North American Oak Woodland

California's oak woodlands are a unique ecosystem providing habitat to nearly half of the state's vertebrate animals. Historically they have provided food and fiber to their human inhabitants. They are crucial to water quality protection because most of the runoff from higher elevations flows through this ecosystem and most of the state reservoirs are in the oak-woodlands. Yet they are threatened by urban encroachment and conversion to crop agriculture. Only wise management and careful land protection strategies can save this ecosystem from further reduction in area. Increasingly policy makers are coming to understand that these are working landscapes that can provide a variety of ecosystem goods and services not only to their owners but to society in general.

 

Location and distribution: 

California’s foothill oak-woodlands cover about 5 million acres along the coast range and in a ring around the central valley. The oak-woodlands form a transition zone between the annual grasslands that surround the agricultural central valley and the mixed coniferous forest at higher elevations. The foothill oak-woodlands include three sub-types: 1) blue oak-foothill pine; 2) blue oak woodland; and, 3) coast oak woodland.

Oak Woodland Distribution

Vegetation structure ranges from a sparse oak-savanna with an annual grass understory to a dense oak-woodland containing shrub and tree layers. The oak woodlands are commonly adjacent to or intermixed with annual grasslands, chaparral or other shrub dominated communities. Coast Range foothill oak-woodlands are dominated by coast live oak and/or blue oak. Valley oak and canyon live oak may also occur in these foothill ecosystems.

Oak Woodland vs. Oak Savanna

Physical characteristics: 

Climate

The climate of the oak-woodlands is Mediterranean, with precipitation falling primarily between October and May, mostly as rain. The dry season averages 6 months but may range from 2 to 11 months. Precipitation in the oak-woodlands decreases from north to south and increases with elevation. Foothill oak-woodlands generally occur at an elevation of 200-2300.

Climate Map

Winter precipitation is primarily the result of cold fronts that move across the Pacific Northwest and northern California. The frequency of these storms decreases from north to south. Rainfall in California ranges from less tha, 10” in southern desert areas to greater than 100” on the north coast. On average foothill oak-woodland precipitation ranges from 15-32”.

Each growing season starts when rainfall is sufficient to start the germination of stored seed. Young annual plants then grow rapidly if temperatures are warm (15.6° to 26.7°C or 60° to 80°F) but more slowly if cooler temperatures prevail (4.4° to 10°C or 40° to 50°F ). There is little growth during winter when temperatures are low (4.4°C or 40°F or less). Rapid spring growth begins with warming conditions in late winter or early spring. Rapid growth continues for a short time until soil moisture is exhausted. Peak standing crop occurs at the point when soil moisture limits growth or when plants are mature.

Topography and Soils

Soils and topography vary greatly across California’s oak-woodlands, with flat to gentle slopes grading into steeper slopes, often with shallower soils. The geology of the oak-woodlands is diverse. In the southern and central Sierra Nevada foothills most of the soils are from granitic parent material. In the northern Sierra Nevada foothills the soils are usually from parent material of volcanic origin. The Coast range soils are sedimentary in origin and more prone to erosion than the Sierra Nevada foothills. Mass wasting during heavy rainfall years is common on some Coast range soils.

Soil Map

Oak-woodland watersheds are predominantly drained by intermittent streams that only flow during the wet season. These intermittent streams feed into higher order permanent streams. It takes several inches of rainfall to saturate a watershed and cause intermittent streams to flow. In dry years these intermittent streams may not flow.

Plants and animals: 

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

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.

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, especially in the upper soil horizons. Removal of oak trees results in loss of soil fertility over a 10 to 20 year period.

Historic or Potential Natural 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 purple needlegrass (Nassella pulchra) were widespread.

Purple Needlegrass

Major Plants

While there are around 2000 plant species in the oak-woodlands, a few tree, shrub and herbaceous species dominate the species composition. Dominant trees in the Coast Range include coast live oak (Quercus agrifolia), blue oak (Q. douglasii), toyon (Heteromeles arbutifolia), madrone (Arbutus menziesii), and coffeeberry (Rhamnus californica)

.Oak Varieties

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.

Oak Savanna ShrubsCommon Trees in the Oak Woodland

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.

Ceanothus

The understory of the oak-woodlands 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.

Common Grasses in the Oak Woodlands

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. Many of these species are on state and federal threatened and endangered lists.

