SHRUBLANDS IN CALIFORNIA: LITERATURE REVIEW AND RESEARCH NEEDED FOR MANAGEMENT

edited by Johannes J. DeVries

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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.

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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, chaparral occurs result, precipitation in California has become principally on the Franciscian formation along the increasingly limited to frontal storms when the summits of the central coastal ranges, including the Polar-front jet stream extends equatorward to the east scarp of the Santa Cruz Mountains, northern region during the winter season. The only check to Diablo Range, and coastal Santa Lucia Mountains. this trend of progressive drying over the Tertiary Large stands also occur on granite in the northern has occurred during Pleistocene full-glacial periods Santa Lucia and Gavalon Mountains and unconsolidated when southward shift of the jet stream increased Miocene and Pliocene sediments in the southern wintertime precipitation (cf. Street and Grove Gavalons, and La Panza and Lopez Mountains in San 1979). Luis Obispo County.

The fossil record provides strong evidence that Chaparral is relatively infrequent on the west during most of the Tertiary, the world's climate was slope of the Sierra Nevada Mountains owing to the mainly warm temperate, and free of zones of extreme prevalence of old erosion surfaces on this westward aridity at subtropical latitudes (Lamb 1979; Frakes dipping tilted block. North of the Tuolumne River, 1979). From the Eocene to the Miocene, southern stands are found mostly on southern exposures of California and areas eastward to New Mexico were major gorges or on hogback ridges near Placerville covered by oak-laurel-pine forests, thorn scrub, and and San Andreas. As the west slope steepens toward subtropical dry forests strongly resembling modern the south, chaparral increases into a narrow zonal vegetation in northwestern Mexico south of Sonora belt from the Tuolumne River to the Kern River, where warm-season rain is abundant (Axelrod 1979). expanding locally on southern exposures of the Many progenitors of modern chaparral flora in Merced, San Joaquin, Kings, and Keweah Canyons, and California were already recorded in Eocene sub- west and east flanks of the Greenhorn Mountains. tropical dry forests and oak-laurel forest floras in Scattered patches occur along the Tehachapi Ranges. New Mexico, including Rhus, Quercus, Cercocarpus, Most Sierra Nevada chaparral covers granitic Prunus, Fremontodendron, and Rhamnus (Axelrod 1979). substrate of the batholith, although small populations also occur on Mesozoic volcanics and During the middle Tertiary, evergreen Franciscian sediments (some serpentine) on the sclerophyllous shrubs spread westward with the Placerville hogbacks, and in Paleozoic metasediments expanding dry climate to Nevada and California and metavolcanics east of Chico. (Axelrod 1975). Most shrubs first appear in abundance in California during the mid-Miocene, as Chaparral is most widespread in the highly evidenced by the Tehachapi, Mint, and Mill Creek dissected mountains of southern California, forming floras which contain many nearly modern species almost unbroken cover along the Transverse Ranges (Axelrod 1979). These include Arctostaphylos from the Cuyama Gorge east to the San Bernardino (pungens), Ceanothus (cuneatus, crassifolius), Mountains, and the Peninsular Ranges from the San Cercocarpus (betuloides, ledifolius) Quercus Jacinto and Saqta Ana Mountains southward to the (turbinella, wislizenii), Rhamnus (californica, Mexican Border and beyond to the Sierra Juarez and crocea), Rhus (ovata), Fremontodendron Sierra San Pedro Martir in northern Baja (ca-Mornicum), and Prunus (ilicifolia). These California. In the western Transverse Ranges (Santa shrubs were still prO5iiiiif with oak-laurel and Barbara and Ventura Counties), Channel Islands, subtropical thorn forests, which suggests that cur- Santa Monica Mountains, and coastal Peninsular rent mediterranean conditions were not yet extant. Ranges from the Santa Ana Mountains to near Post-Miocene records, however, show depletion or Ensenada, chaparral occurs on Jurassic, upper extinction of tropical elements and the primacy of Cretaceous, and Eocene sedimentary rocks; and evergreen sclerophyllous shrub and woodland vegeta- Tertiary volcanics (see also Gastil et al. 1971). tion in temperate southern and central California. Much island chaparral has been grazed to destruction Continued cooling acted to more and more confine by feral goats, sheep, and pigs and replaced by chaparral to its present limits of California, exotic grasslands (Minnich 1980a; Brumbaugh 1980). mountainous Arizona, and northwestern Mexico. In the inland ranges it grows on granitic substrate 10 of the southern California batholith or local bodies tions sweep the mountains and coastal basins from of Precenozoic metamorphic and metavolcanic rock the desert interior. These occasionally produce complexes. Stands are absent from mountainous gale to hurricane force winds (20 to 35 m/s) terrain only in areas with deep soils underlain by associated with surfacing mountain waves and foehn unconsolidated Miocene sediments or scattered old flow in narrow jets through low mountain passes erosion surfaces including the Santa Susanna (Fosberg et al. 1966; Schroeder et al. 1969; Ryan Mountains, portions of the Santa Monica Mountains, 1969). The great relief of Sierra Nevada impedes the Puente Hills, Palos Verdes Peninsula, and inland offshore flow over most of central and northern mountain valleys of San Diego County (Santa Ysabel, California, where weather is dominated by persistent Julian, Henshaw basin). tule fogs and ground inversions in the Central Valley and valleys between the coast ranges. Most In the increasingly arid south, chaparral even of these chaparral areas are cool, humid, and foggy, prevails on old erosion surfaces as well as on the in sharp contrast with generally desiccating steep slopes (owing to limited soil development), conditions in southern California. covering broad mountain plateaus of southern San Diego County, the Sierra Juarez and the Sierra San California climate during the warm half of the Pedro Martir. Small patches also cover the highest year is influenced by the coastal marine (trade peaks of central Baja California, including the wind) layer, superior air aloft, and local Sierra Borja (lat. 29° 30'), Sierra Calmalli (lat. mountain-valley wind systems. Air masses passing 28°, 30'), Sierra Santa Monica (lat. 27°), and over California are almost always warmer than the Volcan las Tres Virgenes (lat. 27°). A few species, cold, upwelling waters of the California Current. notably Malosma laurina, Heteromeles arbutifolia, Thus, when regional circulations permit, a layer of and Prunus iliciTOITiToccur with montane thorn cool, moist air with associated stratus forms over scrub as far south as the Cape range near La Paz. the ocean and moves onshore onto the coastal plains and adjoining mountains.

