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 stream terraces, flood plain steps, and alluvial fans with slopes of 0 to 10 percent 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 typically around 20 inches. 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 midspring to midautumn. 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:

Arroyo Seco, a coarse-loamy, mixed, superactive, thermic Fluventic Haploxeroll
Cole, a fine, mixed, superactive, thermic Pachic Argixeroll
Conejo, a fine-loamy, mixed, superactive, thermic Pachic Haploxeroll
Corralitos, a mixed, thermic Typic Xeropsamment
Lockwood, a fine-loamy, mixed, superactive, thermic Pachic Argixeroll
Pleasanton, a fine-loamy, mixed, superactive, thermic Mollic Haploxeralf
Tunitas, a fine, smectitic, isomesic Pachic Argiustoll
Yolo, a fine-silty, mixed, superactive, nonacid, thermic Mollic Xerofluvent

Vegetation dynamics

This ESG covers the areas of the valleys in MLRA 14 that were at one time part of a vast complex of coastal prairies, meadows, and native grasslands with scattered shrubs and oaks. The urbanized landscape in the valleys within this MLRA that exists today makes it difficult to imagine the natural landscape prior to human development. These loamy fans were some of the first areas to be settled and many of these highly fertile and productive ecosystems were drained, leveed, cleared for crops and other agriculture, and urbanized.

As this landscape was de-watered and houses and agriculture took over, the water table for many of these habitats moved deeper and deeper, creating soils that would no longer offer the available soil moisture for many of the plants that had evolved with the hydrologic function of the natural system that no longer existed. These loamy fans may have remained wetter than many of the surrounding soils, due to their prolonged available water capacity and their argillic horizons that perched water for a time before moving off site or deep into the aquifers on the landscape. The variable range in soil textures will dictate the species composition and production, with the finer soils holding more water that results in more native perennials and forbs and higher annual production overall. The coarser textures will dry out more rapidly through both drainage and evapotranspiration in the summer months making them less hospitable for many of the native perennial grasses and more dominated by annual grasses and forbs. Annual production will still be higher than the other ESGs on terraces and uplands, due to the deep, loamy soils which provide decent available water and slightly slower but still well drained soil conditions.

Historically, this site may have been more interconnected to the riparian forests and flood plains, however with the introduction of non-native annual grasses and the impacts from fragmentation, extensive agriculture, channelization of rivers and streams, levees, irrigation ditches, continued de-watering, and human alterations such as homes and roads, this site now reflects a scattered oak savannah with an understory of moderately producing, dry, annual grasslands.

Currently, where this site is not under cultivation or urban developments, it is dominated by non-native annual grasses and scattered overstory of live oaks, California sycamores, buckeyes, other shrubs and vines. These include wild oats, soft chess, ripgut brome, red brome, wild barley, and foxtail fescue. Common forbs include broadleaf filaree, redstem filaree, turkey mullein, true clovers, bur clover, popcorn flower, and many others. Perennial grasses will be found in moist, lightly grazed, or relic prairie areas. Species composition is also related to water availability with greater amounts of relic perennial grasses in areas of greater precipitation or soil moisture.

Information from:

John G. Kie
California Wildlife Habitat Relationships System
California Department of Fish and Game
California Interagency Wildlife Task Group

Major Land Resource Area

MLRA 014X
Central California Coastal Valleys

Stage

Provisional