Common vertebrate species in the oak-woodlands include California quail (Callipepla californicus), Beechey ground squirrels (Spermophilus beecheyi), Botta pocket gopher (Thomomys bottae mewa), 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: 

Within the oak-woodlands vegetation changes associated with the fire cycle, succession and climate change occur over years, decades and centuries. Within the herbaceous layer there are shorter seasonal and annual changes in species composition and productivity. Disturbances such as fire, drought, grazing, and disease also influence the structure of the vegetation.

Long-term Trends

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. Frequent fire tends to result in an oak savanna dominated by an oak-annual grass community. 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.

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. 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 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. 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.

In dry years and following fire filaree tends to dominate species composition. Grass dominates when rainfall is adequate or above normal. Medusahead invades on some soils.

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. 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.

Disturbance Factors

Fire

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 intervals between fires increases the risk of catastrophic fire with far-reaching ecological and economic impacts.

Lightning-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. Researchers have 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. 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.

Oak trees are fire-adapted by virtue of their thick bark and tendency to resprout following fire. While interior live oak (Q. wislizenii) will resprout vigorously, blue oak may not resprout vigorously in some locations. Several shrubs in the oak-woodlands also resprout following fire and some, such as ceanothus, are stimulated to germinate by fire. Grazing and browsing may slow the recovery of woody plants following 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.

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% 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. 

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.

 

Management issues: 

Recognizing that the oak-woodlands provide a variety of ecosystem services and that they are threatened by urbanization, management of these lands has become more complex since the 1980s. Habitat loss and mitigation of loss is important at the landscape scale and restoration, invasive species management and water quality protection are important issues at the ownership and watershed level. Following is a brief introduction to some oak woodland issues and practices.

Carbon Sequestration

Carbon sequestration is a relatively new issue that has relevance for the oak-woodlands. Because many acres of the oak-woodlands have been thinned or cleared to increase forage production, there are opportunities to increase the storage of carbon by replanting oak-trees. Not only is carbon stored in the biomass of trees and shrubs but research has shown that the content of organic carbon is greater in the soils under oak trees than in adjacent grassland patches.

Blue Oak Regeneration

Poor regeneration of blue oak has been an issue since the 1980s. While initially attributed to livestock grazing, research has shown that poor regeneration has several causes: 1) competition from introduced Mediterranean annuals for soil moisture, 2) increased rodent populations, 3) changing fire frequencies and 4) livestock grazing (McCreary 2001). Where seedlings and saplings are scarce, regeneration is an issue. However, on some sites recruitment is sufficient to maintain stand density and regeneration is not considered a problem.

Native Grass Restoration

Needlegrass (Nasella pulchra) and other native perennial grasses occur naturally in this ecosystem but most attempts to restore native perennial grasses fail due to the competition from dominant annual grasses and forbs. California scientists have tried to restore native grasses for more than 50 years but have not found any dependable native grass restoration practices that can be recommended. Therefore native grass restoration should not be a management goal for most of the oak-woodlands.

Invasive Species

While many of the species in the herbaceous layer are alien species, several are particularly troublesome, including medusahead (Taeniatherum caput-medusa), barbed goatgrass (Aegilops triuncialis), and yellow starthistle (Centaurea solstitialis).

Invasive Plants

Urbanization/Suburbanization

Since 1973 conversion to residential and industrial uses has resulted in major oak-woodland losses. This results in increased urban-rural conflicts with farming and ranching operations. It also results in habitat fragmentation and loss of habitat for birds and other species. Some land owners and conservation organizations have collaborated to protect oak-woodlands and other ecosystems by placing land in mitigation banks and conservation easements.

Water Quality

Water quality is an issue in the oak woodlands because most of the state’s reservoirs are in or near the oak-woodlands of the Sierra Nevada foothills and these water bodies are often domestic water sources for urban areas, including San Francisco and Los Angeles. Additionally oak-woodland watersheds are the source of streams and rivers that are habitat for threatened and endangered salmonid species. Sediment, nutrients, pathogens and heat are potential pollutants that may be associated with grazing in oak-woodland watersheds.

In 2004, California’s State Water Resources Control Board adopted policies for regulating non-point source pollution (Gerstein et al. 2006). These policies affect landowners and agricultural producers, including range livestock operations. This new policy replaced the voluntary, education-supported program with regulatory programs, such as implementation of total maximum daily load (TMDL) requirements for nonpoint source discharges from agricultural lands, including grazing land.