CLIMATE During late spring (April through June), weak perturbations in the jet stream sustain a relatively The mediterranean climate of California chaparral deep marine layer (1000 to 2000 m), and coastal shifts along the latitudinal gradient from condi- valleys and mountains are enveloped in moist air tions resembling marine west coast climates in which has been little modified from its beach northern California to a fog desert in northern Baja characteristics, to elevations equivalent to the California. Local and regional differences are layer's vertical extent. Cool, cloudy weather not further influenced by interactions between the only covers the coast ranges, but also spills into general atmospheric circulation, California current the Central Valley to the western flank of the and topography, as well as elevational zonation and Sierra Nevada and into the interior mountains of distance from the coast. The winter rainy season, southern California. Troughs in the jet stream summer drought, and absence of severe cold are the occasionally produce air mass instability and most binding features of the climate throughout the thunderstorms over the highest mountains north of range of this formation. the vicinity of Clear Lake and Lake Tahoe. In southern California, deep marine layer conditions During late fall and winter (November to March), are normally associated with persistent stratus rainbearing frontal disturbances move over overcast between 500 and 1500 m which is related to California from the north Pacific Ocean. The a cyclonic low-level circulation in the southern frequency of storms increases northward toward the California bight, termed the "Catalina Eddy" (Bailey mean winter latitudinal position of the jet stream 1966, Richardson 1975). Most California chaparral at an approximate latitude of 40° to 45°. Stormy experiences cool humid weather except stands along days exceed fair weather periods in northern the margins of the Mojave and Colorado deserts. California, while 5 rainy days per month is Ridging and subsidence may collapse the marine characteristic in the south. Annual precipitation layer, resulting in warm dry weather throughout the through the range of chaparral varies from 400 to state. High surface pressure over the Pacific 1200 mm. Northwest occasionally causes northerly (Santa Ana-like) winds in the northern Sacramento Valley. The orographic lift of moist, southwesterly flow in advance of cold fronts produces strong mountain During summer (June to September), frontal storms precipitation gradients that influence chaparral move north of California and the marine layer is distribution. In northern California, stands tend normally shallow (150 to 500 m) above which to cover lee slopes of the north coast ranges and chaparral-covered mountains are exposed to warm, dry the lowest slopes of the northern Sierra Nevada air masses aloft. The marine layer is also rapidly which benefit least from orographic lift. With de- modified by diabatic heating as it moves inland with creasing latitude, chaparral gradually shifts toward prevailing low level gradient flow and sea breezes, a Pacific slope orientation, with stands covering dissipating along the inland central coast ranges at both windward and leeward slopes of the central the west margin of the Central Valley and interior coast ranges south of the Mayacmas, and in southern ranges of southern California (Edinger 1959, 1963). and Baja California, mostly the Pacific slope. Stands also increase in elevation southward along Considerable Pacific moisture still reaches as the length of the Sierra Nevada into heavier far as the inland coastal mountains because of windward slope orographic precipitation zones. limited mixing associated with the trade wind inver- sion (Edinger et al. 1972). Weak ambient winds Fair weather periods during winter are associated aloft and high insolation permits the development of with ridging and poleward movement of the jet stream upcanyon valley winds along coastal mountains which into northern California or the Pacific Northwest. withdraw marine air, humidity, and cooler air con- Passing storms over the western United States are fined at lower levels by the inversion onto higher often followed by strong northerly jet streams aloft slopes (Edinger et al. 1972, Coffin 1959). Further and high surface pressures over the Great Basin. In movement of valley winds over rough terrain toward southern California, strong Santa Ana wind circula- the central valley or the desert, however, results 11 in rapid mixing with drier air aloft, increasing radiata on Cedros Island. A. sparsifolium is most temperature, and decreasing humidity to levels abundant in the interior Peninsular ranges from the equivalent to upper level air mass characteristics. San Jacinto Mountains southward to the Sierra San Nocturnal air drainage and downslope katabatic winds Pedro Martir. expose most chaparral areas to dry air aloft at night (Schroeder et al. 1967; Fosberg and Schroeder Ceanothus is divided into two subgenera, 1966) except stands within the marine layer along Euceanothus and Cerastes, which have contrasting the immediate coast. ecologies. Euceanothus, the more mesic of the two subgenera, comprises mostly successional shrubs in The influence of the marine layer on the summer conifer forests which are not considered here. climate of chaparral varies over the state. In the Among those occurring primarily in chaparral, north coast ranges and the western margin of the Ceanothus foliosus, C. thyrsiflorus, C. sorediatus, Mojave and Colorado Deserts, where chaparral is C. leucodermis, and C. dentatus are prominent in the mostly confined to lee slopes, and along the length northern and central coast ranges, while only C. of the Sierra Nevada, stands experience mostly hot, leucodermis is widespread in the Sierra Nevada. In dry summers. With decreasing latitude, however, southern California, C. spinosus along the coastal chaparral shifts toward windward flanks of mountain ranges is replaced by C. oliganthus and C. ranges subject to marine air penetration and cooler leucodermis in inland ranges. Shrubs of this summers, especially the inner coast ranges including subgenus extend into northern Baja California but Mt. Tamalpais, Santa Cruz, Santa Lucia, Santa Inez, are uncommon south of central San Diego County; no Santa Monica, and Santa Ana Mountains, the Channel member extends onto desert margins. Islands and the coastal Sierra Juarez near Ensenada in Baja California. Inland coastal ranges, such as In subgenus Cerastes, only Ceanothus cuneatus is the Mayacmas, Gavalon, Transverse, and Peninsular widespread, ranging from Oregon to the Sierra San Ranges are also exposed to marine air through Pedro Martir, including the Sierra Nevada. daytime valley winds as long as the marine layer is Otherwise, only C. ramulosus forms wide cover in the present on their coastal flanks. Summers in the northern and central coast ranges. In southern chaparral to the south are also cooler owing to California, subgenus Cerastes forms a diverse hybrid increasing elevational zonation of stands. continuum from coast to desert, including C. megacarpus var. insularis, on the Channel Islands; Surges of tropical moisture into California are C. megacarpus on the coastal faces of the Santa advected by prevailing southerly winds aloft from Ynez, Santa Monica, and Santa Ana Mountains; C. the equatorial Pacific through the Gulf of verrucosus in the near-coast ranges of San Diego California (Hales 1972). The air masses produce County and northern Baja California; C. crassifolius afternoon mountain thundershowers and numerous in the interior Transverse Ranges; C. greggii lightning fires from early July to mid-September perplexans in the interior Peninsular ranges; and C. (Komarek 1968, Keeley 1982). Thunderstorms are most greggii vestitus along the desert margin. These frequent in southern California from the San shrubs form extensive stands with Adenostoma Bernardino Mountains southward because the eastern fasciculatum south of latitude 35° except C. greggii flanks of these ranges contact very moist, low level vestitus which normally occurs with desert chaparral air masses moving northward into the Coachella- pinyon-Juniper forests along the Mojave Desert Imperial Valley trough from the Gulf of California. margin of the southern Sierra Nevada, and the Increasing dilution of tropical air masses northward Transverse Ranges. is paralleled by declining thunderstorm frequency in the western transverse ranges, the southern Sierras, The genus Arctostaphylos is most diverse in and central coast ranges. Thunderstorms then in- northern and central California, especially along crease in frequency over the high Sierra owing the coast ranges. Important shrubs to be found west primarily to their great height. Most California of the Central Valley, exclusively, include A. chaparral experiences little summer rain except in stanfordiana, A. canescens, A. crustacea, A:—glauca, the eastern San Bernardino Mountains and interior and A. glandulosa. A. manzanita ranges eastward Peninsular ranges where the mean July through mid- into the Sierra Nevada to join A. mewukka and A. September precipitation ranges from 40 to 80 mm. viscida. Numerous closely related local endemic shrubs occur in serpentines of the Franciscian formation. Only A. glauca and A. glandulosa form FLORA important cover in southern California, but these are rarely dominant except above 1200 m or on desert The chaparral exhibits continuous floristic margins. However, A. glauca increases south of the turnover throughout its range (Fig. 1). Few species Mexican border where it occurs with extensive stands occur along the length of the chaparral belt, while of closely related A. peninsularis (Wells 1968) at many others, especially in diverse genera such as higher elevations Of the interior Sierra Juarez and Ceanothus and Arctostaphylos, have limited local the Sierra SantPedro Martir. Three other ericaceous distributions, which are often replaced, with shrubs, Comerostaphylos diversifolia, marginal overlap, by closely-related congeners. Anorthostaphylos oppositifolia, and Xylococcus While many shrubs occur exclusively in coastal, bicolor occur sporadically in the inner coast ranges inland, or desert margins of California mountains, of southern California and northern Baja California. approximately 40 percent range into other environments outside of California. Many have The genus Quercus contains fewer shrub species summer rain, including central Baja California, the having wider ranges than species found in Ceanothus Southwestern desert, and adjoining northwest Mexico. and Arctostaphylos. Quercus wislizenii is extensive on both sides of the Central Valley, but mostly as Adenostoma fasciculatum, the most significant an arboreal element of foothill woodland. South of shrub, ranges widely in coastal and interior ranges Ventura County, Q. wislizenii is a medium shrub to from northern California to northern Baja its southern limit in the Sierra San Pedro Martir. California, with isolated populations extending Q. durata in the northern and central coast ranges further south on the highest peaks of central Baja is replaced by closely related Q. dumosa south of California and beneath fog drip forests of Pinus the Santa Lucia Mountains. Q. dumosa occurs in 12 coastal and interior mountains through California THE FIRE REGIME BEFORE CONTROL and northern Baja California, and forms hybrid continua with Q. turbinella along the western Chaparral has doubtless supported widespread margins of the Mojave and Colorado Deserts, and Q. burning since the mediterranean climate developed on . macdonaldii on the Channel Islands (Q. dumosa x Q. the Pacific coast in recent geologic time because of lobata). Q. turbinella is accompanied by Q. dunnii high stand productivity, abundant fuel accumulation, on desert margins south of the San Bernardino and increasing decadence with stand age (see Reid Mountains, and Q. peninsularis (a close relative of and Oechel, Chapter 3). Most chaparral becomes ex- Q. emoryi of northwest Mexico and southeast Arizona) traordinarily flammable within 30 to 60 years after above 1300 m in the Sierra Juarez and Sierra San the previous burn, depending upon productivity, cli- Pedro Martir. mate, and topography (Green 1981; Philpot 1977).

