California Valley Oak Savanna

Historically, these savannas occurred on alluvial terraces and flat plains, often with deep, fertile soils, throughout the California Central Valley from Lake Shasta south to Los Angeles County. This system is found from 10-1200 m (30-3600 feet) elevation; receiving on average 50 cm (range 25-100 cm) of precipitation per year, mainly as winter rain. 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.). Quercus lobata was the characteristic oak species of these savannas, though other species were present, including Quercus wislizeni, Quercus agrifolia, Quercus douglasii, Aesculus californica, Cercis canadensis var. texensis, Juniperus californica, and Nassella pulchra. There is some evidence that much of the understory prior to the invasion by non-native annual grasses and forbs was composed of native annual herbs such as Hemizonia, Eriogonum, Trifolium, Gilia, Navarretia, Lupinus, Calycadenia, Lessingia, Lotus, Daucus, and Holocarpha spp. There is considerable seasonal and annual variation in cover of understory species due to phenology and intra-annual precipitation and temperature variation.

Quercus lobata was the characteristic oak species of these savannas, though other species were present, including Quercus wislizeni, Quercus agrifolia, Quercus douglasii, Aesculus californica, Cercis canadensis var. texensis (= Cercis occidentalis), Juniperus californica, and Nassella pulchra. There is some evidence that much of the understory prior to the invasion by non-native annual grasses and forbs was composed of native annual herbs such as Hemizonia, Eriogonum, Trifolium, Gilia, Navarretia, Lupinus, Calycadenia, Lessingia, Lotus, Daucus, and Holocarpha spp. There is considerable seasonal and annual variation in cover of understory species due to phenology and intra-annual precipitation and temperature variation.

These savannas historically occurred on alluvial terraces and flat plains, often with deep, well-drained fertile soils, throughout the California Central Valley from Lake Shasta south to Los Angeles County. This system is found from 10-1200 m (30-3600 feet) elevation; receiving on average 50 cm (range 25-100 cm) of precipitation per year, mainly as winter rain. Summers are generally hot and dry. Variable canopy densities in existing occurrences are likely due to variation in soil moisture regime and natural patch dynamics of fire, also intra-annual precipitation and temperature variability result in variability in cover of plants.

Fire regime: frequent surface fires since good fuels of grasses, and carried from adjacent grasslands. Summer to early fall; FRI 5-100+ (Sawyer et al. 2009). Very productive and fire-prone landscape. From Sawyer et al. (2009): Literature describing post-fire natural regeneration and long-term fire recovery of Quercus lobata woodlands is minimal. Plants have the ability to survive fire, and stands probably burned frequently and hot with dry grasses and oak litter carrying surface fires. Larger mature trees are usually resistant to moderate-severity fire because of their thick bark. While seedlings and saplings are top-killed by such fire, juveniles sprout from root crowns. However, older mature trees that are top-killed do not have this same ability. Animals such as scrub jays also facilitate regeneration of Quercus lobata, because they prefer burned areas as acorn-caching sites, and buried acorns usually survive fire (Howard 1992, Wills 2006). Hot surface fires may kill large trees that have extensive internal rot, and usually kill small trees. Crown fires will kill a large number of valley oak of all size classes (Howard 1992). Herbivory from ungulates winter range; ground burrowers; oak regeneration is dependent upon bare soil and dispersal from birds/small mammals burial of seeds. Valley oak regeneration to replace mature trees is lower than in other deciduous oak species (Landfire 2007a). Some studies indicate that this is due to a rare occurrence of necessary climate conditions, such as a warm summer followed by several wet years.

Conversion of this type has commonly come from intensive clearing for irrigated agricultural land, urbanization and other purposes (other development). From Sawyer et al. (2009): it's estimated that approximately 90% of Quercus lobata stands that existed prior to European contact have been destroyed by urbanization and intensive land conversion. What remains of these forests are only remnants of what once existed in the Central Valley, other valleys, and foothill locations in California (Allen-Diaz et al. 2007). The remaining patches occur in a matrix of agricultural, urban and suburban land, and annual grasslands.

Common stressors and threats include land use (fire suppression, livestock grazing, herbivory, etc.); recent oak fungal pathogens; 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). Quercus lobata is dying in some areas due to lower water tables and the accumulation of saline irrigation runoff. Low rates of Quercus lobata regeneration result in low replacement of mature trees and habitat loss. Infrequent fires result in more significant understory of shrubs and non-native herbs (e.g., Bromus spp., Avena spp., Frangula californica ssp. tomentella (= Rhamnus tomentella), Rhamnus ilicifolia, and Heteromeles arbutifolia). 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). Sawyer et al. (2009) continued: Due to fire exclusion, valley oak woodlands frequently contain an understory of shrubs, evergreen oaks and conifer saplings and trees, and a deep litter of oak leaves, needles, and downed woody debris. Prescribed burning in non-drought years could increase oak abundance. More regular fires could reduce or eliminate invasion by evergreen oaks and conifers and open up sites for valley oak seedling establishment or oak sprouting. However, the threat of severe fire in oak stands has increased greatly where valley oak woodlands border conifer forests. Also, deeply fire-scarred trees are susceptible to various heart-rot fungi and to windthrow (Howard 1992).

In the Central Valley, regional climate models project mean annual temperature increases of 1.4-2.0°C (1.8-3.6°F) 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 47-175 mm (1-7 inches) 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: deep-rooted or phreatophytic species under greater stress and death; drop in groundwater table; 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 favoring certain invasive species (Brooks and Minnich 2006); and increased competition for water from all users stresses the already overtaxed water allocation of California agricultural system (PRBO Conservation Science 2011).

Historically, this system was found throughout the California Central Valley from Lake Shasta south to Los Angeles County.