University of California

Ecology

 by M.R. George, L.M. Roche and D.J. Eastburn

Introduction

Ecology is the study of interactions among organisms, their environment and each other.  This chapter summarizes the structure (species, distribution, diversity) and function (vegetation dynamics, nutrient cycling) of the annual grassland, oak-woodland and chaparral ecosystems that make up California’s Annual Rangelands (Figure 1).  The botanical composition of these vegetation types includes a significant component of annual and commonly alien plants that arrived in the new world with the arrival of European settlers.  The earliest Europeans began to describe these rangelands in journals of their travels.  Eventually scientists and conservationists began to classify and map vegetation.  In their surveys they report what species were present and how they were distributed across the landscape.  Keeler-Wolf (2007) reviewed the history of vegetation classification and mapping in California.  To help the reader compare and contrast California landscapes we begin this chapter with a review of important classification systems and mapping systems that have been applied to the annual rangelands.

Fig1.annrng.map

Figure 1.  Location and area of annual rangelands (oak woodlands, annual grasslands and chaparral) and other rangeland types in California.

 

Vegetation Classification and Mapping

Biomes and ecosystems are often named for the vegetation that dominates them.  Vegetation and plant communities are defined and classified on the basis of shared floristic and/or physiognomic characteristics that distinguish it from other kinds of plant communities or vegetation.  Classifying and mapping of vegetation is a critical component of inventory, assessment and monitoring of land and ecosystems.  Several approaches have been used for classifying and mapping vegetation in California resulting in different nomenclatures.  

United States and North American vegetation has been classified and mapped using several systems.  The Küchler system classifies and maps potential natural vegetation.  These maps are often used to teach students the location of major rangeland biomes.  In contrast the USDA Forest Service uses an ecoregion vegetation mapping model to delineate existing vegetation.  In 1994 the Society for Range Management published the Range Cover Types for the United States (Shiflet 1994). Gap analysis has been applied to aerial photography to delineate vegetation and land use patterns of the continental United States.  In recent years The Nature Conservancy and the Ecological Society of America have collaborated to develop a National Vegetation Classification System. 

Vegetation Classification and Descriptions

National Vegetation Classification

The U.S. National Vegetation Classification (NVC or USNVC) is a scheme for classifying the natural and cultural vegetation communities of the United States collaboratively developed by federal agencies, the Vegetation Panel of the Ecological Society of America, and NatureServe, a nonprofit group that manages the NVC for the U.S. government.  The purpose of this standardized vegetation classification system is to provide a common language and to facilitate communication between land managers, scientists, and the public when managing, studying and protecting plant communities (Keeler-Wolf 2007). 

Manual of California Vegetation

In its Manual of California Vegetation the California Native Plant Society has adopted a system for describing vegetation statewide. This classification has been accepted by state and federal agencies, and the principal unit is an Alliance (or series), which is a floristically defined vegetation type identified by its dominant and/or characteristic species.  The California Native Plant Society (CNPS) is an organization of amateurs and professionals united by an interest in the plants of California. Its chief purpose is to preserve the native flora and to add to our knowledge of this flora. Its members participate in monitoring rare and endangered plants, fostering public education, supporting legislation that protects native plants, and providing expert testimony to government bodies.

Ecological Sites (USDA NRCS)

USDA NRCS has divided the states and territories into land resource regions (LRRs), and major land resource areas (MLRA).  The LRRs are made up of geographically associated MLRAs.  There are 28 land resource regions and 278 MLRAs (http://soils.usda.gov/survey/geography/mlra/). Major land resource areas are geographically associated by a particular pattern that combines soils, water, climate, vegetation, land use, and type of farming.  Within the MLRAs,  ecological sites are delineated for the purpose of inventory, evaluation and management.  An ecological site is a distinctive kind of land with specific physical characteristics that differ from other kinds of land in its ability to produce a distinctive kind and amount of vegetation.  Soils with like properties that produce and support a characteristic native plant community are grouped into the same ecological site. An ecological site is recognized and described on the basis of the characteristics that differentiate it from other sites in its ability to produce and support a characteristic plant community.  Ecological sites replaced the traditional range site in the 1990s as USDA NRCS expanded its site descriptions to cover vegetation dynamics (state and transition models) and to include ecosystem services in addition to forage and grazing capacity.  Like soil surveys, development of ecological site descriptions is an ongoing process.  Ecological site descriptions can be found at http://esis.sc.egov.usda.gov/Welcome/pgESDWelcome.aspx

Rangeland Cover Types

Rangeland cover types were described by Shiflet (1994) and published by the Society for Range Management.  These cover types were based on current vegetation that covered fairly large areas but not necessarily in continuous stands.  The cover type names were based on the dominant species and used common names.  There are minimal similarities between these cover types and the Küchler and Ecoregion classification systems.

Oak-Woodland Classification System

Allen et al. (1991) divided California’s oak woodlands into 57 subseries arranged within 7 series defined by the dominant oak species based on a survey of approximately 4300 field plots collected as part of the Vegetation Type Map (VTM) survey conducted during 1919-1940's by the USDA Forest Service Pacific Southwest Station. There are 15 subseries in the Coast Live Oak Series, 12 subseries within the Blue Oak Series, 6 subseries in the Valley Oak Series, 6 subseries in the Interior Live Oak Series, 13 subseries within the Black Oak Series and 3 subseries within the Scrub Oak Series. A Mixed Oak Series is described for which there are 3 or more species of oak occupying 30 percent of the total cover. There are 11 subseries in this Mixed Oak Series.  A key for delineating these series and subseries is available at, http://ucanr.edu/sites/oak_range/Oak_Articles_On_Line/Oak_Woodland_Ecology_and_Monitoring/A_Hardwood_Rangeland_Classification_System_for_California/.

Vegetation Mapping

Vegetation Type Mapping

In the 1930s, forester A. E. Wieslander spearheaded a U. S. Forest Service survey of California vegetation, called the Vegetation Type Mapping (VTM) Project. Its purpose was to create vegetation type maps.  In order to validate some of the broad zones of vegetation they collected species composition and other data along vegetation transects.  Additionally, they collected sample specimens, took photos of many vegetatively distinct locations, and marked the locations of these photos on maps.  While this project never completed all of the maps originally planned, the project left a rich archive of vegetation information that is curated by the University of California, Berkeley (Keeler-Wolf 2007). 

Soil-Vegetation Survey

In 1947 the California Soil-Vegetation Survey program was started with CDF as the lead agency and collaboration from UC, USDA SCS, and the USFS Pacific Southwest Forest and Range Experiment Station (Keeler-Wolf 2007). This survey covered the forests and rangelands (uplands) and the USDA SCS Soil Survey focused on farmland. The survey delineated soils and vegetation at the time of mapping and included botanical composition of range and forest sites.  The Soil-Vegetation Survey was terminated in 1988.

Ecoregions (USDA Forest Service)

The Forest Service has used an ecoregion approach, developed by Robert Bailey ( Bailey and Cushwa 1981), to classify the results of their mapping projects.  This approach uses a hierarchical classification that first divides land areas into very large regions based on climatic factors, and subdivides these regions, based first on dominant potential vegetation, and then by geomorphology and soil characteristics.  In 1993 the Forest Service adopted ecoregions for use in ecosystem management. 