Members of the Anacardiaceae, including Malosma Unfortunately, there is little direct evidence of laurina, Rhus ovata, and Rhus integrifolia, occur past fire in the geologic record. Occasionally, fos- south of Pt. Conception. Rhus integrifolia is sil charcoal is discovered (e.g.-Berger 1980; Byrne found on the Channel Islands and the mainland et al. 1977), but these data yield little coastal strip, while Rhus ovata and Malosma laurina information on the frequency, intensity, or site are widespread from the near-coast to the inland characteristics of past fires. In the same way, the ranges. Other lesser genera are represented by few extrapolation of fire regimes from plant adaptations species in California chaparral. Although most are is hazardous since many responses are generalized to scattered or even rare in most stands, they have many kinds of disturbances. The sprouting habit of widespread distributions both latitudinally and from chaparral shrubs, for example, is not pyrogenic coast to desert. These include Cercocarpus because this trait is common to most angiosperms and betuloides, Prunus ilicifolia, Rhamnus crocea, R. is probably ancient in origin. californica, Heteromeles arbutifolia, Fremontodendron californicum, Fraxinus dipetela, and Investigators of the prehistoric fire pattern in F. trifoliolata. California forests have attempted to reconstruct past fire frequency and stand mosaics from fire scars and dendrochronology (McBride and Lavin 1976; STAND COMPOSITION Kilgore and Taylor 1979) and sea floor varves (Byrne et al. 1977). Hardwood and coniferous trees living It is widely recognized that chaparral varies Tifth the chaparral (Pinus attenuata, P. coulteri), greatly in stand composition along environmental however, are subject to stand replacement fires gradients created by elevational zonation and (Vogl 1977; Talley and Griffin 1980; Vale 1979; diversity of slope, exposure, and elevation of the Minnich 1978, 1980a), stump sprout, as in Quercus. rough terrain on which this community is normally chrysolepis (Plumb 1977; Minnich 1980b), or survive found. Extensive sampling of this formation has on precipitous slopes and cliffs which often escape been conducted only by the Vegetation Type Map (VTM) fire altogether (Pseudotsuga macrocarpa, Abies survey (Weislander 1935; see also Colwell 1977, bractiata; Minnich 1980b; Griffin 1982) and quadrat data on file with William B. Critchfield, consequentlydo not lend themselves to fire scar Pacific Southwest Forest and Range Experiment analysis. Station, Berkeley, CA). This data has not been analyzed except for a few quadrangles (cf. Bradbury Since suppression has been a longstanding policy, 1974). Hanes (1977) and Payson et al. (1982) there is no uncontrolled landscape in California sketched rough classifications based largely on where suppression has not been practiced, thus pre- generic mixes along moisture gradients. Quercus cluding direct reconstructions within the state. chaparral and "mixed" chaparral (some combination of The impact of suppression on chaparral was deter- Quercus, Arctostaphylos, and Ceanothus) tend to mined by comparisons between the fire histories of dominate mesic sites, especially northern exposures. southern California and of northern Baja California Adenostoma fasciculatum ("chamise") chaparral occurs (where fire control is de facto not practiced), on more xeric southern exposures, often as pure using definitive evidence taken between 1972 and stands. Many stands are converted to "Ceanothus 1980 from Land-sat imagery (Minnich 1983). The chaparral" after burns, regardless of antecedent analysis reveals a pattern of numerous small burns composition (Hanes 1977). in northern Baja California and scattered large burns to the north. There was also a sharp decline In northern and central California, incomplete in the area burned across the International Border VTM map coverage suggests that most chaparral stands northward where environmental gradients are insuf- tend to be dominated by species in Arctostaphylos ficient to explain such a disparity. It was and Quercus with Adenostoma fasciculatum confined to concluded that fire control reduces the number of dry southern exposures. South of latitude 35°, fires, but not the burned hectarage; fires conse- Adenostoma fasciculatum is widespread with members quently increase in size, as stand mosaics became of Ceanothus cerastes on southern exposures below ever more coarse. In Baja California, the intricate approximately 1400 m. Most classifications take stand mosaics created by numerous burns tend to special notice of Adenostoma sparsifolium chaparral preclude large fires. Suppression also affects fire in the interior Peninsular Ranges of southern behavior because there is a built-in selection for California and island chaparral on the channel large fires to escape early control efforts in the islands (Philbrick and Haller 1977; cf. Minnich most severe fire weather environments of the 1980a). More open stands of desert chaparral, which climatic range. Most southern California fires, for consist of a peculiar mix including Quercus example, have occurred during autumn Santa Ana turbinella, Q. dunnii, Cercocarpus betuloides, winds, while fires in northern Baja California have Ceanothus greggii vestitus, Rhamnus crocea, Prunus burned in ordinary summer weather (Minnich 1983). ilicifolia, and Arctostaphy1-6T-67-iika, rim the Since Santa Ana winds rarely affect northern and western margin of the Mojave and ColoradoDeserts. central California, however, recurrent fires there have burned largely during summer heat waves.

13 2 The contrast in fire regimes across the discharge rates as high as one ignition/km /5 years, International Border invites the question of whether i.e. where suppression action is taken by fire the modern Mexican fire regime existed in California crews. Most strikes are not viable because pinyon before fire control. Historic accounts of southern forests mature to flammability only after approxi- California mountains in the late nineteenth century mately 100 years (Minnich 1978). A rare lightning (cf. Leiberg 1899, 1900; Mendenhall 1930), including ignition in chaparral, however, may be enormously a detailed analysis of 3 burns on Mt. Wilson between efficient. The correlation between ignition con- 1896 and 1900 which was culled from newspapers, centration and fire frequency, however, is probably suggest that chaparral fires festered for months, debatable. Thus, although thunderstorms are storing for long periods in large fuels, or crept seasonally rare along the coast in the short run, down slopes by means of falling coals and brands, they are sufficiently numerous to be a ubiquitous and only occasionally formed hot runs on steep source of combustion within the maturation period of slopes during hot days (Minnich, submitted). Yet, chaparral. most burns were relatively small by modern standards owing to stand mosaics seen widely at the time in Moreover, fires sputtering in the interior ranges the San Gabriel, San Bernardino, and San Jacinto during summer can also spread to the coast with Forest Reserves. Most of them occurred in summer autumn Santa Ana winds. Such fires would be and early fall, as presently occurs in northern Baja encouraged by cured grasslands growing •in coastal California (Minnich 1983). and inland valleys since herbaceous cover does not form fuel mosaics (Minnich 1983). (Many fires have Initial attack suppression efforts by a few ran- spread from the interior Transverse Ranges to the gers thereafter had an immediate impact on the fire Santa Monica Mountains and Pacific Ocean in recent regime, as conflagrations as large as 25,000 ha decades.) Given the enormous endurance of brush spread across southern California chaparral by 1919, fires over a period of months (even whole seasons) owing to the disappearance of the stand mosaics. before fire control, complex fire mosaics could From 1920 to 1950; national forests in southern develop throughout the chaparral belt exclusively on California experienced a marked decline in small lightning ignitions. According to Katsuo Nishikawa fires, accompanied by rare conflagrations. Total (personal communication), who has chased down many area burned declined from 206,000 ha in the 1920's fires in the uninhabited Sierra San Pedro Martir by to 92,500 ha in the 1940's. Continued limited burro and on foot, fire mosaics seen throughout this occurence of small fires after 1950 was accompanied range result mostly from lightning ignitions. by numerous extensive conflagrations, which in- creased the area burned to 200,000 ha in the 1960's. Unfortunately, the impact of man on the chaparral The two largest burns, the 70,100 ha Laguna fire in before fire control is difficult to establish San Diego County and the 89,000 ha Matalija fire because evidence is so fragmentary. There is an northeast of Santa Barbara, represent 14 percent of extensive literature on both Indian and early the total area burned area in southern California European burning and vegetational changes in national forests since 1911 (Minnich 1983). California, but few authors have recognized the fact that man cannot always capriciously burn the vege- Many researchers have suggested that the burning tation at will. Plant species have necessarily of chaparral and this ecosystem's extensive evolved canopy morphologies that propagate lethal distribution over the state is anthropogenic in fire frequencies that are compatible with their origin (cf. Axelrod 1976). Among evidences cited generation times. Chaparral shrubs invest most are included the current dominance of human- photosynthate into ligneous structures (wood). initiated conflagrations (see Keeley 1982). Since Without artificial manipulation such as cutting, lightning ignitions are necessarily associated with slashing, or grazing, stands may therefore be