CHAPTER 2

MEDITERRANEAN CLIMATE

CALIFORNIA CHAPARRAL IS a product of the mediterranean climate in which it grows, where summers are hot and dry and winters mild and wet. Less than three percent of the world's land surface has a mediterranean climate. This climate type is found in California, the Mediterranean Basin, central Chile, the southern tip of Africa, and parts of Australia (map 2). Each of these areas has an evergreen shrubby vegetation commonly referred to as chaparral, maquis, garrigue, matorral, fynbos, or heath. The environmental conditions that produce a mediterranean climate result from the confluence of three factors: latitude, a cold ocean, and a large high-pressure air mass that extends from the ocean across the land of the adjacent continent. These factors come together on the western edge of continents between roughly 30 and 40 degrees latitude on either side of the equator. The high-pressure air mass shifting north and south with the seasons produces the alternating cool, wet winters and hot, dry summers characteristic of this climate type.

The Pacific High

The Pacific High air mass controls California's mediterranean climate. This slowly circling and gradually descending mass of air produces high pressure from the central Pacific Ocean all the way to California. Areas beneath this air mass are largely protected from storms half the year, producing rainless summers and falls. From midspring until late fall the Pacific High covers a large portion of California's coast and nearby inland areas. In winter, the sun moves to the south and the Pacific High travels with it. As the Pacific High drifts south, storm systems from the North Pacific can move progressively farther down the California coast, eventually reaching as far south as Baja California. Other storms from the east, over the Great Plains, for example, can also travel west to California because the Pacific High no longer deflects them. Because of the immense barrier of the Pacific High, the driest time of the year in California is typically in June, July, and August (fig. 3). The Pacific high exerts its influence over southern California for a longer period of the year than over northern California. Consequently, the rainy season is somewhat longer and more intense in the north.

Map 2. Mediterranean climate is found in five locations worldwide between 30 and 45 degrees latitude as shown in red. (Scale reflects distances at equator.)

Rainfall—Always Unpredictable

The amount of rainfall varies greatly from winter to winter. This variability occurs over the entire state, so even though northern California is wetter overall than southern California (fig. 3), in some years areas in the north may be as dry or drier than areas much farther to the south. Figure 4 shows rainfall measurements taken at the Los Angeles Civic Center over a 20-year period between 1982 and 2002. The average rainfall over this period was approximately the same as over the past century: 15 inches per rainfall year (July 1 to June 30). Twelvemonth totals are measured from the middle of the calendar year so that the November through April rainy season is totaled together. This makes ecological sense as well, since most chaparral plants grow between late winter and early summer in response to the preceding rainy season. It is not only the multiyear average rainfall that matters, but also the range of variation from year to year. This can be large in California. For example, in the 2001/02 rainfall year, the total was less than one-third of the average amount, while the 1997/98 rainfall year had more than twice the average amount. Notice that the total fluctuates widely among years, and that the amount in any given year is not a predictor of how wet or dry the next year will be.

Figure 3. Average monthly rainfall in San Francisco, Los Angeles, and San Diego from 1961 to 1990. Northern California has a slightly shorter period of summer drought than southern California and receives more rainfall during the rainy season. (Data from Department of Meteorology, University of Utah.)

Figure 4. The annual seasonal rainfall (July 1–June 30) at the Los Angeles Civic Center over 20 years.

The spacing of storms, their duration, and their intensity are also highly variable and unpredictable. One storm may provide one-third to one-half of the total for a rainfall year all at once, while another may drop only enough moisture to dampen the surface. For example, in April 1926 a world record was set in the San Gabriel Mountains of southern California when almost an inch of rain fell in one minute. After this deluge, the region received no rain at all for several weeks. In January 1943 another record was set, also in the San Gabriel Mountains, when 26.12 inches of rain fell during a 24-hour period. Read about the impact on people of these intense storms in chapter 6.

