Hardwood Rangeland Habitats
Although there are numerous ways to classify California's oak-dominated woodlands, the five vegetation types used in the California Wildlife Habitat Relationships System (CWHR) are the most commonly used and are described below. The CWHR types are based on the dominant tree species, and include Valley Oak Woodland, Blue Oak Woodland, Blue Oak-Foothill Pine Woodland, Coastal Oak Woodland, and Montane Hardwood Forest.
- Valley Oak Woodland
- Blue Oak Woodland
- Blue Oak-Foothill Pine Woodland
- Coastal Oak Woodland
- Montane Hardwood Forest
Valley Oak Woodland
Vegetation Composition and Structure: These widely scattered but sparsely occurring woodlands are dominated by valley oaks. Associated tree species in the Central Valley include California sycamore, California black walnut, California boxelder, Oregon ash, interior live oak, California buckeye, and blue oak. In the Coast Range, foothill pine and coast live oak occur in valley oak woodlands, while California black oak occurs with valley oaks at higher elevations. At low elevations close to water, valley oak is associated with Fremont cottonwood and tree willows. Valley oak woodlands vary from open savannahs to closed canopy forests. Dense stands occur along natural drainages in deep soils. Tree density tends to decrease as one moves from lowlands to uplands. The understory shrub layer can be dense along drainages and very sparse in uplands. Understory grasses and forbs are mostly introduced annuals. Mature valley oaks have well-developed crowns and reach maximum heights of 50 to 120 ft. The massive trunks (often up to 6 feet DBH) and branches of mature trees dominate valley oak woodlands.
Ecological Processes: In many areas, there is little valley oak recruitment to replace mature tree losses due to both natural and human causes. This is presumed to be related to moisture competition with grasses and forbs, wild and domestic animals feeding on acorns and seedlings, and flood control projects. Also, fire suppression has encouraged live oak and pine invasion in upland valley oak sites. Valley oaks tolerate flooding and young trees will sprout when damaged by fire. Valley oak woodlands should be able to maintain themselves with natural disturbances such as fire and flooding. However, suppression of fire and flooding has adversely affected sustainability of valley oak woodlands.
Locational Characteristics: Valley oaks are endemic to the state, meaning that they are found only in California. They occur in a patchy distribution throughout most major lowland valleys including the Sacramento-San Joaquin and those valleys occurring in the Coast Range and Transverse Range. Many valley oak woodlands occur as isolated stands in areas where surrounding habitats have been modified by agricultural, urban, and suburban activities. Annual grasslands, riparian forests, and other oak woodland types occur adjacent to valley oak woodlands. Conversion of valley oak woodlands to irrigated agricultural land uses has had the largest effect on the acreage decline of this type.
Physical Characteristics: Valley oak communities generally occur on deep, well-drained alluvial soils found in valleys and foothills below 2,400 feet. However, valley oaks can occur up to 5,600 feet as components of other vegetation types in the south Coast Range and Tehachapi Mountains.
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Blue Oak Woodland
Vegetation Composition and Structure: Blue oak woodlands are highly variable with blue oak comprising 80-100 percent of the trees present. Foothill pine, California buckeye, valley oak, interior live oak, coast live oak, canyon live oak, and California black oak are common associates of blue oak. The overstory of blue oak woodlands range from sparsely scattered trees on poor sites to nearly closed canopies on good quality sites. Annual grasses form most of the understory in open woodlands. Characteristic shrub species in this community include poison-oak, California coffeeberry, and several species of ceanothus and manzanita.
Ecological Processes: Blue oaks are relatively slow-growing, long-lived trees. Most blue oak stands exist as groups of medium to large trees with few or no young oaks. This may or may not indicate there is a regeneration problem (see chapter 9). However, there is concern that in areas of poor regeneration, blue oak woodlands may be slowly changing into savannas and grasslands as trees die and are not replaced. Since young, vigorous blue oaks can stump sprout readily, while older, decadent trees cannot, younger stands are more likely to regrow after fires. Poor blue oak recruitment from acorns occurs for several reasons. Introduced annual grasses out-compete blue oak seedlings for soil moisture. In addition, acorns and seedlings are eaten or damaged by insects, domestic livestock, and wildlife. Blue oak is also somewhat intolerant of shady conditions, and is unable to survive under dense overstory canopies. Disturbances with small openings may be needed for seedlings to survive and grow sufficiently to promote a broader age class distribution. Furthermore, reduction in annual grass biomass through fire, limited grazing, or weeding may increase seedling growth and survival.