humid weather responsible for convection, fires are relatively nonflammable for decades after a burn easily extinguished by suppression crews; their until stems, litter, and stand senescence result in expansion into conflagrations is unlikely in the sufficient dead fuel accumulation to support short run until drier weather prevails. combustion (Countryman and Philpot 1970; Philpot Anthropogenic ignitions occur in dry weather as well 1977; Countryman 1964). Conversely, subshrubs and as moist, and may quickly trigger a mass fire before herbaceous plants largely invest most photosynthate initial attack procedures can be implemented. into fine stems and foliage that may sustain repeat Suppression, therefore, inherently skews the burns within a few years, but these are mostly numerical importance of human-caused ignitions. vigorous colonizers that reestablish in a short period of time (see Wells 1962). Before control, lightning was doubtless a potent source of wildfires. Ignitions can store for weeks Chaparral taxa can withstand short fire periods. in large fuels (logs, snags) until dry weather Sprouting species accumulate sufficient stored arrives. Thunderstorms are most frequent during the reserves to effectively resprout within a few years. summer drought when the vegetation is dessicated. Obligate seeders germinate within 1 to 2 years and Although lightning is most concentrated in forests reach seed production with 3 to 5 years. The at high elevations, where convection normally. replacement seed pool develops within approximately originates (Komarek 1968; Keeley 1982), clouds move 10 years (see Reid and Oechel, Chapter 3). Repeated long distances and lightning fires are continually burnings at such rates, however, would still cause being extinguished throughout the chaparral belt. stand deterioration (cf. Sampson 1944; Bentley 1967; Zedler et al., 1983). Stand dead fuel content, Some investigators have concluded that lightning however, rarely permits burn intervals less than 25 ignition distribution strongly influences local fire to 50 years (Philpott 1977; Minnich 1983). Thus, frequencies on the questionable assumption that each although most chaparral species either resprout or lightning strike has equal impact (Keeley 1977b, germinate immediately after fire, stand resistence 1981; Kilgore 1981). In prime ignition areas, to fire during the early decades of succession may events may be too numerous relative to the matura- be more important to the stability of the ecosystem tion and flammability period of many communities. against short period firing. It follows that since Pinyon Forests on Sugarloaf Mountain in the San chaparral provides limits to burning, the demonstra- Bernardino Mountains, for example, experience site tion of real changes in the fire pattern must also 14 establish corresponding changes in the fuel struc- The dynamics of the grassland-chaparral interface ture of the vegetation, particularly the invasion of however is limited by substrate and soil formation herbaceous cover capable of supporting short-term (Wells 1962). Shallow, diffuse root systems of repeat burns. Unfortunately, most speculations of grasses appear ill-adapted to coarse textured rocky prehistoric and Indian burning in brushlands have soils where water quickly percolates to considerable focused on the nature of ignitions to the exclusion depths. Since thin stands of successional herbs of vegetation as a source of fire. (fire annuals) that establish after a brush fire often provide insignificant fuels to carry a short- It has been proposed by Axelrod (1976), for term hot ground fire, chaparral shrubs receive an example, that California chaparral is a recent zonal important respite from repeat fire. Dense stands of vegetation type that replaced oak-laurel, mixed grasses on heavy soils provide an annual source of evergreen forests, and conifer forests, as a result fuel for fire and associated destruction for shrubs. of reckless burning by man, especially European man. Heavy clays are also unfavorable for deqs-rooting Such a transformation, however, seems incongruous woody plants compared to grasses, which can survive with the much longer appearance of the mediterranean summer drought through dormancy. It is probably for climate since the Miocene, during which time condi- these reasons that chaparral shrubs tend to occur on tions conducive for fire (fuel accumulation, natural unstable slope surfaces, and avoid deep soils and ignitions, fire storage in large fuels, and long- poorly-drained flat areas throughout its range. The term seasonal burn duration) have always been tolerance of chaparral taxa to low nutrient soils present. By focusing on the sources of ignition, may explain its occurrence in a wide range of Axelrod ignores the potential for fire latent in the substrate, including improverished serpentines. vegetation that was doubtless liberated at a rela- tively high frequency over the California landscape By comparing modern vegetation with landscape far back into the past. Moreover, his interpreta- descriptions of Crespi during the 1769 Portola tion is inconsistent with Miocene records of many expedition, Dodge (1975) hypothesized that recent progenitors of modern obligate chaparral seeders in expansion of chaparral into grasslands in San Diego Ceanothus and Arctostaphylos, which at least par- County was due to recent decreases in fire frequency tially require fire for reproduction (Axelrod 1979). since the demise of Indians. However, whether such To be sure, the vegetation was different before the a change actually occurred there is uncertain owing Holocene, but this was not due to the absence of to ambiguous locations given in Crespi's diary. A fires, but rather to differences in climate. recent discovery of extensive Chumash burning along Species migration attendant with climate change the Santa Barbara coastal plain, based on a trans- arose partially due to geographic shifts in the fire lation of a previously unknown original Crespi regime to which they had been long-adapted. diary, suggests that Indians had significant impact on the biogeography of native grassland and coastal The influence of Indian burning is unclear and sage scrub, but chaparral was apparently not has met with divided opinion due to the paucity of impacted (Timbrook et al. 1982). information provided by early explorers and set- tlers. Scenarios have ranged from a widespread Chaparral-grassland dynamics also involves the burning regime due to natural ignitions, usually recent spread of exotic European grasses which have lightning (Rundel et al. 1977), to one of highly- established wider distributions than indigenous modified vegetation due to conscious Indian burning perennial bunch grasslands and thereby altered the for the promotion of desirable landscapes for fire regime of woody perennial communities hunting and gathering purposes (Steward 1951; Lewis regardless of culture. Native grasslands, for 1973). Investigators advocating major vegeta- example, were not reported by early explorers in the tive alteration by Indians and early European man arid southern San Joaquin Valley where more simply deduced changes without any direct evidence. drought-tolerant exotic grasslands now grow (Wester The record of deliberate ignitions alone was 1981). Zedler (1977) suggests that Cupressus sufficient. Despite early recorded accounts kept forbesii on Tecate and Otay mountains in San Diego between initial European contact and the year 1850, County has declined owing to increasing fire as well as ethnobotany and consultations with frequencies caused by anthropogenic ignitions. informants, Sampson (1944) and Aschmann (1959) were Since these mountains are fairly low, however, such unconvinced of the extensiveness of Indian burning an increase may have been created by exotic European or of major impacts from it. However, Lewis (1973) grasses which have also resulted in the decline of came to the opposite conclusion using these data. several chaparral shrubs there (Zedler et al. 1983). He cites both the openness of forests and the Likewise, the spread of exotic grasses and high-fire juvenile state of chaparral in the nineteenth frequencies in coastal sage scrub is producing a century as a priori proof of an Indian burning general deterioration and floristic improverishment system. Unfortunately, Lewis (1973) fails to of this community in the Perris Plain near Riverside. segregate the effects of Indian burning from the (Minnich, in preparation). influence of natural fires, early European logging and grazing, and the changes which have been In contrast, the chaparral in northern Baja introduced de novo by modern fire suppression (see California is stable despite extensive deliberate also Bean and Lawton 1973). burning associated with ranching because early stand development is not characterized by enduring Man can readily increase fire frequencies in herbaceous cover capable of propagating short-term herbaceous vegetation because grasses will support repeat burns (Freedman, 1984). Local increases in short intervals between fire periods and because fire-event frequency only reduced fire size due to cover is universally flammable regardless of age stand mosaic elements of previous burning history class structure; lightning is rare in context with impeding the progress of active fires. Increases in the short-term flammability period of grasses. fire events resulted in modest increases in burn Accelerated burning also selects for herbaceous area, but fire intervals averaged 60 years (Minnich, cover at the expense of woody perennials which in preparation). Thus, even if European man and sprout and reproduce poorly under high fire Indians had deliberately or accidentally started frequencies (Sampson 1944; Howe and Carothers 1980; fires in chaparral, the natural fire regime was Zedler et al. 1983; Gautier 1982). probably unaffected. Speculations on low frequency