Each rainfall year, many weeks and sometimes months have no detectable precipitation. There are occasionally even 12 consecutive months with no rain. Drought is an annual summer event in the chaparral, but it is also unpredictable in its duration and severity. A study of rainfall patterns in central California showed large variations during the winter months over a 20-year period. During that time, 12 winter months had no rainfall at all. Droughts of several years' duration are not uncommon, as exemplified between the years 1986 and 1991. These fluctuations between wet and dry are a recurrent feature of California's mediterranean climate and are often linked with cyclical variations in winds and ocean currents in the equatorial Pacific Ocean. These equatorial cycles affect weather patterns in many parts of the world. In fact, the idea of a “normal” year based on average rainfall over many years is misleading, because it fails to take into account year-to-year variability. Those who have experienced water rationing during droughts, or floods and mudslides in wet years can testify to the side effects of these wide climatic variations.

Winds That Carry Water or Take It Away

Wind is also a basic component of California's mediterranean climate and, like rain, can be either life sustaining or life threatening. The prevailing wind pattern that provides moisture in winter and cool sea breezes in summer moves from the ocean toward the mountains and inland valleys. These winds are called the Westerlies. Periodically winds flow from the east, reversing the normal wind direction. These latter winds have been variously called Santa Anas, Diablo or Devil winds, and Sundowners.

 

The Westerlies

The Westerlies flow regularly from the ocean onto the land all along California's coast during all seasons. These winds carry moisture inland, where it often precipitates as fog drip, rain, or dew when it reaches the foothills and mountains. The western slopes of most mountain ranges in the state are greener and the shrubs taller and thicker than on the east side of the mountains because the preponderance of precipitation brought there by the regular flow of the Westerlies is deposited on the windward side. Westerlies also moderate air temperatures, an important ingredient of California's mediterranean climate.

The winds from the ocean are not always gentle, however. During winter storms the Westerlies may carry so much water that when they reach the foothills the downpours described above can cause flash floods. If the chaparral on steep slopes has been recently burned off, the force of the rain can wash large quantities of soil and debris off the mountainsides and flush out canyons into the valleys below. The fire, besides removing the standing vegetation, often burns up the organic matter in the soil as well, causing it to become powdery and unstable. This combination of factors can result in a slug of thick mud and debris that can surge out of the canyons, over barriers, and out onto the valley floors. The force of this mud is sufficient to toss cars off roads, and crush and bury houses, leaving little evidence of former human habitation in its wake (see chapter 6).

Fire-Fanning Winds from the East

Strong winds from the east periodically change the atmosphere over chaparral completely. These are the winds responsible for fanning wildfires into raging and unstoppable blazes. They are unlike the prevailing westerly winds in every respect, whistling down mountain passes from the east, blowing hot, dry, gusty breaths through the summer-parched vegetation. Classified as foehn winds by meteorologists, they are generated by interior high-pressure air masses that are forced down mountain slopes and valleys (fig. 5). They are called Santa Ana winds in southern California, reputedly because they blow strongly through the gap in the Santa Ana Mountains and out across coastal Orange County. They are known as Diablo or Devil winds in northern California because they originate near Mount Diablo. In the Central Coast Ranges near the city of Santa Barbara, winds called Sundowners develop in the hours immediately after sunset as hot air suddenly pours down over the mountains from the nearby Santa Ynez valley. While producing a similar-feeling east wind, a Sundowner is not technically a foehn wind. In the Bay Area the Diablo winds, like the Santa Anas of the south, can drive large wildfires. Two of the most destructive wildfires in the history of the United States were driven west by downslope winds out of the vegetation of the Berkeley Hills and into the East Bay cities of Oakland and Berkeley (see chapter 6).

Figure 5. Santa Ana (south) and Diablo (north) winds blow strongly from high-pressure areas over the Great Basin toward the coast of California.

Surprisingly, in California these very dry winds are often connected with storm systems. Santa Ana–type winds blow most often between September and December; however, they can occur any time of the year except summer. As the stable high pressure that anchors the long California summer begins to drift southward in fall, it allows a cavalcade of low-pressure storm systems from the Bering Sea to move across the state. In early fall, as the southern fringe of this storm track begins traveling through California, the trailing edges of storm fronts lightly brush the state but are too weak to produce rain. These weather fronts are often closely followed by high pressure that clears the sky while moving in a southeasterly direction toward the center of the continent. As these high-pressure areas drift over the Great Basin, they push a westerly flow of air back toward the Pacific Ocean (fig. 5). This river of air accelerates as it flows and tumbles down California mountain slopes. The steep drop in elevation (approximately a mile) from the Great Basin to the coast compresses, heats, and dries the air. As the winds cross the mountains, the canyons and passes become wind tunnels, constricting the flow of air through narrow passages and increasing its velocity. The interplay of rugged topography and the shifting of the high-pressure center causes sudden changes in wind direction and velocity. In concert, these physical variables produce hot, dry, erratic winds that blow across slopes clothed in chaparral and down chaparral-filled canyons.