Gap Analysis Program

Gap analysis supports state and national wildlife conservation efforts to identify gaps in landscapes where significant plant and animal species and their habitat or important ecological features occur.  Gap analysis is used by conservationists and scientists to assess the adequacy of wildlife habitat protection.   California Department of Fish and Game uses gap analysis as the basis for identifying and classifying habitats in its Wildlife Habitat Relationships Data Base.

Nationally the Gap Analysis Program (GAP) combines land cover data generated for the Southwest Regional Gap Analysis project completed in 2004, the Southeast Regional Gap Analysis Project completed in 2007, the Northwest Regional Gap project, and the updated California Gap project completed in 2009. For areas of the country without an Ecological System level Gap project, data created by the Landfire Project was used. All these projects use consistent base satellite imagery, the same classification systems and similar mapping methodology allowing for the creation of a seamless national land cover map.

Annual Grasslands

There are several vegetation or habitat classification systems that have been used in the annual grasslands.   Annual Grassland habitat has been described as Valley Grassland (Munz and Keck 1959, Heady 1977), Valley and Foothill Grassland (Cheatham and Haller 1975), California Prairie (Küchler 1977), Annual Grassland Ecosystem (Garrison et al. 1977), Brome grass, Fescue, Needlegrass, and Wild Oats series (Paysen et al. 1980), and Annual Grass-Forb series (Parker and Matyas 1981).  Some of these classes include oak-woodlands.  In 2002 Jackson and Bartolome identified and described a Coast Range Grassland subtype based on TWINSPAN classification of species composition data from three to five years of data from 9 sites ranging from the north coast to the southern San Joaquin Valley. 

Plant and Animal Communities

Current Plant Communities

Most of California’s grasslands are dominated by non-native grasses and forbs of Mediterranean origin (Heady 1977, Baker 1989, Keeley 1990), although alien taxa in California come from all parts of the world (Hickman 1993).  The annual grasslands are the product of severe disturbance of the former native grassland (see Vegetation Dynamics section).  While climax-based models of vegetation dynamics may have predicted plant community change in the former native grassland, the current annual dominated grassland is not predicted by the traditional range succession model (Dyksterhuis 1949, Dyksterhuis  1958,  Ellison 1960). Instead it is heavily influenced by annual and intra-annual weather and grazing management (Jackson and Bartolome 2002).   Following the state and transition procedure for describing vegetation dynamics of Westoby et al. (1989), George et al. (1992) proposed a non-equilibrium model for the annual grasslands and oak-woodland understory.  Jackson and Bartolome (2002) provided data support for non-equilibrium state and transition models of vegetation change when they identified a large number of states and transition over 9 sites distributed latitudinally in the annual grasslands and oak-woodlands understory.  They concluded that non-equilibrium state-transition models offer potential for developing and testing vegetation change hypotheses but that it will require large spatially and temporally replicated datasets.

Plant communities within this ecosystem have not been well defined beyond the classifications of Valley Grasslands and Coastal Prairie.    Soft chess brome (Bromus hordeaceus) and broadleaf filaree (Erodium botrys) are common in areas with 65-100 cm (25-40 in) of rainfall, and red brome (B. madratensis)  and redstem filaree (E. cicutarium) are common on southern sites with less than 25 cm (10 in) of precipitation (Bartolome et al. 1980).  Native perennial grasses are more common on deep soils with high rainfall.  Vernal pools, found in small depressions with a hardpan soil layer, support downingia (Downingia spp.), meadowfoam (Limananthes spp.) and other species (Parker and Matyas 1981).

Historic or Natural Potential Communities

The pre-settlement composition of Mediterranean-type grasslands and the understories of associated shrublands and woodlands, now dominated by non-native annual species, are uncertain.  Classical ecologist Fredrick Clements first proposed that the vegetation of the Central Valley, the central and southern Coast Ranges, and the valleys of southern California was perennial grassland (Clements 1920) and proposed that these were dominated by Stipa spp.  Clements relied on observations of scattered patches of purple needlegrass (Nassella pulchra) along railroad rights-of-way (Keeley 1990, Hamilton 1997). It since has been suggested that several other perennial grasses (e.g. Poa secunda, Leymus triticoides, Melica spp., Muhlenbergia rigens) were historically more important constituents in some environments (Keeley 1990, Holland and Keil 1995, Holstein 2001, Schiffman 2007). 

The hypothesis that many of California’s current grasslands were formerly dominated by woody vegetation and not "pristine" prairie (Cooper 1922) has been less popular, but has received some scientific support (Hamilton 1997). Cooper noted numerous examples where repeated burning, often intentionally, was sufficient to eliminate woody vegetation and replace it with weedy annuals. Some annual grassland sites may previously been dominated by coastal scrub (Hopkinson and Huntsinger 2005) or native annuals (Solomeschch and Barbour 2004) and not perennial bunchgrasses.  Keeley (1993) compared site characteristics of grasslands with significant native perennial grass stands and sites lacking native perennial grasses and concluded that in the absence of disturbance by fire and livestock grazing, sites often were re-colonized by shrubs.

While the pre-settlement grassland commonly included native perennial grasses, the composition (species and amounts) of the pre-settlement grassland is uncertain.   Invasion of non-native annual species is well documented beginning with European exploration and settlement as early as the late 1600s (Hendry 1931).  The major period of invasion was in the 18th century and many of these species were well established by the following century (Keeley 1990) and invasion and expansion continue today.

Major Plants

Introduced annual grasses and forbs (Figure 2) dominate the annual grasslands. Soft chess (Bromus hordeaceus, formerly B. mollis), ripgut brome (Bromus diandrus, formerly B. rigidus), wild oats (Avena fatua and A. barbata), red brome (B. madratensis, formerly B. rubens), wild barley (Hordeum spp), and foxtail fescue (Vulpia myuros) are common grasses (Table 1). Common forbs include broadleaf filaree (Erodium botrys), redstem filaree (E. cicutarium), turkey mullein (Eremocarpus setigerus), true clovers (Trifolium spp), bur clover (Medicago polymorpha), popcorn flower (Plagiobothrys nothofulvus), and many others. California poppy (Eschscholzia californica), the State flower, is found in the annual grasslands.  Native grasses, such as Purple needlegrass (Nasella pulchra) and blue wildrye (Leymus glaucus) and native forbs can be found throughout the annual grasslands.  Native perennial grasses are more common on northern sites with mean annual rainfall greater than 150 cm (60 in). Soft chess and broadleaf filaree are common in areas with 65-100 cm (25-40 in) of rainfall, and red brome and redstem filaree are common on southern sites with less than 25 cm (10 in) of precipitation (Bartolome et al. 1980). 

Fig2.anngrass

Figure 2.  Soft chess brome, ripgut brome and wild oats are present in most annual grassland and oak woodland ecosystems in California.

Table 1.  Frequency of the 20 most common annual grassland and oak woodland understory species in quadrats along 455 transects located from Mendocino and Shasta Counties to Kern and Ventura Counties (Alonso 2008)

Common Name

 

Scientific Name

 

Frequency (%)

Soft brome

Bromus hordeaceus L.

91.9

Ripgut brome

Bromus diandrus Roth

71.6

Wild oats

Avena spp. L.

58.7

Annual ryegrass

Lolium multiflorum Lam.

47.3

Stork's bill

Erodium L'Hér. ex Ait.

43.7

Annual fescue

Vulpia K.C. Gmel.