15 fire regimes in prehistoric southern California Bay. Annual precipitation in these areas may be as chaparral (Byrne et al. 1977; Keeley 1977, 1981, low as 200 to 300 mm (California 1980; Secretaria de 1982; Zedler 197717 therefore, may prove to be Agricultura y Recursos Hydraulicos, 1948 to 1983). correct, but infrequency of ignitions was probably Chaparral seems to be most productive under low not the cause. precipitation in coastal environments, such as the Santa Inez Mountains (Gray 1982). In inland ranges Chaparral is also remarkably stable under fire subject to hot summers, the chaparral-coastal sage suppression despite probable reduced fire scrub border rises to 700 m in the Transverse and frequencies, larger fire size, and higher Peninsular ranges of southern California (California intensities (Minnich 1983). The occurrence of very 1929 to 1937) and 1400 m in the Sierra San Pedro old stands with wide floristic diversity throughout Martir where annual precipitation ranges to as high California demonstrates that most taxa are as 500 mm. In many areas of northern Baja long-lived and flexible to long fire intervals. California, one can descend westward below the Individual shrub seed production does not decline mountain chaparral belt, only to return to sea level with age (Keeley 1981). Thus, the sprouting stands on hillsides near the coast. potential of many shrubs and latent seed reservoir in the soil permits efficient stand establishment Chaparral also ranges to very high elevations whether burns are short (40 years) or long (100 from approximately 1300 m in northern California to years). However, some change in composition may be 2400 m in the Sierra San Pedro Martir of northern caused by changes in fire frequency. Chaparral is Baja California. More intemperate climate, with also plastic to fire size because all shrubs either elevation, may not be a limiting factor to this sprout or have seed available in the burn site. ecosystem. Chaparral shrub photosynthesis is mostly insensitive to temperature (Oechel and Lawrence 1981). Mean summer temperatures of 10° C required for foliar growth, occur as high as 3000 m in EXAMPLES OF ENVIRONMENTAL CONTROLS ON California (Minnich 1984). Few taxa are vulnerable CHAPARRAL DISTRIBUTION to a hard freeze except for Malosma laurina which suffered severe dieback during major cold waves in January 1949 and December 1978. Poor frost Elevational Limits hardiness probably explains why this shrub is limited to south of Pt. Conception. Perhaps the primary adaption of chaparral shrubs in mediterranean climates is sclerophyllous foliage Moreover, increasing orographic precipitation which reduces transpiration under high evaporative with elevation encourages greater chaparral demand (Poole and Miller 1975; Mooney and Dunn 1970; productivity, leaf surface area, and canopy Poole and Miller 1981). Relatively low photosynthe- transpiration rates. In fact, mixed chaparral soil tic rates owing to sclerophylly, prolonged drought, drought is more severe than chamise chaparral or and low soil nitrogen content (Mooney 1981) is com- coastal sage scrub at lower elevations (Harrison et pensated for by year-round photosynthesis (Oechel al. 1970; Miller and Poole 1979). The increase in and Mustafa 1979, 1982; Houpis 1984). chaparral vigor with elevation suggests that stands may establish at even higher elevations if it were At lower elevations south of a latitude of not for competition 0 from adjoining conifer forests approximately 35 , evergreen sclerophyllous shrubs whose shade would suppress these photosynthetically decline in favor of coastal sage scrub, consisting inefficient shrubs (Mooney 1981). of subligneous mesophytic subshrubs (Mooney 1977). As a result of longer soil drought, the drought- The geography and ecology of chaparral-conifer deciduous form is more competitive because forest limits have evoked many hypotheses among photosynthetically efficient subshrubs can grow, researchers concerning their dynamics and competi- flower, and fruit rapidly during short periods of tive relations. One observation on chaparral-mixed abundant moisture. The rainy seasons become conifer forest biogeography is the elevational ever-shorter with decreasing latitude. Moreover, parallel between the lower limit of mixed conifer since most mountain soils probably have minimum forest and reliable winter snow pack development field capacities of about 200 mm (Rowe and Coleman (Bailey 1949). This trend appears to be related to 1951), soil percolation to the regolith where deep- feedback between vegetal physiognomy, phenology, and rooting chaparral species secure moisture during snow melt. Snow melts slowly in forest shade summer drought would occur only during wettest years (Anderson 1956) but rapidly when intercepted on in the drought deciduous coastal sage zone. chaparral shrub canopies owing to exposure to high solar intensities and ambient mixing from the under- The lower elevational limit of chaparral has canopy. (Snow normally persists longer in recent often been assumed to be associated with annual burns than adjacent mature stands, even in full precipitation amounts of approximately 400 mm sun.) An accumulating snowpack in forests functions (Harrison et al. 1971). However, Oechel and Mustafa effectively as water storage which helps water- (1982) report high photosynthetic rates of chaparral demanding trees survive the summer drought. stands occurring at 70 m elevation on the coast Immediate melt of intercepted snow in brushfields which receives about 250 mm of precipitation. The accelerates water flow in a manner that accommodates elevation of the chaparral-coastal sage scrub the late spring-early summer chaparral growth cycle. boundary varies enormously with distance from the Pacific coast, annual precipitation, variations in Snow storage may also explain the rapid decline seasonal transpiration rates (especially in summer), or disappearance of temperate mixed evergreen and soil characteristics. In near-coast ranges, forests such as those composed of Pseudotsuga where cool foggy summers reduce drought stress and macrocarpa, Pinus coulteri, Umbellularia mild winters permit long growing seasons, chaparral californica, Lithocarpus densiflora, Arbutus descends to near sea level over portions of the menziesil (see Sawyer et al. 1977) growing dis- Santa Lucia Mountains, the Santa Barbara coastal junctly in the chaparr'iT in southern California, plain, Santa Monica Mountains, Channel Islands, and despite the occurrence of more mesic mixed conifer the Todos Santos Range at the south end of Ensenada forests growing further southward into increasingly 16 arid Baja California. Habitats suitable for mesic FIRE GRADIENTS AND CHAPARRAL FLORISTIC COMPOSITION temperate trees, however, are wanting in Baja California because mesic conditions there have Previous research has implied that fire become limited to cold, snowy climates on forested frequencies (although temporally variable) are geo- mountain plateaus owing to extreme annual and graphically homogeneous in which mature stands seasonal precipitation variability. South of the persist in a vigorous state commensurate with the border, only Quercus agrifolia is abundant where it fire interval (cf. Hanes 1971, 1977; Philpot 1977; occurs primarily in riparian settings. .0. chrysoTepsis Rundel and Parsons 1980). There is geographic evi- is a smell-leaved shrubby ecotype, sympatric dence, however, that chaparral productivity, fuel with mixed conifer forests and rarely extends accumulation rates, stand biomass, and fire fre- downslope into the chaparral belt (Minnich 1982b). quency vary enormously, particularly in response to precipitation gradients. In northern Baja The upper limit of chaparral may also be related California, for example, chaparral burh area to shifts in fire regime from one of intense brush increases gradually with increasing precipitation fires capable of regularly burning out conifers in from the Sierra San Pedro Martir at the margin of the chaparral to ground fires in the conifer the Sonoran Desert to the Mexican Border (Minnich forest zone. Many conifers are vmpatric with 1983). (Suppression in California, however, has chaparral including the closed-cone pines (Pinus reduced fire event distribution to a point of spa- attenuata, P. muricata, P. radiata), Cupressus spp., tial randomness, and fire gradients are consequently Pinus coulteri, P. sabiniana, and P. torreyana. unrecognizable on a short-term basis.) Site fire Since these taxa normally experience stand period may also be dependent upon shrub morphology. replacement fires, chaparral understory regenerates The fine fasciculate foliage, and complex inter- in full sun during early succession before seeding lacing stem structure of Adenostoma fasciculatum, conifers reestablish canopy closure (Wright 1966 a, for example, makes this shrub vulnerable to fire 1966b; Vogl et al. 1977; Vale 1979; Tally and periods of 20 to 30 years (Philpot 1977; Countryman Griffin 1980; Minnich 1980b; Wilson and Vogl 1965; and Philpot 1970), while broad-leaved, simple- Griffin 1982). Other forest types experience branched taxa in Quercus and Arctostaphylos usually frequent ground fires including mixed conifer permit repeat burns onlyafter longer periods of forests (Weaver 1974; Rundel et al. 1977; Kilgore approximately 40 to 60 years. 