing by these small herbivores that causes the bare zone. Birds also use the shrub-grassland zone for feeding in the open while utilising the shrubs for protection.
California quail are found in abundance in this area because it contains the four habitat features they require. These are: (1) shrubs for daytime, protected resting places away from hawks and eagles; (2) a daily freshwater source; (3) open, grassy places for seed gathering; and (4) taller shrubs and trees for night time roosting, out of reach of hunting coyotes, foxes, and bobcats. All four habitat requirements must be in close proximity to each other.
Human interaction with the chaparral biome has frequently been unfortunate. Building homes within chaparral and then attempting to permanently exclude fire has never worked. The awesome sight of a home instantly becoming a total fireball, as the 1200 degree Fahrenheit heat wave reaches it, is hard to forget. Only in the last few years have humans tried to live with the fire cycle by zoning and purposely setting fires to fit the natural ecological rhythm of the chaparral, and thus reduce the enormous fuel build up that results from total fire exclusion.
To summarize, the chaparral biome occurs in the Mediterranean climate regions of the world. Chaparral consists of small, hard-leaved shrubs and trees, often in dense stands. The climax community is reached in a few years, and after a ten to one hundred year period it is reduced by fire to the bare ground, whereupon the cycle starts anew. Many of the chaparral plants have specific fire requirements as part of their life cycles. Chemical growth inhibitors are produced by many of the dominant plant species.
The chaparral biome community has a rapid cycle of succession from bare ground to climax community, compared to other biome types. However, the biological community interactions are just as complex and complete as they are in other biomes, where the climax stage may last thousands of years.