42.6

Barley

Hordeum spp. L.

37.1

Red brome

Bromus rubens L.

28.1

Italian thistle

Carduus pycnocephalus L.

26.6

Rose clover

Trifolium hirtum All.

26.4

Medusa head

Taeniatherum caput-medusae (L.) Nevski

20.4

Tarweed N

Hemizonia spp. DC.

19.1

Purple needlegrass NP

Nassella pulchra (A.S. Hitchc.) Barkworth

17.8

Purple false brome

Brachypodium distachyon (L.) Beauv.

17.4

Bristly dogs tail grass

Cynosurus echinatus L.

17.4

Bur clover

Medicago polymorpha L.

16.9

Wild oats

Avena fatua L.

13.4

Silver hair grass

Aira caryophyllea L.

13.2

Spreading hedge parsley

Torilis arvensis (Huds.) Link

12.5

Rat-tail fescue

Vulpia myuros (L.) K.C. Gmel.

12.3

Animals 

Of  the 694 terrestrial vertebrates (amphibians, reptiles, birds, and mammals) native to California, over 285 species utilize annual grasslands for reproduction, cover, and/or including at least 97 species of mammals, 130 species of birds and approximately 73 species of amphibians and reptiles (CDFG 2011). Many of these species are on state or federal threatened and endangered lists.

Many wildlife species use the annual grasslands for foraging, but some require special habitat features such as cliffs, caves, ponds, or habitats with woody plants for breeding, resting, and escape cover. Characteristic reptiles that breed in annual grassland habitats include the western fence lizard (Sceloporus occidentalis), common garter snake (Thamnophis sirtalis), and western rattlesnake (Crotalus viridis oreganus) (Basey and Sinclear 1980). Mammals typically found in this habitat include the black-tailed jackrabbit (Lepus californicus), California ground squirrel (Spermophilus beecheyi), Botta's pocket gopher (Thomomys bottae mewa), western harvest mouse (Reithrodontomys megalotis), California vole (Microtus californicus), and coyote (Canis latrans (White et al.1980). The endangered San Joaquin kit fox (Vulpes macrotis mutica) is also found in and adjacent to the annual grasslands (U.S. Fish and Wildlife Service 1983). Common birds known to breed in annual grasslands include the burrowing owl (Athene cunicularia), short-eared owl (Asio flammeus), horned lark (Eremophila alpestris), and western meadowlark (Sturnella neglecta) (Verner et al. 1980). This habitat also provides important foraging habitat for the turkey vulture (Cathartes aura).  Images and descriptions of most of these animals are available on Wikipedia.org.

Vegetation Dynamics

Long-term Trends and Changes

While  yearly and within year variation in productivity and species composition is heavily influenced by prevailing weather,  long-term change in annual grassland productivity, species composition and ecosystem processes has been influenced by continuing waves of invasion (DiTomaso et al. 2007).  Structural changes in invaded plant communities typically cause reduced native species richness and diversity and changes in canopy structure. 

Invasive plants have altered ecosystem structure and function including hydrologic, fire and nutrient cycles.  Replacement of deep rooted native perennial grasses by annual grasses and forbs that are largely rooted in the top 12 inches (30 cm) of the soil has changed patterns of soil moisture depletion leaving a soil moisture niche for invading summer annuals such as yellow starthistle.  Additionally loss of deep rooted perennials has reduced the transfer of nutrients stored below 12 inches to the surface soil.

Fire frequency has changed from frequent burning by native Americans and early ranchers to infrequent burning today.  Frequent fire would have reduced thatch build up, and grass dominance resulting in a shift in species composition toward forb domination. 

Climate change and air pollution may also influence annual grassland species composition.  Studies have shown that forbs may increase under elevated CO2, warming and precipitation and nitrogen deposition from air pollution appears to have enabled nonnative annual grasses to invade serpentine grasslands in the San Francisco Bay area (Zavaleta et al. 2003 a, b). 

Yearly and Seasonal Variation

As germination, seedling establishment and plant growth progress during the growing season, species composition changes depending primarily on the timing and amount of precipitation and temperature (George et al. 2001a).  Consequently, understory and open grassland species composition varies seasonally and annually.  Unlike many perennial dominated grasslands, kinds and amounts (weight or cover) of herbaceous species are not stable and predictable from year to year.   Grass dominated years occur when rainfall is well-distributed or greater than normal.   Filaree years occur in low rainfall years or when residual dry matter (Bartolome et al. 2002) is low.  Drought, heavy grazing and fire result in filaree dominated understory.  Following a fire filaree may dominate the site for up to three years (Parsons and Stohlgren 1989, McDougald et al 1991).  Medusahead (Taeniatherum caput-medusa), goatgrass (Aegilops triuncialis) and yellow starthistle (Centaurea solstitialis) invasions may occur on some sites, especially on deep clay soils and more northern sites with higher rainfall. 

Disturbance Factors

While livestock grazing has been implicated as a primary reason for conversion of California’s former native grassland to one dominated by non-native annuals (Biswell 1956, Baker 1978, Minnich 1980, Sims 1988, Jackson 1985, Schoenherr 1992, Holland & Keil 1995, Hamilton 1997), some recent studies suggest that, in many areas, tillage associated with crop agriculture may have been the primary cause of the conversion (D’Antonio et al. 2007). In these areas, livestock grazing may have been the initial stressor but cultivation was probably the primary stressor leading to reduced distribution and dominance of native perennial grasses (D’ Antonio et al. 2007).  Vegetation type conversions, for the purpose of increasing forage production and reducing fire hazard, have also been responsible for conversion of woodlands and shrublands to grasslands (Love et al. 1952). 

Severe droughts in 1828, 1862 and 1864 have also contributed to the conversion to non-native annual-dominated grassland (Baker 1978, Keeley 1990, Heady 1977, D’Antonio et al. 2007).  At least eight multiyear droughts have occurred in California since 1900.  Droughts that exceed three years are uncommon, though occurrences in the past century include 1929-1934, 1947-1950, and 1987-1992. 

Some researchers have suggested that high frequency burning first by native peoples and later by Europeans may have made the former grassland susceptible to invasion by non-native species (Hervey 1949, Zavon 1982, Ahmed 1983, Keeley 1990, Fossom 1990), but Keeley and Fotheringham (2001) concluded that the effects of pre-European anthropogenic fires “were likely limited due to low population density and reduced mobility.  More recently, Malmstrom et al. (2006) have implicated grass infections with barley yellow dwarf virus in the susceptibility of native grasslands to invasion.  While all of the grassland was not subject to identical stresses the various combinations of drought, fire, cultivation and grazing can reasonably be implicated in the transition from native grassland to non-native annual-dominated grassland.

Oak Woodlands

Plant and Animal Communities

Current Plant Communities

The current oak-woodlands have tree and shrub species composition similar to historic communities but the understory is now dominated by introduced annual grasses and forbs.  Native annual and perennial grasses and forbs are present in this annual dominated understory but many are remnants of their former composition

The oak-woodlands are a multi-layered mosaic of tree, shrub and grassland patches (Figure 3).  In some locations these mosaics have been correlated with geological substrate (Cole 1980) and soil characteristics (Harrison et al. 1971). However, other researchers have found each of these vegetation types on most soil depths, slopes, aspects and all geological substrates suggesting that disturbance (fire) and/or biological factors (competition, grazing and browsing) are important determinants of the patchy distribution of these vegetation types (Wells 1962, Callaway and Davis 1991) at a scale smaller than an ecological site or even a soil mapping unit.  Given this mosaic of multi-layered vegetation types there is wide amplitude in expected species composition and amounts on the same soil series or association within an ecological site. 