Practices and uses: 

Since European settlement the primary use of the oak-woodlands has been for livestock production. More than 70% of the oak-woodlands are in private ownership and provide more than 80% of the state’s grazed forage. While urbanization is driving the loss of oak-woodland communities, orchards, wine grapes and irrigated pasture have displaced some of the oak-woodlands in some counties.

Grazing Management Practices

Most of the oak-woodlands are suitable for grazing by all classes of livestock at any season. Sierra foothill and coast range oak-woodland carrying capacity is commonly in the range of 10 to 30 acres per animal unit per year. However, forage quality declines below the nutritional needs of many kinds and classes of livestock during the 6 to 8 month dry season.

Forage Sources and Seasonal Use

Each ranch has a different mix of forages sources to support production. Forage sources are seasonal in their availability, productivity and quality. Season of grazing is controlled by seasonal availability and lease agreements. The annual grasslands and oak-woodlands are the largest source of grazed forage in the state.

Being dominated by annuals, these rangelands begin to grow with fall rains, providing green feed in late fall, winter and spring and low-quality dry feed the rest of the year. Traditionally ranches have moved livestock to high-elevation public and private rangelands or irrigated pasture during the summer to keep livestock on high quality forage. Many ranches provide supplements to livestock on annual range when green forage is in short supply.

Grazing Systems

Continuous grazing and seasonal suitability grazing are commonly used on annual rangelands. These grazing systems are the result of research and adaptive management (trial and error) over several generations. Some have speculated that desirable plants, particularly grasses, will be grazed excessively under continuous grazing. However, research does not support this view when proper stocking rate is implemented. With continuous grazing, stocking rate must be very light during the growing season because adequate forage must be left to carry animals through the dormant season. Under light stocking, animals are allowed maximum dietary selectivity throughout the year. For example, cattle and sheep preferentially select forbs (i.e., broad-leaved plants) during certain times of the year, which can greatly reduce grazing pressure on grasses. Rotation systems that restrict livestock from part of the range during the growing season can waste much of the forb crop because some forb species complete their life cycle quickly and become unpalatable after maturation. So the forbs have dried up and even shattered by the time some of the pastures are grazed in the rotational grazing system. Seasonal suitability grazing is a system that describes how many ranchers manage grazing and forage. It has a flexible rotation schedule that fits the needs of the ranch operation. Often the ranch is subdivided into different vegetation types such as fencing meadows from uplands. It may include installation of riparian pastures so that riparian areas can be managed separately. A few ranchers accomplish rotation without internal fences. Instead they have several water troughs and they rotate by alternately opening and closing (filling and emptying) the troughs forcing the animals to move for water.

Grazing systems are specialized grazing management practices that facilitate rest periods between grazing periods or deferment for two or more pastures. Comparisons of seasonal and rotational grazing with continuous grazing have also concluded that these specialized systems offers no forage or livestock production advantage over continuous grazing.

Animal Nutrition and Forage Quality

Typically, four nutrients are of primary concern to managers of animals on California’s annual-dominated foothill and coastal rangelands: protein, energy, carotene (the precursor of vitamin A), and phosphorus. Additionally, certain minerals may be deficient or toxic at certain times or locations. Annual range forage may be deficient in copper. A high amount of molybdenum aggravates copper deficiency. Potassium and zinc may also be deficient in mature weathered forage. Other minerals such as selenium may be found in deficient or toxic levels in certain areas of the state.

Early in the growing season, annual plants contain the highest protein content: over 15% in grasses, over 25% in filaree, and nearly 30% in bur clover. This declines to about 10% protein in grasses and 15-20% in filaree and bur clover when plants flower.

Grazing Protien Content by Month

In the 1950s researchers described three seasons based on the adequacy of annual range forage for weight gains. The inadequate-green season begins with the fall germination of stored seed. Cattle grazing this forage may lose weight, hence the term inadequate-green forage. The onset and length of this period depend on prevailing weather conditions. Rapid spring growth begins with warming conditions in late winter or early spring. This also is the period when animal performance improves. The period is commonly called rapid spring growth or the adequate-green forage season. This forage usually is nutritionally adequate for maintenance, growth, and gestation. Rapid spring growth continues for a short time until soil moisture is exhausted. This period is followed by the summer dry season when the forage is a fair energy source but is low in protein, phosphorus, carotene, and other nutrients. Some classes of livestock can be maintained on dry feed while other classes may perform poorly during the inadequate-dry season. During this summer period ranchers commonly provide supplements or transport their stock to green feed at higher elevations or on irrigated pasture.