1981) and Pseudotsuga macrocarpa Quercus chrysolepis forests (Bolton and Vogl 1969; McDonald and Littrell The fire cycle, of course, is not regular owing 11976; Minnich 1980b), and chamise or mixed to the vagaries of weather, fire behavior, and stand chaparral rarely occurs in the mostly permanent mosaic configuration. Since local burning regimes shade of these communities. respond to the vegetation as a fuel, however, site fire regimes over time should equilibrate toward a Analysis of fire damage and vegetation dynamics mean state that reflects the static aspects of for 36 years in the San Bernardino Mountains habitat, including climate, terrain, and vegetal (Minnich 1978, 1980b) suggests that the biogeography structure. Moreover, chaparral taxa have wide- of both chaparral and conifer forests responds to ranging adaptive modes (see Reid and Oechel Chapter terrain-fire behavior interactions, including a 3) and establishment efficiencies after burns. proportionate relationship between shrub cover and Thus, in view of possible feedbacks between shrub biomass, slope steepness, and the lower elevational morphology, regeneration, and fire frequency, it limits of mixed conifer and Pseudotsuga macrocarpa follows that gradients in fire regimes may impose a forests. Chaparral and fire pines such as Pinus strong selective force in chaparral composition coulteri ascend to high elevations on terrain depending upon the divergent adaptive modes of conducive to high fire intensities. Typical are individual taxa. steep, smooth, concave slopes, well-exposed to the ambient wind field that supports orographic channel- Keeley (1977b, 1981) for example, hypothesizes ing of superheated air from fire runs. Conversely, that obligate seeding species evolved under mixed conifer and Pseudotsuga macrocarpa forests periodically low fire frequencies which selects for descend to low elevations on gentle surfaces, allu- increasing soil seed pools and higher sprouter vial basins, and dissected convex canyons and scarps mortality rates (owing to hotter fires), lower which either fail to support orographic fire runs or sprouting shrub density and superior competitive interrupt the momentum of passing conflagrations. advantage (cf. Wells 1969). To explain the Although the occurrence of these communities on biogeography of obligate taxa, Keeley (1977b) northern exposures and flats has been attributed to postulates a proportionate relationship between greater soil moisture (cf. Horton 1960), both des- lightning ignition density and fire frequency which cend below prevailing limits on xeric south-facing predicts higher obligate seeder diversity at lower cliffs and precipitous canyons having minimal shrub elevations of coastal ranges in lightning impover- cover capable of burning conifer overstory. ished southern California (Keeley 1977). He also suggests that fleshy-fruited sprouters disperse into The smoulder and run behavior of chaparral fires disjunct mesic sites on northern exposures where in southern California interior ranges before fire either fire frequencies are lowered by stand escape control (Minnich, unpublished), which was compatible fire islands or fire intensities are locally with Pseudotsuga macrocarpa mixed conifer forests in reduced. Under these conditions, sprouters are able the past, has been supplanted by infrequent, intense to mature sufficiently to sprout efficiently and conflagrations that have caused large declines of fleshy fruits can be dispersed among disjunct these communities under modern suppression (Minnich habitats. Obligate seeders have no need to disperse 1978). Perhaps the near absence of conifers in the in more continuous xeric habitats (Keeley 1981). mesic Pacific slopes of the Santa Ynez range, Santa Monica, and coastal Santa Ana Mountains (California No model can provide a comprehensive biogeo- 1929 through 1937) suggests a prehistoric fire regime graphic explanation of individual taxa and evidence of intense Santa Ana wind-blown fires from the in current distributions often suggests alternative interior ranges (see Byrne et al. 1977). scenarios. Obligate seeders are abundant at all

17 elevations below 1800 m throughout coastal and Hanes 1971), and perhaps capable of living in near- interior mountains of southern California. In the desert habitats, the long infrequency of burning in . run, lightning is an ubiquitous source of desert chaparral may select combustion against taxa relying on' (see above). Sporadic, very low fire seed storage and dormancy for reproduction. intervals (e.g. 100 Under a years; Keeley 1981) may not have 200-year fire interval, for example, shrubs may been necessary for obligate seeders since minimum ultimately die of old age, the seed pool may be frequencies were limited by long stand flammability depleted by predation (Keeley 1977, 1981), or periods, stand by thinning occurs continuously during lost viability. Sprouters continually disperse succession, and obligate seeders are more efficient fleshy-fruited seeds (Keeley resource 1982) or windborne seed exploiters than sprouters (Parker, in (Cercocarpus) and reproduce without press),. fire. Ceanothus Moreover, the evolution of these taxa over greggii vestitus occurs sporadically time scales in recently of millions of years involves rearrange- burned sites and dies at around 30 ment of to 50 years after mountainous terrain (the Peninsular Ranges fire, although viable seed, unpreferred of southern by animals California, for example, are less than (Martin 1951), may be stored in one the soil throughout million years old), convection and lightning the desert chaparral belt. This ignitions, shrub establishes and fuel (vegetal) distribution. Thus, en masse after pinyon fires despite the speciation the absence of of southern California obligate adults to the burn (Minnich 1978). seeders (Keeley 1977b) Vasek doubtless occurred in and Clovis (1976) found that Arctostaphylos glauca different regions than they now occupy. on the desert slope of the San Bernardino Mountains has a prostrate form, and flowers and The abundance fruits spar- of fleshy-fruited sprouters on ingly. In the absence of regular fire northern recycling, A. exposures is probably unrelated to lower glauca adapts to desert conditions by fire intensities. layering and Greater chaparral biomass is vegetative propagation. Observations of A. glauca paralleled by increased transpiration and equivalent stand depletion after short period fires in drought stress the San to xeric stands on other exposures. Dimas Experimental Forest (Riggin and Dunn Thus, although 1982) may occasional fire escapes are possible, also explain its greater abundance on fire intensities the desert are probably higher when stands do slope in southern California. Cercocarpus burn. Yet most shrubs are vigorous sprouters regard- betuloides also occurs on the Pacific slope, but less of fire intensity (Zedler 1981). Fleshy-fruited mostly on cliffs and geomorphically sprouting active slopes taxa, e.g. Quercus dumosa, Heteromeles (see VTM maps, California 1929 to 1937), arbutifolia where it are not restricted to mesic disjunct normally escapes fire for lack of fuel, environments an ideal on northern exposures, especially above habitat for a species with wind-dispersed seed. 900 to 1200 meters. Some species including Prunus ilicifolia, Rhamnus crocea, Malosma laurina, Rhus In the same way, the chaparral on the Pacific ovata, R. integrifolia, Xylococcus bicolor, and slope of northern Baja California south of the Comerostaphylos diversifolia, occur widely on Ensenada area is dominated by sprouters, despite southern exposures. relatively high annual precipitation amounts of 400 to 600 mm (Secretaria de Agricultura Recursos Perhaps the most instructive examples demon- Hydraulicos, 1948 through 1938). In the Sierra strating the relationship between fire gradients and San Pedro MArtir, for example, obligate seeders chaparral flora can be found at the margin of the are confined largely to local northern exposures chaparral belt adjacent to the Mojave and Colorado (Ceanothus Verrucosus), water courses Deserts, and in northern Baja California. Hanes (Arctostaphylos glauca), or sparingly at high (1971) was impressed with the differences in the elevations above approximately 1600 m (C. greggii species composition between chamise chaparral and var. perplexans). This mountain range ig so desert chaparral along sharp precipitation gradients marginal to,i4inter cyclonic storms that a high between coastal and desert slopes of the San Gabriel frequency of rainy seasons have no significant and San Bernardino Mountains. On coastal slopes, storms (about 1 in 5), and soil drought may where annual precipitation ranges from 500 to 1000 persist more than a year. Assuming, hypo- mm, stands of chamise and mixed chaparral are thetically, a mean fire period of 50 years, the dominated by Adenostoma fasciculatum, Ceanothus failure of obligate seeders to establish after crassifolilus, Ceanothus leucodermis, Arctostaphylos fires is possible every few centuries. During glandulosa, A. glauca, Quercus dumosa, and Q. this time, dispersal of largely immobile seed wislizenii. As precipitation cgaTiTie-s to i5out 200 into the burn site may be exacerbated by even mm on the desert slope, coastal chaparral modest fire size. During reconnaissances in the communities grade into increasingly open stands of Sierra San Pedro Miktir from 1974 to 1979, I desert chaparral dominated by Quercus turbinella, observed few seedlings of obligate seeders, and even Cercocarpus betuloides, Arctostaphylos glauca, Adenostoma fasciculatum, in many burns ranging Rhamnus crocea, Prunus ilicifolia, Rhus ovata, and from 1 to 10 years. Thus, it appears that under Ceanothus greggii var. vestitus. Forest Service these conditions, the evolutionarily more fire history records since 1911 (Sniegowski n.d., conservative sprouting mode would be more San Bernardino National Forest) reveal a sharp successful in shrub establishment since adults decline in fire frequency from about 40 years in can withstand year-round drought (Harvey and chamise chaparral to 100 plus years in desert Mooney 1964). Wells (1968), for example, postu- chaparral. Thinner desert chaparral stands among lates that obligate Arctostaphylos glauca is re- Pinus monophylla forests experience fire intervals placed by burl-sprouting A. peninsularis in Baja Tess than200 years (Minnich 1978). Decreasing fire California because the rainy season is so unreliable. frequencies toward the desert are paralleled by a decline in obligate seeding taxa and sprouters which For similar reasons, arid chaparral sites on the seed en masse after fires (Adenostoma, Ceanothus, fire-prone Pacific mountain slopes in California Arctostaphylos). may Therefore, desert chaparral taxa be completely dominated by Adenostoma fasciculatum comprise mostly sprouters having fleshy fruits or because it both establishes seedlings atter fires achenes or capable of long-distance wind dispersal. sprouts in case of severe drought. The occurrence of sharp coastal sage scrub-chaparral (mostly Although many seeders are extraordinarily Adenostoma fasciculatum) boundaries in northern drought-tolerant Baja (e.g., Adenostoma fasciculatum, California and San Diego County (Bradbury 1974,