Fig3.multilayer

Figure 3.  The oak-woodlands are often a mosaic of oak, grass and shrub patches.

Oak trees are an important component of the ecosystem serving a valuable role in retention of nutrients which in turn contributes to long-term ecosystem sustainability (Figure 4).  Nutrient cycling studies have shown that oak trees create islands of enhanced fertility through organic matter incorporation and nutrient cycling. Compared to adjacent grasslands, soils beneath the oak canopy have a lower bulk density, higher pH, and greater concentrations of organic carbon, nitrogen, total and available P, and exchangeable Ca, Mg, and K (Figure 5), especially in the upper soil horizons (Dahlgren et al. 1997).  Removal of oak trees results in loss of soil fertility over a 10 to 20 year period (Kay 1987, Dahlgren & Singer 1994).

Fig4.nutrientcyc

Figure 4.  Nitrogen cycling with major pools of nitrogen (lbs./acre) for an oak woodland-grassland ecosystem in the Schubert watershed at University of California Sierra Foothill Research and Extension Center northeast of Marysville, CA (Dahlgren et al.  2003, California Agriculture 57:42-47).

 

Fig5.nutrients

Figure 5.  Selected soil quality and fertility parameters for the 0 to 5 cm surface soils beneath an oak canopy and adjacent grasslands for three oak-woodland sites (Dahlgren et al.  2003, California Agriculture 57:42-47).

 

Historic or Natural Potential Communities

Most native tree and shrub species are still present in oak-woodland communities but probably in different amounts due to changes in fire frequency, grazing pressure, harvesting and other disturbances.  The species composition of herbaceous vegetation in the oak-woodlands prior to European contact is unknown.  It is commonly held that native perennial grasses such as the bunchgrass Nassella pulchra were widespread (Clements 1934, Heady 1977).  However others have made the case that native forbs were once dominant, especially in drier parts of the woodland (Hamilton 1998).  With the introduction of domestic livestock grazing and invasion of alien species during the Spanish colonization herbaceous cover has changed from perennial to annual and from native to exotic (Holmes 1990).  Fire interval and intensity have increased (McClaren and Bartolome 1989).  Overstory cover has generally increased (Holzman and Allen-Diaz 1991).  Soil moisture late in the growing season has decreased, and soil bulk density has increased due to compaction from large herbivore numbers grazing during the rainy season (Gordon et al. 1989).

Major Plants

While there are around 2000 plant species in the oak-woodlands a few tree, shrub and herbaceous species dominate the species composition.  Blue oak, interior live oak and coast live oak are dominants in the oak woodlands (Figure 6).  Coast live oak (Quercus agrifolia) and blue oak (Q. douglasii) are common dominant trees in the coast range.  Other trees include toyon (Heteromeles arbutifolia), madrone (Arbutus menziesii) and coffeeberry (Rhamnus californica).  The shrub layer, if present, may include narrowleaf goldenbush (Ericameria linearifolia), chamise (Adenostema fasiculatum), black sage (Salvia mellifera), and coast sagebrush (Artemesia californica).  In the Sierra Nevada foothills dominant trees include blue oak (Q. douglasii), interior live oak (Q. wislizenii), and foothill pine (Pinus sabiniana).  Black oak (Q. kelloggii) occurs at upper elevations in the transition to coniferous forest.  Dominants in the shrub layer, when present, may include wedgeleaf ceanothus (Ceanothus cuneatus), manzanita (Arctostaphylos spp.) and poison oak (Toxicodendron diversilobum).  At lower elevations and lower rainfall the oak-woodlands are often an oak savanna. With increasing elevation and slope the interior live oak and shrub component increases.

Fig6b.oaks

Figure 6.  Blue oak, interior live oak and coast live oak are dominant species in the oak woodlands.

 

The understory is dominated by annual grasses and forbs of European origin. Soft chess brome (Bromus hordeaceus, formerly B. mollis), ripgut brome (Bromus diandrus, formerly B. rigidus) and wild oats (Avena fatua) are the most prevalent grasses in the foothill oak-woodlands and filaree (Erodium spp) is the most prevalent forb.  Native perennial grasses such as California needlegrass (Nasella pulchra) and blue wildrye (Leymus triticoides) may also be present.  Patches on shallow soils are often dominated by filaree or other low growing forbs.  Deep soils with higher water holding capacity are often dominated by wild oats and other tall annual grasses.  Oak canopies influence species composition of the understory.  Studies have shown that oak canopies favor wild oats, soft chess and ripgut brome (Holland 1980 and Ratliff et al. 1991).   

Animals 

Of  the 632 terrestrial vertebrates (amphibians, reptiles, birds, and mammals) native to California, over 300 species use oak-woodlands for food, cover or reproduction, including at least 120 species of mammals, 147 species of birds and approximately 60 species of amphibians and reptiles (Tietje et al.  2005). Many of these species are on state and federal threatened and endangered lists.

California quail (Callipepla californicus), Beechey ground squirrels (Spermophilus beecheyi), Botta pocket gopher (Thomomys bottae mewa), are common in the oak woodlands as are Audubon cottontail (Sylvilagus audubonii vallicola), and deer (Odocoileus spp).  The rich rodent and lagomorph population is an important food source for common predators including: bobcat (Lynx rufus californicus), coyote (Canis latrans) and the Pacific rattlesnake (Crotalus viridis oreganus).  The value of this site for food or cover changes seasonally with the vegetation.  In habitat planning each plant community and each species needs must be considered individually and collectively.

Vegetation Dynamics

Long-term Trends and Changes

Fire is a natural part of oak-woodland ecosystems and a driving force behind vegetation change. Following fire the woodland often has a savanna structure until shrubs and small trees begin to fill the space between the existing trees. Competition between the species that germinate or resprout following fire or other disturbances, mediated by weather and soil moisture conditions, greatly influence the vegetation states present in the oak-woodlands.  On some soils, geological substrates, and aspects; tree, shrub and grass patches are all possible vegetation states (Figure 3).  Shallow soils, coarse and rocky soils and southern aspects sometimes limit vegetation to shrub dominated states.  Frequent fire tends to result in vegetation states dominated by an oak-annual grass community (Figure 7).  Protection from fire and grazing results in a gradual increase in shrubs contributing to increased fuel loads.  As the shrub canopy reaches into the tree canopy the potential for crown fires increases.  Protection from browsing reduces hedging allowing the oak canopy to reach the ground layer increasing the chances for ground fires to become crown fires. Crown fires can top-kill oak trees.  While interior live oak (Q. wislizenii) will resprout vigorously, blue oak may not resprout vigorously in some locations.   Grazing and browsing may slow the recovery of woody plants following fire (Johnson and Fitzhugh 1990).  Vegetation dynamics for many oak woodland sites have been compiled in state and transition models and published by USDA NRCS in ecological site descriptions (http://esis.sc.egov.usda.gov/Welcome/pgESDWelcome.aspx). 

Fig7.oakST.jpg

Figure 7.  Frequent fire tends to result in oak-woodland vegetation states dominated by an oak-annual grass community.  Protection from fire and grazing results in a gradual increase in shrubs contributing to increased fuel loads and increased risk of crown fires.

