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Fire in California's Ecosystems

University of California Press

Chapter One

NEIL G. SUGIHARA AND MICHAEL G. BARBOUR

In California, vegetation is the meeting place of fire and ecosystems. The plants are the fuel and fire is the driver of vegetation change. Fire and vegetation are often so interactive that they can scarcely be considered separately from each other. M. G. BARBOUR, B. PAVLIK, F. DRYSDALE, AND S. LINDSTROM, 1993

During the last decades of the twentieth century and early years of the twenty-first century, fire ecology has emerged as a rapidly expanding area of study. Since the first article on fire ecology appeared in Scientific American in 1961 (Cooper 1961), several books have been published on the subject (Kozlowski and Ahlgren 1974, Wright and Bailey 1982, Agee 1993, Whalen 1995, Arno and Allison-Bunnel 2002). However, no book has been published that focuses specifically on the ecological role of fire in California. Wildland fire is a complex, dynamic, and often spectacular force that plays a richly complex role in California's diversity of ecosystems. Human interactions with fire have developed around our need to simultaneously protect ourselves from its harm and use it as a tool. As we get more effective at controlling wildfire, we are gaining an appreciation of the value that fire has to ecosystems and biological diversity.We are also recognizing that exclusion of fire from wildlands is not always the most effective way of protecting ourselves from fire or managing ecosystems. Managing wildland fire has developed into one of the largest ecosystem restoration efforts ever undertaken. Our need to understand fire and the consequences of its occurrence-or exclusion-have become great. This book is the first effort at a comprehensive synthesis of our knowledge of fire in California ecosystems.

Fire as an Ecological Process

Part I of this book is an introduction to the study of fire ecology and provides a basic framework and perspective on fire as an ecological process. Much of California has a mediterranean climate conducive to the occurrence of fire (Pyne et al. 1996), with long dry summers and periods of thunderstorms, low relative humidity, and strong winds. These patterns vary through an extremely wide range of climatic zones and complex topography.

Fire is a physical process as well as an ecological process. The heat it produces, the rate at which it spreads, and the effects it has on other ecosystem components are all part of the physical process. Watersheds, soils, air, plants, and animals are affected in one way or another by fire. Water quality and quantity, erosion, smoke, and plant and animal mortality are some of the more obvious effects. Other ecosystem effects are less obvious, but perhaps even more important. Dead biomass accumulates in mediterranean ecosystems because weather conditions are favorable for growth while decomposition is active for a relatively short part of the year. Fire complements decomposition in these systems by periodically removing debris through combustion. Fire has a differential effect on plant species mortality, allowing those that are best adapted to fire to be perpetuated.

Pyrogenic vegetation has evolved with recurring fire and includes species that tolerate or even require fire in order to complete their life cycles. There is a feedback loop between fire and vegetation. Fire feeds on vegetation as fuel and cannot reoccur without some minimum burnable, continuous biomass; and vegetation cannot maintain its occupation of a site without recurring fire. Fire and vegetation are often so interactive that they can scarcely be considered apart from one another. Indeed, the properties of any fire regime-its seasonality, fire return interval, fire size, spatial complexity, fireline intensity, fire severity, and fire type-require specific responses by the vegetation to persist.

Animal populations and communities have developed in habitats where fire has been the dynamic perturbation. The distribution of animal species on landscapes has been driven by the patterns of fire, controlled by climate, weather, and topography, over space and time. Perpetuation of California's biological diversity certainly requires fire to be present as a vital ecological process.

It is difficult to overstate the importance of fire in California ecosystems. A central theme of this book is that wildfire is a pervasive, natural, environmental factor throughout much of the state, and ignoring its role in ecosystems will seriously limit our ability to understand or manage wildlands in a sustainable, ecologically appropriate manner.

Bioregions and the California Landscape

The diversity of the California landscape is well known; from the mist-shrouded mountains of the north coast to the searing heat of the southeastern deserts, and from the sun-drenched beaches of the south coast to the high Sierra Nevada, the range of climate, geomorphology, and vegetation mirrors this diversity. Similarly, fire's role in each of these bioregions of the state is equally diverse.

If we add up the areas of vegetation types generally regarded as fire maintained, about 54% of California's 39,400,000 ha (985,000,000 ac) requires fire to persist (Barbour and Major 1988). Only desert scrub, alpine tundra, subalpine woodland, and a few other, less widespread, vegetation types are not fire dependent. Even some wetlands-such as tule marsh, riparian forest, and California fan palm (Washingtonia filifera) oases-have experienced fires set by both indigenous human populations and lightning strikes (Anderson 2005). Knowledge of how fire operates as an ecological process within the state's various bioregions is part of the foundation for wise management and conservation of California's natural heritage. In Part II of this book, we examine each bioregion in detail to see how that bioregion's physical features influence the interactions among fire, vegetation, and other ecosystem components. But first we take a statewide look at the bioregions and the vegetation within them.