Extreme fire weather is not confined to fall, even though this period is normally regarded as the chaparral wildfire season. The catastrophic episodes of widespread chaparral wildfires that visit California with distressing frequency are almost always whipped up by the easterly winds. They can drive choking clouds of smoke across populated valleys and far out to sea, as shown by photographs taken from space (pl. 12). Wind-driven chaparral wildfires have been described from the time Europeans first arrived in California. In December 1793, from his ship, the English explorer George Vancouver saw the following, just south of San Diego:

Plate 12. Smoke and ash from chaparral wildfires blown out to sea by Santa Ana winds, October 2003. The largest cloud (right) is from San Diego County, the smaller cloud (center) is from the San Gabriel and San Bernardino Mountains of Los Angeles and San Bernardino Counties, and a third (upper left) is from Ventura County. There are much smaller plumes visible from Baja California to the south, and San Luis Obispo County to the north.

During the forenoon immense columns of smoke were seen to arise from the shore in different parts.…These clouds of smoke containing ashes and dust soon enveloped the whole coast…The easterly wind still prevailing, brought with it from the shore vast volumes of this noxious matter, not only uncomfortable to our feelings, but adverse to our pursuit, as it intirely [sic] hid from our view every object at the distance of an hundred yards…by the prevalence and strength of the north-east and easterly wind, spread to a very great extent. Large columns of smoke were still seen rising [the following day] from the vallies [sic] behind the hills, and extending to the northward along the coast….Under these circumstances it cannot be a matter of surprize that the country should present a desolate and melancholy appearance. (Lamb 1984, 1115–1116)

Sometimes rain does not come until well into winter, prolonging the fire season past the end of the calendar year. There have been large chaparral wildfires in California every month of the year. In March 1964, at the time when moisture in the soil and in the plants would normally make chaparral shrubs virtually fireproof, simultaneous chaparral wildfires driven by Santa Ana winds in three southern California mountain ranges burned a total of 11,000 acres.

Temperature

A mediterranean climate may feature moderate or extreme temperatures any time of the year. The most moderate climate prevails near the ocean where there may be only a 10 degree F fluctuation in average temperature over the entire year. This is one reason why many people prefer to live along the coast. Westerlies carry air from the ocean inland varying distances that depend on topography. The moderating influence of the sea air is blocked by mountains, and gradually diluted by inland air. Away from the immediate influence of the ocean, the temperatures may reach extremes and are often unpredictable from one day to the next. For example, although uncommon, frost can occur any day of the year in the inland mountain chaparral, and snow may be frequent at times in winter (pl. 13). Large daily fluctuations in temperatures are common as well and are superimposed on the seasonal patterns. In the chaparral of central California, for example, it is not uncommon for the springtime dawn temperature to be freezing, whereas the noontime temperature reaches over 100 degrees F. Winter temperatures rarely stay below freezing for more than a short time, however, particularly in southern California. This is the reason that oranges and other citrus crops can be grown in the primarily gentle climate of the valleys below chaparral-covered hillsides (pl. 14). Occasionally, when the temperatures suddenly plummet in agricultural areas, smudge pots, fans, or electric heaters must be used to prevent freezing and destruction of the crop. It is advisable to have a sweater or jacket with you at all seasons of the year in the chaparral, as a warm spring afternoon can quickly become a cold evening.

Plate 13. Snow-covered manzanita chaparral with cypress trees in the background (Guatay Summit, San Diego County).