 

Yearly and Seasonal Variation

Species composition and productivity of the annual dominated understory grasses and forbs vary greatly within and between years and is greatly influenced by the timing and amount of precipitation and the amount of residual dry matter (George et al. 2001a).  Grass dominated years occur when rainfall is well-distributed or greater than normal.   Filaree years occur in low rainfall years or when residual dry matter (Bartolome et al. 2002) is low.  Drought, heavy grazing and fire result in filaree dominated understory.  Following a fire filaree may dominate the site for up to three years (Parsons and Stohlgren 1989, McDougald et al 1991).  Medusahead (Taeniatherum caput-medusa), goatgrass (Aegilops triuncialis) and yellow starthistle (Centaurea solstitialis) invasions may occur on some sites, especially on deep clay soils and more northern sites with higher rainfall. 

As germination, seedling establishment and plant growth progress during the growing season, species composition changes depending primarily on the timing and amount of precipitation and temperature (George et al. 2001a).  Consequently, understory and open grassland species composition varies seasonally and annually.  Unlike many perennial dominated grasslands, kinds and amounts (weight or cover) of herbaceous species are not stable and predictable from year to year.  Grass dominated years occur when rainfall is well-distributed or greater than normal.   Filaree years occur in low rainfall years or when residual dry matter (Bartolome et al. 2002) is low.  Drought, heavy grazing and fire result in filaree dominated understory.  Following a fire filaree may dominate the site for up to three years (Parsons and Stohlgren 1989, McDougald et al 1991).  

Disturbance Factors

Fire

Fire is a natural process in oak-woodland ecosystems.  Fire influences community structure, nutrient cycling, regeneration from seeds and resprouts, habitat,  and livestock grazing.  As in other ecosystems, fire effects are governed by the frequency, timing, intensity and landscape complexity.  Adjacent communities, especially chaparral and forests, influence oak-woodland fire regimes.  Increasing interval between fires increases the risk of catastrophic fire with far reaching ecological and economic impacts.  

Lightening caused fires, while infrequent, have surely influenced the structure of the oak-woodlands.  Native Americans used fire as a management tool to enhance habitat and to manage food and fiber plants.  McClaren (1986) and McClaren and Bartolome (1989) estimated fire return intervals of about 25 years prior to European settlement.   After settlement the return interval was around 7 years due to burning by settlers.   In the 1940s Sampson (1940) estimated that oak-woodland burning by ranchers resulted in return intervals of 8-15 years.  While prescribed burning continues today, urbanization and air quality concerns have reduced the use of fire as a management tool.  Today fire frequency is more likely to be on the order of 25 to 50 years or longer.  Prescribed burning, mechanical and chemical brush control have been used to remove the shrub and tree layers but have been used infrequently since the beginning of the 21st century (Murphy and Crampton 1964, Murphy and Berry 1973).

Oak trees are fire adapted by virtue of their thick bark and tendency to resprout following fire.  Several shrubs in the oak-woodlands also resprout following fire and some, such as ceanothus, are stimulated to germinate by fire.  Fire is also important because it reduces ladder fuels reducing fire hazard and it kills diseases and pests that infest oaks and other species (Allen-Diaz et al. 2007)

Drought

California’s annual rangeland forage production varies greatly over short distances due to variations in rainfall, soil characteristics and topography.  The coastal areas of a county may have adequate rainfall but drier inland locations may have low rainfall and forage reductions exceeding 50 percent during dry years.

At least eight multiyear droughts have occurred in California since 1900.  Droughts that exceed three years are uncommon, though occurrences in the past century include 1929-1934, 1947-1950, and 1987-1992.  Severe droughts in 1850-1851 and 1862-1864 have been implicated in the conversion of the former native perennial grassland to a grassland dominated by annual grasses and forbs (D’Antonio et al. 2007). 

Grazing

Grazing has positive and negative effects on oak-woodland ecosystem sustainability.  Positive grazing effects include reduced moisture competition between oaks and herbaceous understory, reduced habitat for rodents that consume oak seedlings and acorns and elimination of ladder fuels that increase the risk of crown fire.  Negative effects of grazing include increased soil compaction due to grazing during the wet season, consumption of acorns and oak seedlings and reduced soil organic matter.

Disease

Sudden Oak Death is the common name of a disease caused by the plant pathogen Phytophthora ramorum. The disease kills oak and other species of tree and has had devastating effects on the oak populations in coastal California and Oregon. Symptoms include bleeding cankers on the tree's trunk and dieback of the foliage, in many cases eventually leading to the death of the tree.

Ecosystem Services

Ecosystem services are the benefits humanity obtains from the environment, and are generally categorized into four service types: provisioning, regulating, habitat, and cultural (TEEB; MEA 2005). California’s annual grasslands and oak woodlands provide multiple benefits to society, including forage and livestock production, wildlife habitat, recreation, carbon sequestration, and drinking water supply (Table 2). Management and conservation of rangelands is critical in maintaining ecosystem function and capacity to support goods and services over time. Services can be provided locally by an ecosystem, but the benefits to human well-being can also accrue across multiple scales (de Groot et al. 2010). For example, agricultural production can provide food at the local and global levels; managed watersheds and open space provide water and nutrient cycling and community value at the regional level; and conservation practices can provide carbon sequestration and climate regulating functions at the global level.

Across California’s annual grasslands and oak woodlands, there has been a historical focus on agricultural production, with the goal of sustaining the state and national food supply; however, there is increasing societal demand for provisioning agricultural goods (e.g., livestock and forage production) and additional services (e.g., abundant and high quality water, wildlife habitat) through the management and conservation of these lands (Briske 2011). Balancing tradeoffs between agricultural production and the maintenance of ecosystem services will be a key challenge. Here, we highlight an example framework for understanding multiple ecosystem service provisioning across a managed oak woodland-annual grassland system.

Case Study

During the mid-20th century, approximately 1.9 million acres of oak woodland were cleared to create productive, open grasslands (Biswell 1954; Murphy and Crampton 1964; Bolsinger 1988). The UC Sierra Foothill Research and Extension Center (SFREC)—located in the northern Sierra Nevada foothills in Yuba County, California—has been a natural laboratory for oak woodland research (McCreary 2010). At SFREC, woody species (predominantly Q. douglasii, Q. wislizeni, Ceanothus spp., and Toxicodendron diversilobum) were actively cleared during the 1960s for forage improvement objectives, and selective woody species removal continued throughout the 1970s and late 1980s. The resulting gradient of woody cover (i.e., cleared open grassland, thinned savanna, and unthinned woodland) has served as a model managed landscape to assess tradeoffs and synergies between multiple ecosystem service-based goals across different management scenarios.

State-and-transition models have been proposed as a framework to explicitly assess tradeoffs and win-wins for ecosystem management options (George 1992; Eastburn et al. In prep). Spider diagrams are one approach to simply illustrate relative quantities of goods and services associated with different ecosystem management options (e.g., alternative vegetation states in a state and transition model). Figure 8 demonstrates the tradeoffs and win-wins in ecosystem response based on alternative vegetation states adapted from George et al. (1992) and Huntsinger and Bartolome (1992) for the Sierra Nevada foothill gravelly-loam ecological site.