California's Bioregions: Climate and Geography

Bailey (1996) and Bailey et al. (1994) developed an ecosystem classification based on climate, as affected by latitude, continental position, elevation, and landform. Located at the mid-latitudes, California receives a moderate amount of solar radiation, placing it in the temperate thermal zone with both a summer and a winter. California's position on the western edge of the North American continent provides the western portion of the state with a moderate marine climate. Bailey (1996) included this area in the Humid Temperate Domain. East of the crest of the Cascades, the Sierra Nevada, and the Peninsular ranges, where the influence of the ocean is much decreased, lies the Dry Domain.

In California, the Humid Temperate Domain comprises two divisions: the Mediterranean Division and the Mediterranean Regime Mountains Division with alternating wet winters and dry summers (Bailey 1996). In the mountains, the climatic regimes differ from the adjacent lowlands and results in climate zones that change with elevation. Two divisions make up the Dry Domain as well: the Tropical/ Subtropical Division occurs in the southeastern desert portion of the state, whereas the Temperate Desert Division includes areas of the Great Basin.

Bailey (1996) further divided these divisions into provinces based on macro features of the vegetation. Ecological subregions of California including provinces, sections, and subsections were described by Miles and Goudey (1997). They divided the Mediterranean Division into the California Coastal Chaparral Forest and Shrub Province, the California Dry Steppe Province, and the California Coastal Steppe, Mixed Forest, and Redwood Forest Province. The Mediterranean Regime Mountains Division comprises the Sierran Steppe-Mixed Forest-Coniferous Province and the California Coastal Range Open Woodland-Shrub-Coniferous Forest-Meadow Province. In the Dry Domain in California, the Tropical/Subtropical Desert Division has only the American Semi-Desert Province. The Temperate Desert Division includes the Intermountain Semi-Desert and Desert Province and the Intermountain Semi-Desert Province.

The next level in Bailey's (1996) classification is the section. A section is defined by landform, the overall shape of the surface. For example, mountain ranges are differentiated as are coastal steppes, deserts, and the Central Valley. We have combined the 19 sections described by Miles and Goudey (1997) that comprise California into nine bioregions based on relatively consistent patterns of vegetation and fire regimes (Map 1.1). In the northeastern portion of the state, tall volcanoes and extensive lava flows characterize the Southern Cascade Range and Northeastern Plateaus bioregions. To the west of the Cascades lies the Klamath Mountains bioregion, a complex group of mountain ranges. Numerous valleys and steep coastal and interior mountains are typical of the North Coast, Central Coast, and South Coast bioregions. The Sacramento and San Joaquin Rivers flow through broad interior valleys with extensive, nearly flat alluvial floors. These valleys constitute the Central Valley bioregion. Immediately east of the valley is the Sierra Nevada bioregion, a high range of north-to-south trending mountains. Finally, the vast southeast corner of California constitutes the Southeastern Desert bioregion. Table 1.1 lists the sections from Miles and Goudey (1997) that are included in each bioregion.

Coastal California is characterized by a long chain of steep, geologically complex mountains known collectively as the Coast Ranges. In general, the climate (and vegetation) progress from mesic to dry on a gradual gradient from north to south and a much more abrupt transition from the coast to the interior. A montane gradient, with cooler and wetter conditions at high elevations, is also present. The Coast Ranges are composed of three bioregions: the North Coast, Central Coast, and South Coast.

The North Coast bioregion supports north coastal scrub and prairie, north coast pine forest, and Sitka spruce (Picea sitchensis) forest on the immediate coast. Upland forests and woodlands that are farther away from the marine influence include coast redwood (Sequoia sempervirens), Oregon white oak (Quercus garryana), and mixed evergreen. At the higher elevations, the vegetation is typically mixed conifer lower montane forest and Shasta red fir (Abies magnifica var. shastensis) upper montane forests.

The Central Coast bioregion is an area of transition between the bioregions to the north and south. Ridge tops are generally less than 1,200 m (3,800 ft) in elevation, but a few peaks rise up to 1,800 m (5,700 ft). The region supports a mixture of the vegetation types found to the north and south in a more complex mosaic including coastal prairie, north and south coastal scrub, redwood forest (in isolated locations), mixed evergreen forest, chaparral, oak woodland, and some mixed conifer forest in the lower montane belt of the Santa Lucia Range.

The South Coast bioregion contains the east-west-running Transverse Range and the north-south-oriented Peninsular Range. Except for the alpine zone, both ranges have a full complement of montane zones. Elevations extend from sea level to over 3,500 m (11,400 ft). In addition to montane vegetation types, low-elevation vegetation includes interior grassland, south coastal scrub, chaparral, foothill woodland, and mixed evergreen forest. Despite the fact that Coastal California is greatly urbanized, including the San Francisco Bay Area, Los Angeles, San Diego, and their adjacent urban centers, these bioregions do still have large areas of wildland with relatively low human population densities.

