California Coastal Live Oak Woodland

These Quercus agrifolia-dominated woodlands occur throughout the Pacific coastal areas from Sonoma County, California, south to Baja California. Occurrences vary in canopy cover from dense conditions that support sparse understory vegetation of Rubus ursinus, Symphoricarpos mollis, Heteromeles arbutifolia, and Toxicodendron diversilobum, to more open conditions with perennial bunchgrass understory. The latter typically occur on south-facing slopes with soils of variable depth. 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.).

This system is found mainly below 500 m elevation in foothill environments (but up to 1200 m) on alluvial terraces, canyon bottoms, streambanks, slopes, and flats. It is typically found within 100 km of the coast, largely within the coastal fog belt (Allen-Diaz et al. 2007). Soils are moderately to well-drained, deep, sandy or loamy with high organic matter. More open occurrences with perennial bunchgrass undergrowth are typically on south-facing slopes with soils of variable depth. Annual precipitation is 40-80 cm, with January mean minimum daily temperatures of 5-10°C and July mean maximum daily temperatures of 18-23°C.

From Sawyer et al. (2009): Dominant tree root system contains both roots that tap groundwater and extensive surface-feeding ones (Callaway 1990, as cited in Sawyer et al. 2009). It is the most susceptible of the California oaks to soil drought.

Fire is the dominant disturbance mechanism. Fire severity can range from high in oak woodlands with a high shrub component to moderate or low in open woodlands and savannas with a grass understory. Historically, fire occurred frequently, and the dominant oaks are resistant to low-intensity surface fires (Allen-Diaz et al. 2007). Lightning-ignited fires are uncommon but human-ignited fires may have occurred frequently given the propensity of aboriginal cultures to burn foothill environments (Keeley 2002, Landfire 2007a). Fire history does exert some effect on fire mosaic turnover, although the effect appears to be short-lived. Also, productivity (e.g., high cover of flammable shrubs and grasses) does not seem to be as strong a control on fire occurrence as meteorology (i.e., hot, dry wind events) in these systems (Landfire 2007a).

From Sawyer et al. (2009): Large trees are exceptionally fire-resistant with the thickest bark of any California oak. They generally recover well from a fire, although severely burned crowns, trunks, and root crowns may require several years to sprout. Smaller trees are less resistant, but even low to moderately severe fires often kill seedlings and saplings. Stands may attain 80 to 100% of their pre-fire densities within 10 years after fire, though fire-return intervals in natural conditions vary widely (Steinberg 2002b, Sugihara et al. 2006).

Conversion of this type has commonly come from residential and urban development; conversion to agriculture (clearing for rangeland and pastures). Common stressors and threats include widespread mortality of oaks from exotic pathogen sudden oak death syndrome (Phytophthora ramorum) (Allen-Diaz et al. 2007); land use (fire suppression, livestock grazing, herbivory, etc.). Frequent fires may create shrublands or limit oak invasion of chaparral and grasslands (Mensing 1998), while long fire-free intervals may have allowed an oak expansion (Van Dyke et al. 2001). Some studies suggest that low levels of recruitment may be related to competition from exotic grasses and forbs (Wills 2006), drought, rodent and insect damage, grazing by cattle, seedling and acorn predation by wild, and domestic animals (Landfire 2007a).

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).

In many coastal regions, the interaction between oceanographic and terrestrial air masses may be ecologically important. Intensifying upwelling along the California coast under climate change may intensify fog development and onshore flows in summer months, leading to decreased temperatures and increased moisture flux over land (Snyder et al. 2003, Lebassi et al. 2009, as cited in PRBO Conservation Science 2011). Coastal terrestrial ecosystems could benefit from these changes. However, current trends in fog frequency along the Pacific coast from 1901-2008 have been negative (Johnstone and Dawson 2010, as cited in PRBO Conservation Science 2011), thus the effect of climate change on coastal fog remains uncertain (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).

Pacific coastal areas from Sonoma County, California, south to Baja California.