Shrublands in California: Literature Review and Research Needed for Management
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SHRUBLANDS IN CALIFORNIA: LITERATURE REVIEW AND RESEARCH NEEDED FOR MANAGEMENT edited by Johannes J. DeVries GJANNIN(R44DAT1ON OF • AG RICUL-TURAL4fpNOM LIBRA PY, i JAN a 0) 985 CALIFORNIA WATER RESOURCES CENTER University of California Contribution No. 191 ISSN 0575-4941 November 1984 2. Biogeography and Prehistory of Shrublands' Abstract: Chaparral covers dissected, eroding mountains of wide substrate diversity from northern R. Minnich and L Howard' California to northern Baja California. The mediterranean climate of this ecosystem is characterized by decreasing precipitation, but mostly cooler summers, as stands shift from the interior mountains toward the Pacific coast and increase in elevation with decreasing latitude. Stand species composition varies greatly with important differences at the genus level between northern California and southern California. Unless This chapter will concentrate on evergreen scrub or herbaceous vegetation establishes permanently during "hard chaparral" because of its extent and its postfire succession, chaparral is stable under a importance in management. Less attention will be paid wide range of fire regimes. These include to coastal sage scrub or "soft chaparral," which suppression, which yields infrequent large intense generally burns with more frequency but less intensity fires, and no fire control (with deliberate than does the evergreen scrub chaparral. burning), which yields complex stand mosaics from mostly small low intensity fires. Given current The evergreen scrub chaparral formation covering the lightning incidence, long-term successional mountains and foothills of California (Figure 1) is flammability periods, and season-long fire comprised of deep-rooted, evergreen sclerophyllous endurance, complex stand mosaics could develop shrubs 1 to 5 m tall, interwoven in carpet-like stands without fire control from natural ignitions alone. on infertile coarse-textured soils. Shrubs are compact Examples are given to illustrate the possible role with numerous stiff stems. Leaves are mostly small, of moisture and fire gradients on chaparral simple, and relatively hard. Herbaceous vegetation is biogeography. mostly absent in mature stands. Important genera include Adenostoma, Arctostaphylos, Ceanothus, Ouercus, Cerocarpus, Rhus, Rhamnus, Prunus, Garryg, and Heteromeles. The climate is mediterranean with mild, wet winters and warm dry summers. The growing season is normally from March to June after soils are quasi-mature equilibrium states (climax) in response saturated by winter storms and temperatures have to climate, and more locally, to geology and to warmed, but before the summer drought. During summer topography. Many presumed stable systems however, and fall, shrubs maintain limited metabolic activity, may in fact survive in time because they are relying on small moisture reserves in the regolith flexible to disturbance. Only recently has the until initial autumn or winter rains. Wildfire is a prospect that chaparral taxa evolved in different recurrent feature to this ecosystem in which stands habitats than they now occupy been given serious repeatedly undergo cycles of pryolysis and regrowth. consideration. The understanding of chaparral biogeography, that It is also assumed that organisms comprising a is, why member taxa live where they do, is an elusive natural community have had a shared history when in task requiring information on ecology, plant fact each species has a unique history of evolution physiology, and plant dynamics (prehistoric and and migration. Sauer (1977) states, "...community contemporary). Such information is either rarely evolution is a consequence of individual species available or so sparingly sampled that.provacative evolution and individual species migrations, which explanations are the best that can be offered. together result in loose, changeable associations Sparse macrofossil and pollen data reveal only that among species at any given place or within any a crude picture of chaparral ancestral histories. specified habitat." Indeed, the modern ranges of Ecological analyses of current vegetation is many taxa are not "faithful" to chaparral as necessarily multivariate, utilizing systems approaches, evidenced by their occurrence in forests and deserts and equilibrium models rarely extrapolate very far beyond the mapped range of this ecosystem from the sample sites. The problem also concerns Figure 1). beliefs held by those studying the nature of ecosystems, which are normally classified on the basis Since the primary purpose of this chapter is to of some combination of vegetational and environmental describe modern chaparral distributions, we do not criteria. The plant communities are perceived to reach intend to treat this explanatory theme comprehensively. Nevertheless, it is pertinent to provide examples illustrating the divergent ecological nature of chaparral taxa over a wide range of environments. Such must be appreciated by ecologists and land managers who must focus upon the uniqueness of their respective areas. Some 1Chapter 2 in arublands in California: Literature literature cited in this section is treated more Review and Research Needed for Management, Water comprehensively in Reid and Oechel, Chapter 3. Resources Center, Contribution No. 191, University of California, Davis, November 1984. 