Southern California Oak Woodland and Savanna

These oak woodlands and savannas occur in coastal plains, intermountain valleys, and low mountains (such as the San Jacinto Mountains) from Ventura County, California, south into Baja California, Mexico. Quercus agrifolia, Quercus wislizeni, Quercus engelmannii, Quercus kelloggii, and/or Juglans californica dominate a mixed closed or open canopy. Southern chaparral species such as Adenostoma fasciculatum, Artemisia californica, Rhus integrifolia, Rhus ovata, Rhus trilobata, Ceanothus spp., Ribes spp., and Arctostaphylos spp. are also characteristic. These woodlands may occur as remnant patches on offshore islands, where they include endemic species such as Quercus tomentella and Lyonothamnus floribundus. The California central coast region may have open stands of just Juniperus californica, with a grassy understory. These stands belong here due to proximity to other oak stands or chaparral, and due to the heavy native or non-native grass cover. This is distinguished from Great Basin pinyon-juniper stands, which have little herbaceous understory, and Pinus monophylla mixed with Juniperus californica. These stands of only juniper are caused by repeated removal of the oaks by humans and feral pig populations. Variable canopy densities in existing occurrences are likely due to variation in soil moisture regime, natural patch dynamics of fire, and land use (fire suppression, livestock grazing, herbivory, etc.). Most of these woodlands and savannas have been heavily altered through urban and agricultural development throughout southern California.

Quercus agrifolia, Quercus wislizeni, Quercus engelmannii, Quercus kelloggii, and/or Juglans californica dominate a mixed closed or open canopy. Southern chaparral species such as Adenostoma fasciculatum, Artemisia californica, Rhus integrifolia, Rhus ovata, Rhus trilobata, Ceanothus spp., Ribes spp., and Arctostaphylos spp. are also characteristic. These woodlands may occur as remnant patches on offshore islands, where they include endemic species such as Quercus tomentella and Lyonothamnus floribundus.

This system occurs in coastal plains, intermountain valleys, and low mountains (such as the San Jacinto Mountains). Soils are moderately to well-drained, deep, sandy or loamy with high organic matter. Elevation ranges from sea level to 2200 m, but generally at less than 1500 m elevation. It is found on variable aspects and topography with rainfall between 13-102 cm (5-40 inches).

Variable canopy densities in existing occurrences are likely due to variation in soil moisture regime and natural patch dynamics of fire. Fire reduces the survivorship and growth of juvenile Quercus engelmannii, with seedlings especially sensitive. Stands with grassy understories tend to suffer minimal damage, but those with shrubby understories tend to top-kill the trees, which may sprout and survive (Sawyer et al. 2009). Natural fire-return intervals are 30-100 years, and occur primarily in summer to early fall. From Landfire (2007a): Typical regime is frequent, low-severity fire that likely exert positive influences on overstory productivity and canopy resilience to fire damage. Infrequent isolated areas of stand-replacement fire create gaps of grasslands that require patch-gap recruitment and edge recolonization over time. Grass fuels allow very frequent fire, up to annually. A high proportion of seedlings and saplings are top-killed in low- to moderate-severity fires. Mortality rates of different size trees decrease with increasing height and dbh. Mortality may be as much as 50-60% for trees less than 40 cm (15.7 inches) dbh. In plants that survive fires, there is a significant amount of resprouting (Lathrop and Osborne 1991, Lawson 1993, Steinberg 2002b).

Most of these woodlands and savannas have been converted through urban and agricultural development throughout southern California. Common stressors and threats include residential development, increase and spread of exotic species, fire-suppression effects, and widespread mortality of oaks from exotic pathogen sudden oak death syndrome (Phytophthora ramorum). Cutting or logging and feral pigs repeatedly remove the oaks resulting in structural changes and loss of mature trees. Modified water patterns and non-native plants have affected most remaining stands. Problems facing managers include lack of sapling recruitment, loss of mature trees because of lowered water tables, and saline irrigation runoff. Mature trees are sensitive to overwatering, pruning, grade changes, and asphalt covering their root systems. Feral pigs cause considerable damage (Howard 1992).

From Landfire (2007a): Excessive burning or grazing may result in less canopy cover and more significant understory of herbs and shrubs (e.g., Bromus spp., Avena spp., Eriogonum fasciculatum, Rhamnus ilicifolia, and Artemisia californica). Research by Principe (2002 and unpubl. data) confirmed the findings of Osborne (1989) and Lathrop and Osborne (1991) that fire, even relatively frequent fire (return interval of 3 or more years), does not seem to be as important a mortality factor as others (drought and herbivory). Also, grazed areas appear to have lower numbers of juvenile oaks than ungrazed areas.

In the west central coast regions, regional climate models project mean annual temperature increases of 1.6-1.9°C by 2070. The projected impacts will be warmer winter temperatures, earlier warming in spring and increased summer temperatures. Regional models project a decrease in mean annual rainfall of 61-188 mm by 2070. While there is greater uncertainty about the precipitation projections than for temperature, some projections call for a slightly drier future climate relative to current conditions (PRBO Conservation Science 2011). Potential climate change effects could include (PRBO Conservation Science 2011): deep-rooted or phreatophytic species under greater stress and death; drop in groundwater table; more and larger fires; increased fire frequency due to warmer temperatures resulting in drier fuels; increased invasive species due to lack of competition from native species whose vigor is reduced by drought stress, and increased fire intervals favor certain invasive species (Brooks and Minnich 2006); and increased competition for water from all users, and stresses on the already overtaxed water allocation of California agricultural system (PRBO Conservation Science 2011).

This system occurs in coastal plains and intermountain valleys from Ventura County, California, south into Baja California, Mexico.