18 1978; Mooney 1977), however, may also be related to over wide areas of coastal sage scrub. This may be drought-induced seedling mortality. Sharp coastal due to lower fire intensities and less competition sage-chaparral borders decline to the north because during development in these areas. Many fleshy- the probability of rainless winters is so low and fruited species also occur widely in the South- dispersal into the rare areas of local seedling western Deserts, and in central and southern Baja extinction, is sufficient to maintain shrub California (Fig. 1). This occurrence suggests that population levels. they can also survive without fire. The distribu- tions of chaparral seeders, on the other hand, tend The dominance of sprouters in desert chaparral to be confined to the fire-prone chaparral belt. and chamise chaparral in Baja California suggests that these taxa, particularly fleshy-fruited and wind-dispersed species, are better generalists than RESEARCH PROBLEMS AND GOALS postfire seeding species, both sprouters (Adenostoma, Ceanothus euceanothus and some Where appropriate, the research on chaparral Arctostaphylos), and nonsprouters (Ceanothus should develop a biogeographic perspective, so that cerastes, some Arctostaphylos). Given their low ecological information is compared and made fire mortality rates, sprouters may survive several consistent with actual biotic distributions and fires and live for centuries. Such longevity would physical geography. Although such a goal is often diminish the importance of a seed reservoir, and elusive, this broader perspective will help in the dispersal may be more important to long-term extrapolation of ecological models and hypotheses to stability (see Keeley 1981). The southern the entire chaparral ecosystem, specifically: California channel islands provide an important example because the harvest of fuels by feral (1) The inventory of chaparral floristic herbivores (goats, sheep, pigs) has eliminated fire composition over the state is at best sparse and for approximately 150 years (Minnich 1980a, 1982; published information has not been compiled. The Brumbaugh 1980; Coblentz 1980). Grazing pressures Vegetation Type map survey of the 1920's and on eastern Santa Catalina Island have also decreased 1930's has quadrate data on file at the Pacific owing to fencing and urbanization at Avalon. Southwest Forest and Range Experiment Station at Berkeley, California (William B. Critchfield, A comparison of historic and modern aerial curator). Although the data is old, comparison of photography, and old ground photographs, reveals this data with modern chaparral cover in several major chaparral regeneration but without any Quadrangles (Bradbury 1974, Minnich 1978) reveals burning. Most species involved in stand expansion that chaparral composition has not significantly are fleshy-fruited sprouters including Heteromeles changed during the interim. This data set, arbutifolia, Rhus ovata, R. integrifolia, Malosma therefore, should be used to provide an efficient laurina, Rhamnus crocea sip. pirifolia, and Prunus means of providing a comprehensive inventory of ilicifolia ssp. lyonii. (Quercus dumosa reproduction chaparral floristic composition throughout the is still inhibited by pig acorn predation and range of this ecosystem. rooting.) Adenostoma fasciculatum and Ceanothus megacarpus, which were formerly widespread on the (2) There is some debate whether fire fre- island, exhibited no increase despite the presence quency responds to stand senescence or to fuel . of scattered populations as seed sources. Fleshy- accumulation rates responding to climatic gradi- fruited sprouters in heavily grazed areas show ents. Research should develop a wider sample of remarkable endurance in the absence of fire. In chaparral fuel dynamics than what is currently fact, current mortality rates project an average published. life span of approximately 700 years (Minnich 1982). (3) Attempts should be made to develop better - It appears, therefore, that fleshy-fruited taxa precontrol fire history in California chaparral by adapt to brush fires primarily through sprouting. means of fire scar analysis. To date, there is However, with the exception of Quercus dumosa and Q. nothing published on this theme. Historical wislizenii stands on northern exposures, sprouters studies from newspapers, old photographs, and form only scattered cover and consequently are not other historical sources should also be pursued. as successful as obligate and nonobligate postfire seeders. Low sprout mortality (Zedler 1981) may be offset by low reproductive rates and limited-chance SUMMARY seed dispersal into temporally and spatially rare burn sites (where competition with seeders is California chaparral is comprised of deep- reduced). Seeders proliferate and dominate stands rooted, evergreen sclerophyllous shrubs 1 to 5 m after burns. The dominance of sprouters in oak tall, interwoven in carpet-like stands on infer- chaparral on northern exposures (Hanes 1971), on the tile coarse-textured soils. Wildfire is a other hand, may reflect the development of recurrent feature to this ecosystem in which relatively nonflammable (low fire period), rapidly stands repeatedly undergo cycles of denudation and growing Quercus dumosa, Q. wislizenii, and succession. Heteromeles arbutifolia shrubs in mesic sites with sufficient stature to shade out most seeding The paleobiogeography of chaparral involves the species. The exception may be the short-lived migration and evolution of drought-adapted shrubs forest successional species such as Ceanothus with developing mediterranean climate, which came integerrimus and C. palmeri, which establish to full expression along the California coast immediately after burns from latent soil feed. after the late Miocene. Many progenitors of modern chaparral flora found in California were The generalist behavior of fleshy-fruited already in existence as early as the Eocene. sprouters, such as Heteromeles arbutifolia, Rhus During the middle Tertiary, evergreen ovata, R. integrifolia, Malosma laurina, Prunus sclerophyllous shrubs spread westward with ITTETfoTia (lyonii), Rhamnus crocea (pirablTi), expanding dry climate. Most chaparral taxa first Comerostaphylos and Xylococcus appear in California during the middle Miocene. bicolor, is also evidenced by their distributions

19 Although chaparral occurs on a wide diversity of Chaparral exhibits large variability in stand substrate, stands are characteristically restricted composition along environmental gradients created by to dissected mountain ranges having steep, rapidly slope, exposure, and elevation of the rough terrain eroding slopes. Although northernmost populations on which it is normally found. Extensive sampling are widely scattered, chaparral becomes more wide- of this formation has been conducted only the the spread in the highly dissected mountain ranges VTM survey in the central and south coastal ranges. having steep, rapidly eroding slopes. Although northernmost populations are widely scattered, The precontrol fire regime is poorly understood. chaparral becomes more widespread in the highly Hardwood and coniferous trees living with the dissected ranges surrounding Clear Lake and along chaparral are subject to stand replacement fires, steep ridges between Santa Rosa, Napa, and Berryessa stump sprouting, or surviving on precipitous slopes Valleys. South of San Francisco Bay, chaparral which escape fire altogether, and consequently do occurs on dissected summits of the south coast not lend themselves to fire scar analysis. The fire ranges. Stands are relatively on the mostly histories of southern California and northern Baja undissected west slope of the Sierra Nevada, California (where suppression is not practiced) were although it forms a narrow zonal belt from the compared for 1972 to 1980 from Landsat imagery. The Tuolumne River to the Kern River. Stands are most fire regime in northern Baja California is one of widespread in the Transverse and Peninsular Ranges numerous small burns whose size was limited by an of southern California and northern Baja California. intricate stand mosaic created by antecedent fire history. Since previous burns remove fuels for The mediterranean climate of California chaparral several decades, active fires in old growth stop at shifts along a latitudinal gradient from conditions former burns; numerous small fires preclude large resembling marine west coast climates in northern ones. Fires largely spread in summer under mild California to a fog desert in northern Baja onshore sea breezes and valley winds. Historic California. Chaparral distribution is strongly accounts of southern California mountains in the influenced by orography. In northern California late nineteenth century suggest that brush fires chaparral tends to cover lee slopes of the north festered for months, storing for long periods in coast ranges and lowest slopes of the