Ecological site group description

Provisional. A provisional ecological site description has undergone quality control and quality assurance review. It contains a working state and transition model and enough information to identify the ecological site.

Physiography

This ESG is found primarily on floodplains, alluvial fans and terraces, old alluvial fans and both stream and marine terraces with gentle slopes from 0 to 10% from 30 to 2100 feet elevations.

Climate

The average annual precipitation in this area is 11 to 53 inches (272 to 1,353 millimeters), this site is on the lower end of the range however. The higher amounts of precipitation occur at the higher elevations in the area north of San Francisco. Most of the rainfall occurs as low- or moderate-intensity, Pacific frontal storms during winter. This area is very dry from mid-spring to mid-autumn. Snowfall is rare. The average annual temperature is 54 to 61 degrees F (12 to 16 degrees C). The freeze-free period averages 315 days and ranges from 265 to 365 days. It is longest near the coast, and it becomes shorter with elevation.

Soil features

This ESG is found on variable soils that are generally very deep, moderately well drained to well drained soils in alluvial material from alluvial sources.

Some representative soils include:

Botella, a fine-loamy, mixed, superactive, thermic Pachic Argixeroll
Campbell, a fine-silty, mixed, superactive, thermic Cumulic Haploxeroll
Danville, a fine, smectitic, thermic Pachic Argixeroll
Flaskan, a fine-loamy, mixed, superactive, thermic Typic Argixeroll
Rincon, a fine, smectitic, thermic Mollic Haploxeralf
Sorrento, a fine-loamy, mixed, superactive, thermic Calcic Haploxeroll
Zamora, a fine-silty, mixed, superactive, thermic Mollic Haploxeralf

Vegetation dynamics

This ESG is characterized by valley soils that support open stands of Valley Oak woodland that will include live oaks across alluvial fans, flood plains and terraces. The shrub layer is best developed along natural drainages, becoming insignificant in the uplands with more open stands of oaks. Valley Oak woodland occurs in a wide range of physiographic settings but is best developed on deep, well-drained alluvial soils such as those associated with this ESG, usually in valley bottoms. Most large, healthy valley oaks are probably rooted down to permanent water supplies.
The valley oak (Quercus lobata) woodlands are a rapidly disappearing ecosystem in California. Valley oak stands with little or no grazing tend to develop a partial shrub layer of bird disseminated species, such as poison-oak, toyon, and coffeeberry. Ground cover consists of a well-developed carpet of annual grasses and forbs. Canopies of these woodlands are dominated almost exclusively by valley oaks. Tree associates in the Central Valley include California sycamore, Hinds black walnut, interior live oak, boxelder, and blue oak. The shrub understory consists of poison-oak, blue elder, California wild grape, toyon, California coffeeberry, and California blackberry. Various sorts of wild oats, brome, barley, ryegrass, and needlegrass dominate the ground cover. When reasonably adjacent to streams and higher water tables, Santa Barbara sedge will also be a component of the understory. Foothill pine and coast live oak are associated with valley oak woodlands along the Coast Range.
These areas were and are known to be extremely fertile land for agriculture crops and grazing lands, thus, much of the land suitable for valley oak regeneration has been lost to both habitat destruction and soil moisture availability (Meyer 2002). The valley oak is one of the largest oaks in North America that can grow trunks up to 6 or 7 feet in diameter. They are deciduous trees that require large amounts of water at rooting depth and rich soil created by spring flooding (Pavlick et al 1993). Natural regeneration of these trees has been steadily declining, due primarily, to the loss of habitat and the removal of natural yearly flooding events (Sork et al 2002). Extensive research has been done to determine what can be done to restore valley oaks to riparian woodlands, however many have found that without major changes to current land uses, there is little chance of valley oak restoration success.
Soil moisture, and in turn, soil texture, seem to be the major components in successful seedling germination and establishment for valley oaks (Adams et al 1992). The valley oak needs deep soil that is able to retain soil moisture from spring flood events, into the growing season, in order to ensure survival through the hot temperatures of summer. Flooding is especially important for valley oaks, since they prefer the higher parts of the floodplain, where the water table is further down so as not to saturate the roots, but still offers the soil moisture during the key part of the growing season (Griggs and Golet 2002). This also explains why soil texture is an important component as well. Loamy soil textures have a higher water holding capacity, and will hold water later into the year, whereas sandy soil textures have larger pore spaces, and allow water to flow through rather quickly, making the water-holding capacity very low. Thus, making floodplains with sandy soil textures a more difficult growing medium for new valley oak seedlings than loamy soil textures (Meyer 2002).