Fig8.ES_SpiderDiagram

Figure 8.  Spider diagrams illustrating the quantities of multiple goods and services under different ecosystem management options—resulting in alternative vegetation states (grassland (<10% canopy cover), savanna (10-49% canopy cover), and oak woodland ((<50% canopy cover)). Data on ecosystem service indicators were collected across 5,300 acres of managed oak woodland-annual grassland at the Sierra Foothill Research and Extension Center in Yuba County, California.

 

For each ecosystem service, the maximum distance from the center of the diagram represents the highest level of provisioning (i.e., relativized by maximum observed levels across all three states); therefore, the extent of area covered within each diagram allows for direct visual comparison of trade-offs and win-wins. For example, while the grassland state maximizes agricultural productivity, there are clear trade-offs for soil health and biodiversity and habitat relative to the other management options. The savanna state highlights a local management opportunity to balance multiple ecosystem service goals.

At the landscape scale, maintaining a heterogeneous mosaic of vegetation patches optimizes the benefits of different ecosystem management options—including increased agricultural productivity, maintaining water and nutrient cycling capacity, protecting genetic resources, and enhancing the number of habitat types. Less apparent synergies exist that cannot be directly quantified; notably, conservation of oak woodland-annual grassland landscapes has been linked to socio-economic sustainability (Huntsinger and Hopkinson 1996; Wetzel et al. 2012). Appropriate economic and social valuations for ecosystem services, taking into account tradeoffs and synergies across space and time, remain an open question (de Groot et al. 2010; Villa et al. 2014).

Chaparral

Plant and Animal Communities

Current Plant Communities

Chaparral is composed largely of evergreen, sclerophyllous shrub species that range from 1 to 4 meters in height (Figure 9).  Other growth forms including soft-leaved subshrubs, perennial herbs, geophytes (bulbs and corms), and annual herbs are less abundant in mature chaparral but can be present in abundance in early and late successional stands of chaparral (Keeley and Keeley 1984). Sparse stands of trees can occur within chaparral, typically within transition areas with conifers at higher elevations and oaks at lower elevations (Hanes 1977; Keeley and Keeley 1984). Depending on the species composition and underlying topography and soil, the structure of chaparral can range from low, monotonous, smooth-textured vegetation to more heterogeneous stands approaching the vertical structure of woodlands (Keeley 2000).

Fig9.chap

Figure 9.  Chaparral is composed largely of evergreen, sclerophyllous shrub species that range from 1 to 4 meters (3 to 13 feet) in height.

 

From inland and high elevations to coastal locations, chaparral occurs in both large continuous stands or within a mosaic of vegetation types including coastal sage scrub, annual grasslands, oak woodlands, conifer forests and wetland habitats (Heady 1977; Hanes 1977; Callaway and Davis 1993). Chaparral near the coast tends to occur in disjunct patches occupying more mesic sites whereas coastal sage scrub is distributed more extensively in drier habitats (Kirkpatrick and Hutchinson 1980; Malanson and O’Leary 1994). Mountain foothill and high elevation stands of chaparral are larger and more continuous. Coastal sage scrub occurs in smaller patches generally restricted to steep and south-facing exposures (Keeley 2000; PSBS 1995). Oak woodlands often border chaparral in more mesic areas (e.g. adjacent to stream channels, ravines, north-facing slopes) that have developed deeper soils (Griffen 1977). Oak woodlands are thought to develop within late successional chaparral in areas with more developed soils (Cooper 1922; Wells 1962). The native grassland-chaparral interface is not well understood; however, research has shown cases of type conversion from chaparral to nonnative annual grasslands with frequent fire or mechanical disturbance (Zedler et al. 1983).

Chaparral generally is thought to be a fire dependent system based on the many adaptations of its characteristic species, and its resilience in form and species composition to periodic burning (Keeley 1986; Keeley 1992). Most of the characteristic shrub species in chaparral can be organized into three adaptive strategies related to fire: (1) shrubs that have stems that resprout following fire from below ground burls (Figure 10); (2) shrubs that produce large amounts of dormant seed that persist for long periods of time and germinate by heat or chemical processes initiated by fire (obligate seeders); and (3) plants that apply both strategies (Keeley 1977). Within chaparral vegetation non-shrub plant growth forms may also employ these strategies or fire avoidance to persist within this fire prone system (e.g., geophyte species whose bulbs or corms persist following fire, annual herb species with long seed dormancy and heavy annual seed production, and annuals with the ability to disperse seeds over long distances) (Keeley 1986).

Fig10.chamresprout

Figure 10.  Chamise resprouts from the base of the shrub following fire.

 

The species composition of a particular chaparral stand is largely influenced by fire. Chaparral generally returns to pre-fire structure and composition within a normal fire regime (Keeley 1986); however, considerable research has documented various effects of fire regime on species mortality (Keeley 2000). Frequency of fire has been shown to affect chaparral species composition, where short fire intervals may eliminate obligate seeding species in favor of resprouters (Keeley 1986; 1992). Additional research has shown that fire temperature or intensity also has a strong influence on post-fire species composition (Davis et al. 1989; Rice 1993; Tyler 1995). Stand age following fire is thought to influence the reproduction of species based on reproductive strategies. Research has shown that seedling recruitment is more common for resprouting species in old (> 56 yr.) stands of chaparral whereas seedling recruitment for obligate seeding species was extremely uncommon (Keeley 1986; 1992). This research has led to the conclusion that short interval fires may adversely affect the presence of obligate resprouting species in favor of obligate seeders.

Historic or Natural Potential Communities

The present distribution of chaparral is the result of drying during the Holocene climatic period but chaparral species have their origins in the Miocene period and earlier. Chamise was present in Baja California 10,200 BP (before present). There is little evidence that chaparral is replaced by other vegetation types after a century without fire. Most changes result from changing dominance patterns within the shrub flora. Ceanothus, an obligate fire seeder, varies markedly in its longevity. Some species (e.g. C. tomentosus appear to be relatively short-lived, on the order of 30 to 50 years while others persist longer (e.g. C. greggii). Some obligate fire seeders in the genus Arctostaphylos are much longer lived and persist for a century or more (Keeley and Davis 2007).

Major Plants

The floristic composition of chaparral varies depending on biogeography, local habitat characteristics and fire history. Of the many growth forms present in chaparral, woody evergreen perennials are the dominant plants and, as such, exert the most influence on the habitat. Chamise (Figure 11) is the most common and widespread species within the chaparral vegetation type (Hanes 1971). This species occurs in most stands of chaparral and is the dominant plant in drier habitats (Keeley 2000). The ubiquity of this species is likely explained by its many adaptations to drought, fire and disturbance (Hanes 1977). Other common shrub species include representatives from manzanita (Arctostaphylos spp.), ceanothus, silk-tassel bush (Garrya spp.), oak (Quercus spp.), redberry (Rhamnus spp.), Rhus spp., laurel sumac (Malosma laurina), mountain mahogany (Cercocarpus betuloides), toyon (Heteromeles arbutifolia), holly-leaf cherry (Prunus ilicifolia), and mission manzanita (Xylococcus bicolor) (Holland 1986).

Fig11.chamise

Figure 11. Chamise is the most common and widespread species within the chaparral vegetation type.

