Introduction
Mediterranean climate types can be found along the west side of continents in mid-latitudes from 30° to 50° N and 30° to 40° S latitudes, commonly in a belt of prevailing westerly winds (Fig. 1). Shrub communities are common in the Mediterranean climate type. In Chile and Spain these Mediterranean shrub communities are called matorral, in France it is called maquis and in Italy, macchia. In South Africa it is known as fynbos and in southwest Australia kwongan.Oak woodland and grasslands also occur in this climate type.
These Mediterranean climate types are classified under the Köeppen climate classification system (Kottek et al. 2006) as “Cs”. The “C” stands for warm temperature climates, where the average temperature of the coldest months is 64° F. The “s” stands for a dry season in the summer. In the winter the Mediterranean climate, is mild and moist. During the summer it is very hot and dry.
The annual temperature range is between -1° and 38° C (30° and 100° F). Most of this biome only gets about 25-43 cm (10-17 inches). However, rainfall in California ranges from less that 10 inches in southern desert areas to greater than 100 inches on the north coast. On average foothill oak-woodland precipitation ranges from 15 to 32 inches. Most rain comes in the winter. Winter precipitation is primarily the result of cold fronts that move across the Pacific Northwest and northern California. The frequency of these storms decreases from north to south.
Location and Distribution
California’s Mediterranean-type rangelands include the oak-woodlands, annual grasslands and chaparral. According to the California Wildlife Habitat Relationships Database (Mayer and Laudenslayer 1988) there are more than 29 million acres of oak woodlands, annual grasslands and chaparral (Table 1). The herbaceous understory of these rangelands is dominated by annual grasses and forbs that invaded California during European colonization. California’s annual rangelands occur in an intermittent ring around the Central Valley and at lower elevations in the coast mountains and foothills (Fig. 2).Weather and Forage Production
Four factors—precipitation, temperature, soil characteristics, and plant residue—largely control forage productivity and seasonal species composition. Precipitation and temperature control the timing and characteristics of four distinct phases of forage growth: break of season, winter growth, rapid spring growth, and peak forage production (George et al. 2001). Management decisions may be guided by these patterns, and as the season progresses patterns become set and the outcome becomes more predictable. The climate of the annual rangelands is Mediterranean, with precipitation falling primarily between October and May, mostly as rain (Fig. 4). The dry season averages 6 months but may range from 2 to 11 months. Precipitation in the annual rangelands decreases from north to south and increases with elevation. Foothill oak-woodlands generally occur at an elevation of 200 to 2300 feet.Drought
At least eight multiyear periods of low precipitation have occurred in California since 1900. Droughts that exceed three years are uncommon, though occurrences in the past century include 1929-1934, 1947-1950, and 1987-1992. Tree ring analysis suggests additional extended droughts prior to 1900 (Table 5) (DWR 1993) . Severe droughts in 1850-1851 and 1862-1864, together with other factors, have been implicated in the conversion of the former native perennial grassland to a grassland dominated by annual grasses and forbs (D’Antonio et al. 2007).In some years poor precipitation results in forage production that is 50 percent or more below average. Because the amount and dependability of precipitation increases from south to north and with elevation the frequency of years with forage production less than 50 percent of average varies greatly across the state’s Mediterranean-type rangelands. Analysis of annual forage production data from 26 locations in California’s annual rangelands reveal that a 50 percent reduction in range forage production rarely occurs north of Sacramento (George et al. 2010, Appendix A). Forage losses of 50 percent are more common in the rain shadow of the Coast Range adjacent to the west edge of the San Joaquin Valley.
Literature Cited
Bentley, J. R. and M. W. Talbot. 1951. Efficient use of annual plants on cattle ranges in the California foothills. U.S.D.A. Circular No. 370. 52 p.D’Antonio, C.M., S.J. Bainbridge, C. Kennedy, J.W. Bartolome, and S. Reynolds. 2006. Ecology and restoration of California grasslands with special emphasis on the influence of fire and grazing on native grassland species. A report to the David and Lucille Packard Foundation. University of California, Santa Barbara, CA.
Department of Water Resources (DWR). 1993. California’s 1987 – 1992 Drought. Sacramento, CA: Department of Water Resources. 38 pgs.
Duncan, D.A. and R.G. Woodmansee. 1975. Forecasting Forage Yield from Precipitation in California's Annual Rangeland. Journal of Range Management, 28: 327-329.
George, M. R., C. A. Raguse, W. J. Clawson, C. B. Wilson, R. Willoughby, N. K. McDougald, D. A. Duncan and A. H. Murphy. 1988. Correlation of degree-days with annual herbage yields and livestock gains. J. Range Manage. 41:193-197.
List of Tables
Table 1. Area of annual rangeland vegetation typesTable 2. Influence of normal weather variations on timing of seasonal dry matter (DM) forage production in California's annual rangeland ecosystem.
Table 3. Germination date, monthly and peak standing crop sinec 1979-80 growing season at the University of California Sierra Foothill Research and Extension Center in Yuba County, California.
Table 4. Elevation, production, sample size (n), and frequency of forage losses exceeding 50 percent of average forage production at 26 locations in California's annual rangelands.
Table 5. Sacramento River multiyear droughts (reconstructed from tree rings prior to 1900).