Ecological dynamics

This ecological site is present on elevated mesas formed from latite lava flows. One such lava flow originated near Sonora Pass (Gorny et al., 2009), flowing 80 miles through an ancient river channel, filling the channel and hardening in place. The latite flow is more resistant to weathering than the surrounding rock, so it exists as an elevated, long, thin mesa. The tops of many of these mesas are fairly flat, gently sloping to the west, and characterized by complex undulating topography composed of bedrock outcrops, mounds-and-intermounds, vernal pools, and bedrock fissures. This complex topography formed due to the structural resistance of the latite bedrock, and reworking of surface material by ancient streams, and in present times forms a mosaic of plant communities across the mesa.

Direct rain precipitation is the primary source of water for the mesa. The dense latite bedrock impedes the downward movement of water, causing the soils to become saturated or ponded for short durations in spring. Precipitation infiltrates into the soils or creates surface runoff over the soils and bedrock outcrops. Small ephemeral streams develop over moss covered bedrock drainageways, and vernal pools fill in small depressions after rain events. The vernal pools are flashy, and depending upon size and connectivity, may fill and drain repeatedly over the rainy season. These vernal pools are classified as Northern Basal Flow Vernal Pools (Keeler et al. 1998). After the winter and spring rainy season, the soils dry through the summer, becoming too dry to support shrubs and most perennial species. Therefore, this area primarily supports annuals that flourish for a short period when water is available. A few hardy perennials have endured by adapting to these harsh wet-dry conditions.

This ecological site is dominated by micro-relief swales and grassy hummocks with some channelization occurring among bedrock outcrops (see below, photo of Community Type 5). The percent cover of surface cobbles and gravels varies from site to site (see Representative Soil Features section), with no apparent effect on plant production. Among the mounds and bedrock outcrops there is high variability in soil depth and wetness due to hummocky relief and the formation of basins and swales. The mounds are generally dominated by Pacific fescue (Vulpia microstachys var. pauciflora), non-native brome and wild oats, but a diversity of forbs is also present. In spring there is a colorful wildflower display. Soils are taxonomically the same in the mound and intermound micro-topographies, but have differences that effect the vegetation. The soils on the mounds are typically 10 to 20 inches deep and are well drained, whereas the soils in the intermounds are often less than 10 inches deep over bedrock and are somewhat poorly drained. Deeper soils and better drainage on the mounds supports higher vegetation production. Shallower soils with low permeability on intermounds supports native grasses and forbs such as Pacific fescue, California goldfields (Lasthenia californica), cowbag clover (Trifolium depauperatum), whitetip clover (Trifolium variegatum), and johnny-tuck (Triphysaria eriantha). Vernal pools in broad valleys have moderately deep soils, with aquic soil features. Douglas' meadowfoam (Limnanthes douglasii) is common along the margins of the vernal pools in the interconnecting swales. Within the vernal pools, eryngo (Eryngium spp.) or annual semaphoregrass (Pleuropogon californicus) are dominant. Among the bedrock outcrops are areas with a very a thin layer of soil (< 1 inch), where 20 to 30 percent cover of spikemoss (Selaginella spp.) and a diversity of other forbs and grasses can be found.

Rodents, insects and amphibians rely on the different topographic positions for forage and shelter. The mounds provide drier nesting sites, while the pools provide seasonal water and late season forage. Pocket gopher activity has been associated with mound-intermound development on moderately to very deep soils near Merced California. These soils contained a restrictive duripan, and it was found that gophers preferentially moved sediments from the intermound upslope, onto drier mound positions at a rate two times greater than the rate of natural erosion (Reed and Amundson, 2007). It is unknown whether this same phenomenon is occurring on these elevated mesas. The shallower soils on this site may not provide optimum habitat for pocket gophers
.
Vernal pools are a biologically unique and important component of this of this ecological site. Vernal Pools are defined as: “Seasonally flooded landscape depressions underlain by a subsurface which limits drainage. They result from an unusual combination of soil conditions, summer-dry Mediterranean climate, topography and hydrology, and support a specialized biota, including a relatively large number of threatened and/or endangered species” (Cheatham 1976, Zedler 1987, Holland and Jain 1988). The Environmental Protection Agency (EPA) definition includes: “These wetlands range in size from small puddles to shallow lakes and are usually found in a gently sloping plain of grassland. Although generally isolated, they are sometimes connected to each other by small drainages known as vernal swales” (EPA online, http://water.epa.gov/type/wetlands/vernal.cfm, December 16, 2009).