2Assistant Professor of Geography and Graduate Research Assistant respectively, Department of Earth Sciences, University of California, Riverside. 8 U < U w 0 < w < U < 6 0 U cr < c.) < >- < 0 w 0 < cc < z cc ERICACEAE LL < 0 RHAMNACEAE subg. subg. cearlothus cerastes H „ Z.12 DO a,)7 .zith, g, ? -,,...G0F- , -„,i-Ntn .,,, RO OPR E.H2OW Dani),U Mth.;MAkWi c 'ifts &KWUC.WQ C.i<WQCSciUcleE, cci, .4',icfci .4,i,iqN O000000 ci o i o a a 1 1 i 1 1 N., 4 'I t.,, - / L. (v. I 1 'It/14. 1 I 1 1 \T ! ‘i % 1 , , 1 i . i....., • . ,...rc----.. 1. • $ ,, ,4 I I II1 1 •I 0-11,, , •_,,,... 7. t• 4t 1 N I -7, 1 1 I -I\ sZ. 1 i r III t-b- •71r... - •IT „ , NI1 I I I .,, a, 4%,..*--. !I ill 'CLP.,:',. N!--). IT 1 I 1 1 •.., , ,,, , i•,....,..-,..,, • .,..,„, ,,. , ,._,.Li , • , . , , 1 ' .,. LAT. 35° —,.,......., .T ., Il....- r • 1 1 • Ata. - • • 7.7 GEOGRAPHIC RANGE FROM THE PACIFIC COAST N OF LAT. 35° S OF LAT. 35° COAST RANGES, EXCLUSIVELY NEAR COAST RANGES (SANTA YNEZ, SANTA MONICA, RANGE EXTENSIONS BEYOND CALIFORNIA SANTA ANA MOUNTAINS, CHANNEL isLarios, COASTAL SIERRA JUAREZ) • OREGON INLAND RANGES (WESTERN —•—•—•—•—•—•—•— SIERRA NEVADA, EXCLUSIVELY TRANSVERSE RANGES. SAN 0 CENTRAL BAJA CALIFORNIA GABRIEL. SAN BERNARDINO, SAN JACINTO, NuTA ROSA, SAN YSIORO, VOLCAN, LAGUNA + SOUTHERN BAJA CALIFORNIA ro NTAINS, INTERIOR SIERRA JUAREZ. • SOUTHERN MEXICO SIERRA SAN PEDRO MARTIR) ti ARIZONA, UTAH, NEW MEXICO AND/OR TEXAS BOTH COASTAL AND INLAND MOUNTAINS DESERT MARGINS OF INLAND MOUNTAINS • COASTAL AND INLANO MOUNTAINS. AND DESERT MARGINS ••••••••••••••• iNLAND MOUNTAINS AND DESERT MARGINS Figure 1: Chaparral distribution and geographic ranges of dominant shrubs. Sources- Map: Vegetation Type Map (VIM) Survey (California) and DETENAL aerial photography (Scale 1:50,000, Baja California); Species ranges: Munz and Keck (1959), Wiggins (1983), Moran (1977). 9 PREHISTORY CURRENT DISTRIBUTION The paleobiogeography of chaparral involves the Chaparral covers approximately 4 million ha from migration and evolution of drought-adapted shrubs northern California to northern Baja California with the developing mediterranean climate, which (Fig. 1). Although chaparral occurs on a wide came to full expression along the California coast diversity of substrate over its range (California after the mid-Miocene as the result of several 1977), stands are characteristically restricted to global and continental scale processes (Lamb 1979; dissected mountain ranges having steep, rapidly Frakes 1979). Poleward movement, cooling, and eroding slopes. eventual glaciation of the North American, Eurasion, and Antarctic continents over the Tertiary intensi- The northernmost populations occur on steep fied the latitudinal atmospheric temperature southern exposures on the Trinity River and at gradient, the jet stream, and Hadley subsidence at Shasta Dam on Paleozoic metasediments and granite. subtropical latitudes. The increasing continental- Stands then run more or less continuously along the ity of climate in the interior western United States eastern scarp and adjacent hogback ridges of the was enhanced by Pacific coast mountain building north coastal ranges from Shasta Dam to Clear Lake which cast rain shadows over the Great Basin plateau on lower Franciscian, upper Cretaceous, and Jurassic and confined temperate environments to the Pacific sandstones, shales, and conglomerates, with many coast, lower elevations of Arizona, and northwestern serpentines. Chaparral becomes more widespread on Mexico. The uplift of the Mexican plateau created Franciscian sediments in the dissected ranges another rain shadow by cutting off tropical moisture surrounding Clear Lake and the Mayacmas Mountains to arriving into the region from the Gulf of Mexico in the west, and on Tertiary volcanics along steep summer easterlies. The upwelling of ever-colder ridges between Santa Rosa, Napa, and Bernyessa oceanic bottom waters along the California current Valleys to the south. An isolated large stand (Schopf 1980) stabilized the coastal marine layer, covers Mt. Tamalpais north of the Golden Gate. nearly eliminating convectional precipitation of the Pacific Ocean moisture during the summer. As a South of San Francisco Bay,