Microclimates

Microclimates are variations in climate that occur at a small spatial scale. They are created by localized factors such as slope steepness, presence of rocky areas or ravines, and differences in the exposure of slopes on the south and north sides of mountain ranges. These physical factors affect the quantity of sunlight and heat a slope receives, and therefore how moisture is distributed and retained. North- and east-facing slopes, for example, are shaded from direct radiation during the hottest part of the day, as are deep ravines. These sheltered places are consequently cooler and moister than south- or west-facing slopes or broad, open mountainsides. Because of a relatively high moisture level in these protected locations, the chaparral shrubs there are often taller and more lush than on exposed slopes. These are favored summer resting and feeding places for deer. Daily temperature fluctuations also tend to be less extreme on these more sheltered and moist slopes because the water in the air serves to buffer changes in air temperature. The contrast between shaded and exposed slopes can be significant, as can be seen in pl. 15. On adjacent north- and south-facing exposures, there are differences in the height, cover, and species composition of the shrubs. Similarly, rock outcrops provide shaded areas and protected recesses in the midst of open chaparral. They also affect local temperature and moisture conditions such that ferns and other delicate plants can be found in ledges, crevices, and at the base of boulders. These microclimates provide rich habitats for species of rodents, reptiles, and invertebrates that otherwise would be absent from mature chaparral. In addition, the crevasses and recesses afford shelter to animals that otherwise might be killed by wildfires.

Plate 14. Chaparral-covered hillsides behind the citrus orchards of the Ojai Valley, Ventura County. The vertical white structures in the orchards are wind machines, used occasionally on cold winter nights for frost protection.

Plate 15. Differences in climate on north- and south-facing slopes favor different chaparral shrub species. Chamise (in flower) dominates the south-facing slope on the left, while ceanothus and scrub oak dominate the cooler and moister north-facing slope on the right.

Finally, two different environments in the mature chaparral are created by the plants themselves. The first one is quite visible: the dense, leafy shrub canopy of uniform height and interlacing branches that may be continuous for hundreds of acres in all directions, creating an uninterrupted surface. The air above and around these leaves is dry and hot during the day, cold and somewhat moister during the night. While there is some exchange of air around the leaves, for the most part this top layer takes the brunt of daily and seasonal temperature fluctuations. Below the exposed canopy, on the other hand, is a second environment where the microclimate is cooler because the ground is perpetually shady, and where variations in temperature and humidity are much less extreme than at the canopy. This space beneath the shrub canopy is dimly lit and bare except for a sparse leaf litter (pls. 2, 16). The activity of most animals is concentrated in this understory landscape (see chapter 5). After fire, the ground surface is exposed, resulting in a harsher environment, totally different from its prefire state.

Convergence

The overall effect of climate can be seen in the phenomenon of convergence. Convergence is the similar appearance of unrelated organisms due to common environmental conditions. The climate is the most important factor in shaping organisms' physical appearance. This is because climate determines the major physical factors directly impinging on an organism, such as the temperature, moisture, and seasonal patterns. For the five regions of mediterranean climate (California, central Chile, western Australia, South Africa, and the Mediterranean Basin), convergence in overall appearance of the shrubs is obvious even to the casual observer. Compare the other four mediterranean climate regions in pls. 17–20 to the photographs of California chaparral scattered throughout the text. A blue-green blanket of evergreen shrubs covers the hills equally, whether in Chile or Mediterranean Europe. In all of the mediterranean climate regions of the world the dominant plant species in many areas are drought tolerant, evergreen, sclerophyllous (hard-leaved) shrubs and small trees. Many of these also have the ability to resprout after a fire from roots, stems, or burls. The individual shrub species look alike superficially from one region to the next, even though they are not genetically related. For a thorough and well-illustrated discussion of convergence in plants of mediterranean climate regions worldwide, see Plant Life in the World's Mediterranean Climates, by Peter R. Dallman, listed in the Supplemental Readings and References section.

Plate 16. The top of the shrub is usually hot and sunny, while the under-story remains cooler and shady beneath this manzanita.

Plate 17. Mature fynbos, a chaparral-type shrub vegetation, found in the Cape Province, Republic of South Africa.

Plate 18. Matorral in the Gredos Mountains of central Spain. Chaparral-like vegetation grows in many places around the Mediterranean Basin.

Plate 19. This shrub vegetation of Western Australia has a similar appearance to that of California and other mediterranean climate areas.