There are other factors that could play additional roles in the lack of regeneration of valley oaks including; rodent and deer predation of seedlings, lack of open niche space for new seedlings to utilize, and shading created by the older valley oak canopy. Valley oak woodlands that are highly utilized by deer, rodents, and feral pigs, will have a much more difficult time regenerating due to predation of their seedlings in the first year. It has also been speculated in the research that this is also a factor that can be solved by replacing the natural flooding regimes because the rodents drown, thus reducing the number of seedling predators significantly (Meyer 2002).

Most valley oak woodlands that still remain throughout California are filled with tall, old trees, between 100 and 300 years old, and individual valley oaks may live as long as 400 years. These annual grass and forb understory, such as bromes and wild oats, also create problems for new oak seedlings because they tie up all the available nutrients and water, leaving nothing left for the oak seedlings to begin germination (Adams et al 1992). With the addition of the shade created by the large canopy of the oaks, the seedlings have lost virtually any chance for germination or survival (Griggs and Golet 2002).

Restoration of this ecosystem would be possible; however, it would be costly and require fairly labor-intensive strategies. Without replacing the natural disturbance regime, it would be necessary to manually plant valley oak seedlings with an auger and protect them from predation either with wire-mesh cages or tubing, although tubing does constrict growth (Adams et al 1992). They would also require some sort of irrigation or additional watering, in order to ensure they live through the hot summer season in these Central Coast valleys (Griggs and Golet 2002).

Information from:
Adams Jr., T.E., P.B. Sands, W.H. Weitkamp, and N.K. McDonald. 1992. Oak Seedling Establishment on California Rangelands. Journal of Range Management. 45:93-98.
Griggs, F.T. and G.H. Golet. 2002. Riparian Valley Oak (Quercus lobata) Forest Restoration on the Middle Sacramento River, California. USDA Forest Service General Technical Report. PSW-GTR-184.
Howard, J.L. 1992. Quercus lobata. Fire Information Effects System. Accessed June 30, 2006: http://www.fs.fed.us/database/feis.
Meyer, V.C. 2002. Soil Moisture Availability as a Factor Affecting Valley Oak (Quercus lobata Nee) Seedling Establishment and Survival in a Riparian Habitat, Cosmunes River Preserve, Sacramento County, California. USDA Forest Service General Technical Report. PSW-GTR-184.
Olson, S.K. 1998. Hydrologic Relationships of Valley Oak (Q. lobata, Nee) and their Effect on Seedling Emergence and Seedling to Sapling Mortality, Humboldt State University. Accessed June 30, 2006: http://www.icess.ucsb.edu/esrg/ess_sum97/Students_ESS.1998/Sam_Olson/sam_report.html.
Pavlik, B., P. Muick, S. Johnson, and M. Popper. 1993. Oaks of California. Cachuma Press: 184 pp.
Ritter, Lyman V. 2020. Valley Oak Woodland. California Wildlife Habitat Relationships System.California Department of Fish and Game, California Interagency Wildlife Task Group. Accessed https://nrm.dfg.ca.gov/FileHandler.ashx?DocumentID=67342&inline.
Sork, V.L., F.W. Davis, R.J. Dyer, and P.E. Smouse. 2002. Mating Patterns in a Savanna Population of Valley Oak (Quercus lobata Nee). USDA Forest Service General Technical Report. PSW-GTR-184.

Major Land Resource Area

MLRA 014X
Central California Coastal Valleys

Stage

Provisional