Grazing Graph

Poisonous Plants

Several poisonous plants are present in the annual grasslands, although cattle losses to poisonous plants are rare. Livestock poisoning is often a result of hungry animals being concentrated on toxic plants. Yellow starthistle (Centaurea solstitialis) is poisonous to horses. Pyrrolizidine alkaloids in fiddleneck (Amsinkiaspp.) can cause liver damage in livestock. Several milkweeds (Asclepias spp) may be found on this ecological site including Mexican whorled milkweed. Milkweeds contain several glucosidic substances called cardenolides that are toxic to range livestock. Klamath weed (Hypericum perforatum) occurs in more northerly oak-woodlands. Larkspur (Delphiium spp.) may be present and contains several alkaloids. Cattle consume larkspur most often after plants begin flowering. Acorns and oak leaves are toxic in large quantities and may be consumed in excess if leaves are knocked from the trees by wind or hail or if animals are extremely hungry. The leading cause of livestock poisoning in California is from consumption of oleander (Nerium oleander) used in landscaping.

Brush and Weed Control

Historically, oak and shrub removal has been recommended to increase forage production in the oak-woodlands. From 1950-1980 mechanical and chemical tree and shrub control and prescribed burning were often used to selectively thin the oak-woodlands. In many cases all trees and shrubs were controlled resulting in a conversion from woodland to annual grassland. Seeding and fertilization often accompanied tree and shrub control. In most cases removal of blue oaks (deciduous) to canopy covers less than 25% have resulted in increased forage production. In the central and southern Sierra Nevada foothills blue oaks provide green understory forage earlier and in higher quantities compared to the forage in open areas. In general live oak (evergreen) stands of greater than 25 % canopy cover will have less forage growth than cleared areas.

Invasive plants cause serious ecological damage to California’s wildlands, and successfully addressing this widespread problem requires an approach that integrates diverse program components and control methods. Because public policies limit the use of fire and chemicals for weed control, species like yellow starthistle, medusahead and goatgrass require an integrated approach to weed management that often includes mechanical methods and targeted grazing.

Seeding and Fertilization

Improved forage production and quality can be achieved by seeding annual legumes (rose clover and subterranean clover) and annual grasses such as annual ryegrass. Annual legume production is improved by the application of sulfur and phosphorus fertilizers.

Nitrogen fertilization increases forage and animal production in the 16-32” rainfall zone. Rangelands in lower precipitation zones receive too little rainfall for the fertilization to be effective. Rangelands above 32” receive too much rainfall and fertilizer nutrients are leached from the soil. Because oak-woodland soils are often sulfur- and/or phosphorous- these nutrients must be added with nitrogen to achieve increases in production.

Oak Restoration

Competition for soil moisture from the understory annual plants, acorn and seedling damage by rodents, livestock grazing and changed fire regimes are important factors that can reduce blue oak regeneration. Natural regeneration of blue oaks may be limited because they are weak resprouters on some dry sites and because of a number of factors that limit seed germination, seedling establishment and survival to the tree stage. In the past 30 years researchers have developed successful restoration methods of planting acorns and transplanting seedlings and protecting naturally produced seedlings and saplings.

Native Grass Restoration

Purple needlegrass (Nasella pulchra) and other native perennial grasses occur naturally in the annual grasslands as widely dispersed individual plants and small isolated stands. Restoration of native grasses has been a recurring objective of range managers on California’s annual rangelands since the 1940s. The goal of restoring grasslands and woodland understories to some pre-settlement condition has proven to be unrealistic because not only is there uncertainty about the historical composition and extent of California native grasslands but restoration failure is common (and few showcase their failures). Rangeland and restoration scientists have tried to restore native grasses but have not found dependable native grass restoration practices for use on land that is steep, rocky or highly eroded. On arable land native grasses can be grown for seed and pasture following standard crop production practices. Scientists continue to seek practices to control the annuals and promote native perennials.

Firewood cutting of blue oak, once prevalent, has decreased with increased public awareness of poor blue oak regeneration. Cutting of interior live oak for fire wood is common.

Historical Management

Native Americans have historically used and managed the blue oak-woodlands for food and fiber. This ecological site is in the heart of California’s historic Mother Lode. Mining for gold has occurred in the past. Remnants of historic mining activity can be found on this ecological site including abandoned mines.

Additional materials: 

Wildfire on the Red Hills in Oklahoma

Photo Monitoring on the Santa Rita Experimental Range

The Final Crop

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