Plate 20. The matorral of central Chile looks similar to the chaparral of California.

Convergence also occurs among animals specialized to a particular climate type. For example, species of kangaroo rats (Heteromyidae) of the California chaparral (see chapter 5) have a morphological and behavioral equivalent in the jerboas (Dipodidae) that inhabit arid parts of Eurasia and Africa. Both groups of animals use the same hopping form of locomotion, live in burrows, and depend entirely on seeds for their food and water. Other animals depend on houses or burrows to protect them during the day and forage in the shrubbery at night in similar fashion in each climate region. While each organism is a unique product of its genetics and environment, it is remarkable that the same solutions to the same environmental problems arise independently over and over again. Convergence is a testament to the power of the environment in shaping the plants and animals around us.

The following story illustrates how one chaparral insect responds to the sudden availability of water in an unpredictable environment. A second story about tarantula hawks (Pepsis spp.) in chapter 5 shows another way a native chaparral insect obtains scarce water and food in chaparral. These special adaptations represent fine-tuned responses to the variable climate of chaparral.

Rain Beetles Mate Only When There Is Rain

A success story of survival in the water-limited climate of the California chaparral concerns the rain beetles, a unique kind of scarab (Pleocoma spp.) well adapted to the extended droughts. Around 30 species of rain beetles are found in chaparral and woodlands between southern Washington State and Baja California. Adult females are olive shaped, about two inches long, and males are somewhat smaller and more streamlined. The oval body is shiny on top with dense hairs pointing downward on the underside (pl. 5). Only the males have fully developed wings. Rain beetles spend the greatest part of their lives as larvae in feeding tunnels deep underground, where they are protected from the rigors of heat and drought. The only time these insects are above ground is during winter when it is raining or snowing. Then the air is cool and moist enough that the rain beetles can maintain their water balance and avoid overheating. These beetles are specifically tuned to and dependent upon rain to complete their life cycles. The adult males emerge above ground only in response to rain, and then they fly as the raindrops are falling around them, even in a cold winter rain. Flying in the cold requires a great deal of energy because the males must maintain a body temperature almost as high as ours in order to fly well. Since adults cannot eat (they have no mouths) and must live on fat stored when they were larvae, a male on the prowl has only enough energy reserves to fly for a total of about two hours. Most flights occur at dawn and last only 20 minutes, so males potentially have about six days of rainy flying time in which to find a mate. Whether or not mating has occurred, once the male's energy supply is used up, he dies. When it begins to rain, males emerge at the surface in a matter of minutes. In search of a mate they fly back and forth across the chaparral close to the ground where the females wait at their burrow openings. Females signal their location to the males by emitting an airborne hormone, called a pheromone, as an attractant. When the male discovers the burrow of a female, he enters to mate. Once mated, the female plugs the opening to the burrow and returns to the depths of the soil. Here she waits until early spring when she lays eggs in tightly packed, fine soil. The eggs are large, up to .2 inches in diameter, and require a great deal of energy to produce. When her egg laying is finished the adult female dies.

The entire mission of adult rain beetles is reproduction. These insects do, however, have very long lives below ground. Rain beetle larvae are thought to live eight to 13 years in the soil, where they eat the roots of shrubs and trees. The larvae are also excellent diggers with strong jaws for chewing their way through soil. The larvae grow slowly, molting once a year until they reach maturity. In the final molt they become pupae that persist for a few weeks and then transform into the adult stage. The adults then remain underground until the rains come.

The complex life cycle of rain beetles is adapted well to the environment of the chaparral. Not only can rain beetles survive for long periods without the benefit of rain, but also they respond immediately when it does rain. Rain beetles have one further mechanism that lets them exploit the cold, wet times of the year even when temperatures are below freezing. Male rain beetles can vibrate the muscles of their thorax (the middle body section), thus raising their body temperature to the level necessary for flying. The thick covering of hairs on their undersides acts to preserve this heat in the same way that hairs help insulate mammals from cold temperatures. The hairs also keep wet, cold mud from sticking to their bodies. This kind of a load could weigh them down and require more energy for movement. The rain beetles are truly well adapted to rigors of life in the difficult climate of chaparral, reproducing when there is water and able to wait out years when there is not enough.