Locational Characteristics: Blue oak woodlands form a nearly continuous band along the Sierra Nevada-Cascade foothills of the Sacramento-San Joaquin Valley. They also occur along the western foothills of the Tehachapi Mountains and eastern foothills of the Coast Range. Typically, blue oak woodlands are found below 3,000 to 4,000 feet, but this elevational threshold drops to around 2,000 feet in the northern range, and rises to around 5,000 feet in the southern range. At lower elevations on gentle slopes, blue oak woodlands typically occur as large blocks with highly variable canopy cover. On steeper ground, blue oak woodlands occur in small patches interspersed with other habitats such as annual grasslands, chaparral, riparian forests, and other types of oak woodlands.
Physical Characteristics: Blue oak woodlands occur on a wide range of soils which are often shallow, rocky, infertile, but well-drained. Blue oak woodlands occur in dry, hilly terrain where the water table is usually unavailable to trees. Over the range of blue oaks, there is considerable climatic variation, with rainfall ranging from 10 to 60 inches annually.
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Blue Oak-Foothill Pine Woodland
Vegetation Composition and Structure: Blue oak-foothill pine woodlands have a diverse mix of hardwoods, conifers, and shrubs, and widely variable overstories. Foothill pine (formerly known as digger pine) and blue oak typically form most of the overstory of this highly variable habitat type. Blue oak usually most abundant, although foothill pine is taller and dominates the overstory. Stands dominated by foothill pine have low blue oak density because of its shade intolerance. In the Sierra Nevada foothills, interior live oak and California buckeye are often associated with this type. Interior live oak becomes more abundant on steeper slopes, shallower soils, and at higher elevations. Coast live oak, valley oak, and California buckeye occur with this community in the Coast Range. In the southern Coast Range and Tehachapi Mountains, California juniper also occurs. Shrub associates include several ceanothus and manzanita species, poison-oak and California redbud, and are usually clumped in areas of full sunlight.
Ecological Processes: Blue oak and foothill pine are relatively long-lived, but foothill pine tends to grow faster than blue oak. Historically, fires occurred every 5 to 25 years in this vegetation community. Regeneration is generally thought to be infrequent throughout California. Following fire, young, vigorous blue oaks sprout well, but older, more decadent trees do not. Therefore, younger stands are more likely to replace themselves after fires. Foothill pine is susceptible to severe damage to fire. This is due to the thin bark of young trees and high resin content in the sap. Furthermore, foothill pine does not reproduce by sprouting, so fire management as a tool should be carefully considered.
Locational Characteristics: Blue oak-foothill pine woodlands are found on steeper, dryer slopes with shallower soils than blue oak woodlands. At lower elevations on gentle slopes, these two communities intermix with grasslands. At higher elevations on steeper slopes, the communities are mixed with grasslands and shrublands. Riparian woodlands may bisect these mosaics along permanent and intermittent watercourses. Blue oak-foothill pine woodlands are found throughout the range of blue oak and form a nearly continuous band along the Sierra Nevada-Cascade foothills of the Sacramento-San Joaquin Valley, except for a gap in Tulare and southern Fresno counties. Elevation ranges from 500 feet in the north to 3000 feet in the south.
Physical Characteristics: This woodland type occurs on a variety of well-drained soils. Terrain is hilly and generally dry, and water is unavailable for much of the year.
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Coastal Oak Woodland
Vegetation Composition and Structure: Coastal oak woodlands are highly variable because of their wide distribution along California's coast. Three oak species dominate the range of coastal oak woodlands: coast live oak throughout the central and southern range; Engelmann oak in a small area in southern California; and Oregon white oak in the moister, northern range of this community. Trees associated with Oregon white oak include California black oak, canyon live oak, Pacific madrone, and interior live oak. Species associated with coast live oak on moister sites are Pacific madrone, California bay, tanoak, and canyon live oak, while coast live oak occurs with valley oak, blue oak, and foothill pine on drier sites. In southern California, coast live oak is associated with interior live oak, valley oak, California black walnut and Coulter pine. Where Engelmann oak dominates, it may occur with coast live oak or in almost pure stands. Overstories range from open conditions to nearly closed canopies, resulting in a variable density of understory shrubs, grasses, and forbs. Annual grasses form most of the understory in open woodlands, but are almost non-existent in very dense woodlands. Coastal oak savannas typically occur adjacent to grassland habitats. Shrubs in closed canopy situations tolerate shade, and include toyon, poison-oak, California coffeeberry, and several species of ceanothus and manzanita.
Ecological Processes: Coast live oak, Oregon white oak, and Engelmann oak are relatively long-lived, slow-growing trees, requiring 60 to 80 years to mature to tree size under good conditions. Historically fires frequently occurred in these woodlands. Engelmann and coast live oaks are relatively resistant to low-intensity ground fires, while some mortality occurs to seedlings and saplings. Since coast live oak is fairly resistant to grazing pressure, it appears to be replacing the less resistant deciduous oaks in areas with intense grazing. Coast live oaks regeneration is generally good. However, adequate regeneration of Engelmann oak is not occurring for many of the same reasons affecting blue oaks. In Oregon white oak stands without frequent fires or other disturbances, Douglas-fir and other conifers may grow in the understory and eventually overtop the oaks.