 

Soft-leaved subshrubs are less common in true chaparral than in coastal sage scrub but occur within canopy gaps of mature stands, and may be more prevalent following fire (Holland 1986; Keeley and Keeley 1984; Sawyer and Keeler-Wolf 1995). Common species include California buckwheat (Eriogonum fasciculatum), sages (Salvia spp.), California sagebrush (Artemisia californica), and monkeyflower (Mimulus spp.). Suffrutescent and perennial herbaceous species commonly include deerweed (Lotus scoparius), nightshade (Solanum spp.), Spanish bayonet (Yucca whipplei), rock-rose (Helianthemum scoparium), golden yarrow (Eriophyllum confertiflorum), Bloomeria spp., Brodiaea spp., onion (Allium spp.), sanicle (Sanicula spp.), Lomatium spp., soap plant (Chlorogalum spp.), and bunch grasses (Nassella spp., and Melica spp.) (Holland 1986; Keeley and Keeley 1984; Sawyer and Keeler-Wolf 1995). Vines commonly present in chaparral include wild cucumber (Marah spp.), dodder (Cuscuta spp.), chaparral-pea (Lathyrus spp.), bedstraw (Galium spp.), poison oak (Toxicodendron diversilobum), and honeysuckle (Lonicera spp.) (Keeley and Keeley 1984). Annual species persisting in mature chaparral or in the post-burn flora vary according to geographic location, but typically include lupine (Lupinus spp.), Lotus spp., California threadstem (Pterostegia drymarioides), Claytonia spp., Gnaphalium spp., Phacelia spp., Gilia spp., whispering bells (Emmenanthe pendulflora), fiesta flower (Pholistoma spp.), and many others (Holland 1986; Keeley and Keeley 1984; Sawyer and Keeler-Wolf 1995).

Sampson (1944) separated the California chaparral association into five ecological regions on the basis of climatic variation and differences in floristic composition.  The regions are 1) North Coastal Region, 2) Central Coastal Region, 3) South Coastal Region, 4) North Sierran Region and 5) South Sierran Region.  Each of these regions has the same dominate growth forms in common but there are differences in topography and to some extent distinctive species. 

The North Coastal Region extends from San Francisco Bay to Trinity and Shasta Counties.  The dominant chaparral species in this region are:

Sprouting Species

Nonsprouting   Species

California scrub oak

Quercus berberidifolia

Common manzanita

Arctostaphylos   manzanita

Chamise

Adenostoma fasciculatum

Hoary manzanita

Arctostaphylos   canescens

Eastwood manzanita

Arctostaphylos   glandulosa

Stanford’s manzanita

Arctostaphylos   stanfordiana

Interior live oak

Quercus   wislizeni

Wedgeleaf ceanothus

Ceanothus   cuneatus

Leather oak

Quercus durata

Whiteleaf manzanita

Arctostaphylos   manzanita

Arctostaphylos   viscida

Western Mountain Mahogany Cercocarpus betuloides

 

 

The Central Coast Region extends from the San Francisco Bay to Santa Barbara and Ventura counties.  The dominant chaparral species in the Central Coast Region are:

Sprouting Species

Nonsprouting   Species

California scrub oak

Quercus berberidifolia

Bigberry manzanita

Arctostaphylos glauca

Chamise

Adenostoma fasciculatum

Jimbrush ceanothus

Ceanothus sorediatus

Eastwood manzanita

Arctostaphylos   glandulosa

Parry ceanothus

Ceanothus   parryi

Interior live oak

Quercus   wislizeni

Wedgeleaf ceanothus

Ceanothus   cuneatus

Canyon live oak

Quercus   chrysolepis

Wartleaf ceanothus

Ceanothus papillosus

Greenback ceanothus

Ceanothus spinosus

 

Chaparral whitethorn

Ceanothus   leucodermis

 

 

The South Coast Region extends through Ventura, Los Angeles, and San Bernardino and south to Baja California.  Chaparral is confined to the coastal ranges and the San Bernardino Mountains which border the Mojave Desert on the west.


 

Sprouting Species

Nonsprouting   Species

Canyon live oak

Quercus   chrysolepis

Bigberry manzanita

Arctostaphylos glauca

Chamise

Adenostoma fasciculatum

Hairy ceanothus

Ceanothus oliganthus

Eastwood manzanita

Arctostaphylos   glandulosa

Parry ceanothus

Ceanothus   parryi

Interior live oak

Quercus   wislizeni

Wedgeleaf ceanothus

Ceanothus   cuneatus

Mission manzanita

Xylococcus   bicolor

Bigpod ceanothus

Ceanothus megacarpus

Redshanks or Ribbonwood

Adenostoma sparsifolium

Cup-leaf Lilac

Ceanothus greggii   perplexans

Chaparral whitethorn

Ceanothus   leucodermis

 

Western Mountain Mahogany Cercocarpus betuloides

 

 

The North Sierran Region extends from Butte and Tehama Counties to southward along the Sierra Nevada foothill to Mariposa County.  Chaparral occurs intermittently as isolated stands within open forests.  The dominant chaparral species of this regions are:

Sprouting Species

Nonsprouting   Species

California scrub oak

Quercus berberidifolia

Mariposa manzanita

Arctostaphylos viscida ssp. mariposa

Chamise

Adenostoma fasciculatum

Wedgeleaf ceanothus

Ceanothus   cuneatus

Indian manzanita

Arctostaphylos   mewukka

Whiteleaf manzanita

Arctostaphylos   manzanita

Arctostaphylos   viscida

Brewer Oak

Quercus garryana   var. breweri

 

Toyon

Heteromeles arbutifolia

 

Western Mountain Mahogany Cercocarpus betuloides

 

Woollyleaf ceanothus

Ceanothus tomentosus

 

 

The South Sierran Region extends from Mariposa County to Kern County.  The dominant species in this region include:

Sprouting Species

Nonsprouting   Species

California scrub oak

Quercus berberidifolia

Mariposa manzanita

Arctostaphylos viscida ssp. mariposa

Chamise

Adenostoma fasciculatum

Wedgeleaf ceanothus

Ceanothus   cuneatus

Brewer Oak

Quercus garryana   var. breweri

 

Interior live oak

Quercus   wislizeni

 

Western Mountain Mahogany Cercocarpus betuloides

 

 

Animals 

The abundance and diversity of wildlife in California's chaparral is not commonly recognized. The iconic, but now extinct, California grizzly bear and the majestic California condor, which nearly became extinct and remains endangered, are the chaparral's most famous animal residents.  Chaparral habitat supports nearly 50 species of mammals, but none live exclusively in chaparral.  Some are found primarily in mature chaparral and others in young chaparral and along ecotones between chaparral and other plant communities.  Several prefer riparian areas in and near chaparral.  Predators in California’s chaparral include mountain lions, bobcats and coyotes.  These predators prey on black tail deer, rabbits and ground squirrels (Quinn 1990).

Although many bird species travel over and through the chaparral, only a few reside year-round (California Chaparral Institute 2012). Common birds in chaparral ecosystems include the Wrentit (Chamaea fasciata, observed mostly by call), Western Scrub Jay (Aphelocoma californica), California Towhee, (Melozone crissalis), Spotted Towhee (Pipilo maculatus)  and California Thrasher (Toxostoma redivivum).  Birds especially common in chaparral for several years after a fire include Costa's Hummingbird (Calypte costae, especially spring and summer), Sage Sparrow (Artemisiospiza belli, mostly winter), Rufous-crowned Sparrow (Aimophila ruficeps), Lazuli Bunting (Passerina amoena, April through September), Lawrence's Goldfinch (Carduelis lawrencei), and Black-chinned sparrow (Spizella atrogularis, April through summer months).