The Klamath Mountains bioregion lies between the North Coast bioregion on the west and the Southern Cascade Range bioregion on the east. These mountains are characterized by steep, complex topography dissected by a number of large river valleys. The area is noted for its exceptionally rich flora, which results from several factors. First, the Klamath area is a meeting ground for three regional bioclimates and floras-the Pacific Northwest, California, and the Great Basin-and this increases the number of species present. In addition, the area has served as a refugium for millions of years, hence the presence of many woody species near the edges of their ranges or even restricted to the region. Finally, the diversity of geologic substrates is impressive, ranging from acid granite, to basic marble, metamorphosed shale, and chemically unique ultramafic extrusions (Franklin and Dyrness 1973, Franklin and Halpern 2000). As a consequence, the mosaic of vegetation types does not fall neatly into broadly continuous zones or belts, as it does in the Coast Ranges or the Sierra Nevada (Whittaker 1960, Sawyer and Thornburgh 1988).

The Central Valley bioregion is a wide, flat, low-elevation trough of sediments bounded by the Coast Ranges to the west and Sierra Nevada to the east. The northern part is drained by the Sacramento River; and the southern part, by the San Joaquin and Kern Rivers and their tributaries. The valley floor and adjacent foothills have largely been converted to agriculture or urbanized but were once dominated by a combination of chaparral, foothill woodland, riparian forest, bunchgrass prairie, forb fields, tule marsh, and in the dry southern San Joaquin Valley, saltbush scrub. Recent research has questioned previous assumptions that bunchgrass prairie characterized most of the landscape (Holstein 2001).

The Sierra Nevada and Cascade Ranges form an axis of high mountains east of the Central Valley and Klamath Mountains. The Southern Cascade Range bioregion is the southern end of an extensive chain of volcanoes and volcanic flows extending northward from Oregon and Washington. Mount Shasta and Mount Lassen are the two largest and most well-known Cascade volcanoes in California. The Sierra Nevada bioregion extends south from the Cascade Range 600 km (373 mi) to the Tehachapi Mountains. Vegetation generally occurs in elevation bands with oak woodlands and chaparral in the extensive foothills on the west side of these mountains. The lower montane zone consisting of mixed conifers gives way to an upper montane fir forest and montane chaparral at higher elevations. The highest mountains and ridge tops support subalpine forests and alpine meadows and shrublands.

The western edge of the huge intermountain Great Basin extends into the northeastern corner of California forming the Northeastern Plateaus bioregion. This is a semi-arid region of mountain ranges separated by lower-elevation basins and includes the Modoc Plateau. Major vegetation types include western juniper (Juniperus occidentalis var. occidentalis) woodland, sagebrush scrub, mixed ponderosa pine (Pinus ponderosa), and Jeffrey pine (Pinus jeffreyi) forest, upper montane fir forest, and whitebark pine (Pinus albicaulis) subalpine woodland.

The southeastern portion of the state is extremely arid. Except for isolated desert mountains, rainfall is <25 cm (10 in). Portions of the Mojave and Sonoran warm deserts, and the southwestern tongue of the Great Basin cold desert comprise the Southeastern Deserts bioregion. Major vegetation types include various desert scrubs (creosote bush [Larrea tridentata], blackbrush [Coloegyne ramosissima], sagebrush [Artemisia spp.]), halophytic scrubs in alkaline sinks (greasewood [Sarcobatus vermiculatus], saltbush [Atriplex spp.] ), desert riparian woodland, pinyon (Pinus monophylla) woodland, montane conifer forest dominated by white fir (Abies concolor), and, in the Panamint and White Mountains, a subalpine woodland with scattered western bristlecone pine (Pinus longaeva) and limber pine (Pinus flexilis). Fire is typically limited by the lack of fuel continuity.

California's Floristic Provinces: Evolution of the Vegetation

Floristically, California is divided into three provinces (Hickman 1993). The California Floristic Province corresponds to the Humid Temperate Domain and comprises the portion of California west of the mountainous crest. Both the Great Basin Floristic Province and the Desert Floristic Province are in the area of the Dry Domain. The three provinces are further divided into 10 regions, 24 subregions, and 50 districts. The regions most closely correspond to our bioregions and various combinations of the subregions "or districts" approximate sections. The vegetation in the provinces evolved from different floras. Today, the Arcto-Tertiary Flora dominates in the Northwestern California, Sierra Nevada, Klamath Mountains, and Cascade Ranges regions. Species from the Madro-Tertiary Flora are most common in the Great Central Valley, Central Western California, Modoc Plateau, east of Sierra Nevada, Mojave Desert, and Sonoran Desert regions.

(Continues...)

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