Locational Characteristics: Coastal oak woodlands occur along California's coastal foothills and valleys. Elevations range from sea level to around 5,000 feet. On steep slopes, coastal live oak woodlands occur as relatively small woodland patches in mosaics with annual grasslands, shrublands, and riparian habitats. Blue oak woodlands and montane hardwoods are found with the more interior and higher elevation coastal oak woodlands.
Physical Characteristics: Soils and its underlying rock parent materials are extremely variable. Coastal oak woodlands typically occur on moderately to well-drained soils that are moderately deep and have low to medium fertility. As with other hardwood-rangeland communities, considerable climatic extremes exist.
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Montane Hardwood Forest
Vegetation Composition and Structure: Montane hardwood forests are perhaps the most variable of any California hardwood type. The dominant oak species vary by topography, soils, and elevation. Montane hardwood forests typically lack blue oaks and valley oaks. The characteristic oaks are canyon live oak, interior live oak, California black oak, and Oregon white oak. Many montane hardwood forests are located on fairly productive forest soils, and are not truly "hardwood rangelands", but commercial hardwood forests under the jurisdiction of the California Forest Practices Act (FPA). However, pure stands of black oak, tanoak, and madrone with no evidence of conifer associates are exempt fro the FPA at this time.
Canyon live oak often forms almost pure stands on steep canyon slopes and rocky ridge tops throughout the Coast Range, Klamath Mountains, Sierra Nevada, and Transverse and Peninsular Ranges. They have tremendously variable growth forms, ranging from shrubs with multiple trunks on rocky, steep slopes, to 60 to 70 foot tall trees on deeper soils in moister areas. Throughout the same range, California black oak tends to dominate on gentle topography at higher elevations. It grows to heights of 70 to 80 feet at maturity, with long, straight trunks in closed canopy situations. In open forests, California black oak has larger, spreading branches. Canyon live oak and California black oak are widely distributed and form the montane hardwood habitats throughout much of California's mountain areas. However, these two species are usually not associated with hardwood rangeland sites.
Interior live oak occurs with canyon live oak or alone on steep canyon slopes and rocky, steep slopes throughout the North Coast and Sierra Nevada. Its growth form varies much like canyon live oak. Both of these evergreen oaks have dense canopies. Oregon white oak dominates small amounts of montane hardwood types along the northern Coast Range and northern Sierra Nevada and Cascades. Oregon white oak grows to a height of 50 to 80 feet at maturity, with rounded crowns in open conditions and rather narrow crowns in closed conditions.
Associates of montane hardwood communities at higher elevation, good quality sites include ponderosa pine, Douglas-fir, Pacific madrone, Jeffrey pine, sugar pine, incense-cedar, and white fir. At lower elevations and poor soils with steep slopes, associates include foothill pine, knobcone pine, tanoak, and Pacific madrone. In southern California, many of the same species are found, as well as coulter pine and bigcone douglas-fir. Blue oak and valley oak can be associates at lower elevations. Understory shrub species include poison-oak, ceanothus, manzanita, mountain-mahogany, coffeeberry, wild currant, and mountain misery. Forbs and grasses are not as prevalent as on lower elevation hardwood rangeland types. Montane hardwood forests have a pronounced hardwood tree layer with poorly developed shrub and herbaceous layers.
Ecological Processes: Since oaks of montane hardwood communities are long-lived, the community is rather stable and persistent without stand replacing disturbances. Trees are initially established by acorns, and dispersal by animals plays a major role in planting and survival. Once established, the four dominant oaks - canyon live, interior live, California black, and Oregon white - can sprout vigorously from stumps, allowing rapid re-establishment after a fire. Frequent fires over relatively small areas result in a variety of age classes across the landscape. The large number of hardwood and conifer species allows this type to occupy many environments and locations. The general inaccessibility of these habitats have protected them from many of the human-induced disturbances such as intensive agricultural, residential and commercial development, grazing, and wood cutting.
Locational Characteristics: Montane hardwoods range throughout California from 300 feet near the coast to almost 9,000 ft in southern California. Surrounding habitats include conifer-dominated types, chaparral types, blue oak and valley oak woodlands, and exotic annual grasslands.
Physical Characteristics: A wide range of physical characteristics affect montane hardwood forests. Slopes range from gentle to steep. Soils are mostly rocky, coarse, and poorly developed. However, relatively large California black oak stands occur in mountain valleys on alluvial soils. Exposures tend to be south, west, and east, while conifers tend to dominate on northern exposures. Climates are typically Mediterranean but extremely variable given the wide distribution of this type. Average summer temperatures are moderate, while average winter temperatures range from near freezing to the mid-40's. Snow occurs in the winter at higher elevations, but does not remain as long as on adjacent conifer-dominated habitats.
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