Postfire succession of birds (Alten 1981, Wirtz 1979a), reptiles (Simovich 1979), mammals (Quinn 1990, Wirtz 1977), and insects (Force 1982) has been studied. Currently, information suggests that, in general, wildlife habitat may be optimized by maintaining chaparral in many age classes, by restricting the size of burned or treated areas, by protecting trees, and by enhancing water sources (Quinn 1990).

Vegetation Dynamics

Long-term Trends and Changes

Chaparral has been described as “autosuccessional,” undergoing a rapid succession from largely herbaceous flora immediately after fire to relatively dense woody vegetation in a short time period with minimal loss of species (Hanes 1971; Zedler and Zammit 1989). Immediately after a disturbance, usually fire, the grasses and forbs initially dominate because of their sheer numbers and showy flowers. Within 2 - 5 years the seedlings of chaparral plants and the shrubs resprouting from their crown or germinating in response to fire take over. Their more aggressive root systems exploit deeper water reserves and they will eventually shade out the forbs and grasses and replace them. The shrub phase of succession starts the first year after a fire from crown sprouts and seedlings, and by the fifth year are tall enough to shade out the shorter herbs and approach a climax community.

Early research suggested that without fire, chaparral would develop into oak woodlands or grasslands (Sampson 1944; Wells 1962). Chaparral succession to oak woodlands may occur in mesic situations adjacent to current stands of oak woodlands (e.g., Callaway and D’Antonio 1991) but most research has provided examples of greater than 100 year-old chaparral stands without evidence of physiognomic succession (Zedler 1981; Keeley 1992). This research has shown that in addition to remaining stable and reproductively viable following long periods without fire, some chaparral species (most resprouting species) sexually reproduce largely within older aged stands (Zedler 1981; Keeley 1992). Additional research has shown that high frequency burning of chaparral in the presence of non-native grasses can cause type-conversion from shrublands to non-native grasslands (Wells 1962; Zedler et al. 1983; Keeley 1990). So while chaparral appears to be fire-adapted, it can remain healthy for long periods without fire and too-frequent fire may cause conversion to grassland.

Inhibition of seed germination between fires has been related to chemicals produced by some of the chaparral shrubs. This is termed allelopathy. For example, chamise leaves have a water-soluble compound that washes to the soil and stops germination of numerous herbs. This, in part, explains the bare soil appearance beneath mature chamise. Manzanita shrubs in all parts, leaves, stems, roots, and old leaf litter, contain chemicals, which inhibit seed germination of itself and other plant species. Even upon removal of manzanita stems and leaves, the chemical compounds in the soil continue to inhibit seed germination for some years. Fire is important in burning the compound out of the soil so that seed germination can take place.

In chaparral communities, there is a narrow zone of bare earth between the chaparral and the adjacent grassland communities. When first observed, it was thought to be due to chemical allelopathy. Tests showed that grass and herb seedlings treated with extracts of the adjacent allelopathic shrubs would not grow. However, an animal ecologist disputed this and performed an experiment by placing wire cages over portions of the bare zone. He showed that the herbs and grass would grow if protected from small herbivores, primarily small rodents and rabbits, that use chaparral as protective cover, from predatory hawks.

Yearly and Seasonal Variation

Biological activity in chaparral increases sharply with the first fall rains.  At this time seeds of grasses and forbs germinate, grow through the mild winter and flower by mid-spring. Next season's seed matures by late spring or early summer when the dry season and high temperatures begin. The shrubs and trees do most of their annual growth at the start of the fall rains in October and November. Many are in flower as early as December and January. By flowering this early, there is adequate moisture available for seed maturation before the summer drought begins.

Disturbance Factors

Because chaparral and many of its component species are widely distributed there is no direct threat to chaparral as a vegetation type. Certain stands of chaparral that support sensitive species (e.g., Rainbow manzanita, Arctostaphylos rainbowensis) or unique species composition may be threatened by urban development or type conversion at local scales. Large-scale changes in climate or pollution may affect the distribution of chaparral species but research on the effects of potential changes is not well developed. Fire suppression has been described as a threat to chaparral but this has not been demonstrated over large areas.

The dominant driving force in chaparral is fire. The majority of chaparral species are either adapted to occasional fire or are able to persist in fire prone ecological regimes (Hanes 1977; Zedler and Zammit 1989). The distribution and species composition of chaparral, at the landscape-scale, is largely influenced by varying interactions between fire regime (frequency, seasonal timing, size, and intensity) and physical environment (Zedler 1981; Zedler et al. 1983; Davis et al. 1988; Moreno and Oechel 1991a; Minnich 1995; Keeley 2000). The primary source of wildfires prior to human alteration of the “natural” fire regime was lightning (Keeley 1982). Although lightning-caused fires remain, fire regimes have changed due to increased anthropogenic ignitions and fire suppression (Keeley 1982; Timbrook et al. 1982; Minnich 1995).

Volatilization of nitrogen and to a lesser degree potassium are important fire associated nutrient losses. Some nitrogen is recovered or replaced by nitrogen fixing legumes such as lupine (Lupinus spp.) and deerweed (Lotus spp.), and non-leguminous plants such as California lilac (Ceanothus spp.). The interrelationships among soil microorganisms, heating rates associated with wildfires or prescribed burns, soil moisture at the time of a fire, and various nitrogen-fixing plant species have been studied, but much remains to be learned about the dynamics of nutrients in chaparral systems. Soil erosion following fire results in large losses of all nutrients (Conrad et al. 1986). 

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List of Tables

Frequency of the 20 most common annual grassland and oak woodland understory species in quadrats along 455 transects located from Mendocino and Shasta Counties to Kern and Ventura Counties (Alonso 2008)

List of Figures

Figure 1.  Location and area of annual rangelands (oak woodlands, annual grasslands and chaparral) and other rangeland types in California.

Figure 2.  Soft chess brome, ripgut brome and wild oats are present in most annual grassland and oak woodland ecosystems in California.

 Figure 3.  The oak-woodlands are often a mosaic of oak, grass and shrub patches.

Figure 4.  Nitrogen cycling with major pools of nitrogen (lbs./acre) for an oak woodland-grassland ecosystem in the Schubert watershed at University of California Sierra Foothill Research and Extension Center northeast of Marysville, CA (Dahlgren et al.  2003, California Agriculture 57:42-47).

Figure 5.  Selected soil quality and fertility parameters for the 0 to 5 cm surface soils beneath an oak canopy and adjacent grasslands for three oak-woodland sites (Dahlgren et al.  2003, California Agriculture 57:42-47).

Figure 6.  Blue oak, interior live oak and coast live oak are dominant species in the oak woodlands.

Figure 7.  Frequent fire tends to result in oak-woodland vegetation states dominated by an oak-annual grass community.  Protection from fire and grazing results in a gradual increase in shrubs contributing to increased fuel loads and increased risk of crown fires.

Figure 8.  Chaparral is composed largely of evergreen, sclerophyllous shrub species that range from 1 to 4 meters (3 to 13 feet) in height.

Figure 9.  Chamise resprouts from the base of the shrub following fire.

Figure 10. Chamise is the most common and widespread species within the chaparral vegetation type

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