Sierra Nevada. large fuels, and only occasionally formed hot runs With decreasing latitude, chaparral shifts toward on steep slopes. Most burns were still relatively the Pacific slope orientation with stands covering small owing to stand mosaics seen widely over the both windward and leeward slopes of the central region. Since 1900, suppression in southern coast ranges, and Pacific slopes of southern California has reduced the number of fire events and California mountains. fire frequency, and evidence of a mosaic is almost lacking. Thus, active fires spread with less inter- Fair weather periods in winter are often associ- ruption over enormous areas. Most are high-intensity ated with strong Santa Ana wind circulations in conflagrations driven by autumn Santa Ana winds. southern California and the northern Sacramento Valley. The great relief of the Sierra Nevada, Lightning was a potent source of wildfires before however, impedes offshore flow over most of central fire control because ignitions could store in large and northern California where weather is dominated fuels until dry weather arrived. Thunderstorms are by persistent tule fogs and ground inversions. also most frequent during summer drought when the During summer, chaparral-covered mountains are vegetation is desiccated. Although thunderstorms mostly exposed to warm, dry air masses overlying the are seasonally rare along the coast in the short coastal marine layer. Weak ambient winds aloft and run, they are sufficiently numerous within the high insolation permits development of upcanyon maturation period of chaparral to be a ubiquitous valley winds which withdraw marine air confined at source of combustion. Thus, given the enormous low levels by the trade wind inversion onto higher endurance of brush fires over periods of months, mountain slopes, especially along the coast ranges. complex fire mosaics could develop throughout the The influence of the marine layer on the summer chaparral belt on lightning ignitions, exclusively. climate of chaparral varies over the state. In the north coast ranges, where chaparral is mostly Many researchers have suggested that the burning confined to lee slopes, and along the length of the of chaparral and its extensive distribution over the Sierra Nevada, stands mostly experience hot dry state is anthropogenic in origin. Unfortunately, summers. With decreasing latitude, chaparral shifts most speculations of prehistoric burning have toward windward flanks of mountain ranges having focused on the nature of ignitions to the exclusion cooler, more humid summers. Surges of tropical of vegetation as a source of fire. Man can readily moisture into California from the equatorial Pacific increase fire frequencies in herbaceous vegetation along the Gulf of California produce occasional because grasses have short flammability periods. afternoon mountain thundershowers and numerous However, chaparral is stable under even deliberate lightning fires. burning because stand succession is not character- ized by enduring herbaceous cover capable of The chaparral is characterized by continuous propagating qhort-term repeat burns. Thus, even if floristic turnover throughout its range. Few spe- European man and Indians had deliberately started cies occur over the length of the chaparral belt, fires, the natural fire regime was probably while others, especially in diverse genera such as unaffected. In fact, chaparral is remarkably stable Ceanothus and Arctostaphylos, have limited, often under a wide range of fire regimes. The sprouting marginally overlapping local distributions. Many potential of many shrubs and latent seed reservoir shrubs also occur exclusively in coastal, inland, or in the soil permits efficient shrub establishment desert margins of California mountains. About 40 whether burns are 20 or 100 years apart. Chaparral percent range into other environments outside of taxa are also plastic to fire size since no species California, many with summer rain, including central requires seed dispersal back into the burn site. Baja California, the desert Southwest, and adjoining Perhaps the primary adaptation of chaparral shrubs northwest Mexico. to mediterranean climate is sclerophyllous foliage

20 which reduces transpiration under high evaporative sprouters. It is also believed that fleshy-fruited demand. The elevational zonation of the chaparral sprouters disperse into disjunct mesic sites on coastal sage scrub transition varies enormously with northern exposures because fire frequencies are distance from the coast owing to variations in lowered by fire mosaic escapes and fire intensities seasonal transpiration rates, especially in summer. are locally reduced by mesic conditions. The abun- • In near-coast ranges where cool foggy summers reduce dance of sprouters on northern exposures, however, drought stress and mild winters permit an extended is probably unrelated to lower fire intensities growing season, chaparral descends to near sea since greater stand biomass is paralleled by in- level. In inland ranges that are subject to hot creasing transpiration, drought stress, and higher summers, the transition rises to 900 m in southern fire intensities. Neither is their success related California and 1400 m in northern Baja California. to dispersal into disjunct northern exposures since the distribution of these taxa are more or less con- The upper limit of chaparral ranges to as high as tinuous. In fact, decreasing fire frequency toward 1300 m in northern California and 2400 m in the the desert margin of chaparral is accompanied by a Sierra San Pedro Martir in northern Baja California. decline in obligate seeding taxa and sprouting Despite decreases in mean annual temperature with seeders (Adenostoma, Ceanothus euceanothus); xeric elevation, increasing orographic precipitation desert chaparral comprises dominantly fleshy-fruited encourages greater chaparral productivity, leaf sprouters relying on long distance seed dispersal by surface area, and canopy transpiration rates so that wind or fauna. mixed chaparral soil drought is more severe at these elevations than it is for chamise chaparral or Although seeders are extraordinarily drought- coastal sage scrub at lower elevations. Increasing tolerant, the infrequency of burning along the chaparral vigor in cooler, mesic climates toward desert margin may select against taxa relying on higher elevations suggests that stands could seed storage and dormancy for reproduction. establish at higher elevations if it were not for Fleshy-fruited sprouters continually disperse seed competition from adjoinging conifer forests whose and reproduce without encouragement from fire. shade would be a death knell to photosynthetically Similarly, the chaparral in northern Baja California inefficient sclerophyllous shrubs. The chaparral- is also dominated by sprouters despite relatively conifer forest limit tends to parallel the lower high annual precipitation rates of 400 to 600 mm. limit of reliable winter snowpack development owing Because the area has a high frequency of rainy to feedbacks between vegetal morphology, snow seasons having no significant storms (about one in interception, and melt. Snow melts slowly in forest five), obligate seedling famine after fires is shade, but rapidly when intercepted on chaparral possible every few centuries during which time canopies owing to exposure to high solar intensities dispersal of largely immobile seed into burn sites and ambient mixing from the undercanopy. In fact, may be exacerbated by even modest fire size. snow normally persists longer in recent burns than in adjacent mature stands. An accumulating snowpack The dominance of sprouters in desert chaparral is effective water storage which helps water suggests that these taxa, particularly fleshy- demanding trees survive summer drought. Immediate fruited and wind dispersed species, are better melt of intercepted snow in brushfields accelerates generalists in arid habitats than postfire seeding water flow in a manner that accommodates the late species. Given their low postfire mortality rates, spring-early summer growth cycle of chaparral. sprouters probably survive several fires and live The upper limit of chaparral may also be related to for centuries. Such longevity seems to diminish the shifts in fire regime from moderately frequent importance of a seed reservoir and dispersal may be intense brush fires capable of regularly burning out more important to their long-term stability. conifers in the chaparral to ground fires in mixed However, fleshy-fruited sprouters form only conifer forest. Chaparral tends to rise to high scattered cover in most stands and consequently are elevations on terrain conducive to high fire not as successful as obligate and nonobligate intensities, particularly steep, smooth, concave postfire seeders. Low sprout mortality may be slopes, well-exposed to the ambient wind field that. offset by low reproductive rates and limited chance supports orographic channeling of superheated air seed dispersal into temporally and spatially rare from the flames. burn sites (where competition with seeders is reduced). Seeders proliferate and dominate Chaparral productivity, fuel accumulation rates, chaparral stands after fires. The widespread stand biomass, and fire frequency vary, particularly occurrences of fleshy-fruited shrubs in the in response to precipitation gradients. Site fire Southwestern Deserts, central and southern Baja periods may also reflect shrub morphology. Thus, California suggests that they can survive without gradients in fire regime may be a strong selective fire. The distributions of chaparral seeders tend force on chaparral floristic composition, depending to be confined to the fire-prone chaparral belt. upon the divergent adaptive modes of individual taxa. 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