Vernal pools are important ecological hot spots, which have been vanishing as land is converted for development or agriculture. Estimates vary, but the remaining vernal pools occupy only 5 to 30 percent of their historic area. Vernal pools have several obligate rare and endangered plant and animal species, which have adapted to the harsh wet-dry characteristic of these pools. Several vernal pool plants have adapted by changing the morphology of their leaves for an aquatic stage and a dry stage. For example, vernal pool eryngo species (Eryngium spp.) produce winter leaves that are tubular, hollow and septate. As the pool dries, these hollow tubular leaves are replaced with well developed leaf blades. Arthropods were not inventoried during the development of this ecological site, but some vernal pools support small crustaceans because fish predators are absent. For example, the tadpole shrimp (Lepidurus packardi) is a rare vernal pool species. This shrimp in the order Notostraca, has maintained the same morphology as its ancestral fossils from 250 million years ago. They have survived by laying eggs that can remain dry for up to 10 years or more, immediately hatching when wetted (Goettle, 1997).

Ecological Dynamics:

Fire, recreational use, and cattle grazing have the potential to affect the dynamics of this ecological site. Currently there is no evidence to suggest that these dynamics are occurring, so they are not included in the STM.

The historic fire regime for this ecological site is not known, and the post fire regeneration community was not observed. Studies suggest that fire in the central valley may have occurred on a 2 to 3 year cycle, and is now on a 20 to 30 year cycle (Howard 2006). This site differs from the central valley annual grasslands both in geologic formation and vegetation. Presently this site is dominated by vegetation that provides poor fuels to carry fire. If historically, there were more perennial grasses, perhaps fuels may have been able to carry fire. Since this site is on an elevated mesa it may be more prone to lighting strikes than the surrounding areas. However, due to the low fuels, fire size may be small and of low intensity.

In the event of a fire this site may recover pre-fire composition by the next season. Annual forbs may increase immediately after fire, including the non-native longbeak stork's bill (Gerhardt and Collinge, 2003). Native geophytes such as bluedicks and onions, may not be impacted by fire because their corms are buried deeper in the soil. Pacific fescue will regenerate from soil stored and off site seed after fire (Howard 2006). Seed mortality rates depend upon the type of seed, and the depth of burial. Longbeak stork’s bill, may recover well after fire, because the seeds have a unique appendage that “drills” the seeds into the soil. The seeds of most annual forbs on the site may have high mortality from fire, but regeneration will occur from off-site seed. The effects of fire in annual grasslands are relatively short lived, and the cover of oats (Avena spp.) can recover by the next season (Gerhardt and Collinge, 2003).

This ecological site is used for public recreation and grazing land. Public trails and several private dirt roads cross the top of the mesa. Altered hydrologic conditions such as stream diversions, surface leveling, or infilling of pools due to this use were not observed. The private land is used for low intensity cattle grazing.

In systems where light to moderate grazing is already occurring, there may be stabilizing effects to the plant and animal communities that would be negated if the grazing pressures were removed. For example, Marty (2005) found that native plant species remained higher in continuously grazed vernal pools than in pools where grazing was removed. Marty also documented that cessation of grazing can lead to build up of residual dry matter which in turn tends to decrease species richness. Nevertheless, it may be beneficial for land managers to monitor vernal pools for algal crust formation, which may develop from increased nutrient inputs (eutrophication). In short, land managers and owners should proceed with caution when considering land use alternatives within the vernal pool systems. Functional vernal pools that are ungrazed may experience altered nutrient cycling regimes and soil compaction, leading to changes in native biota, when, and after, domestic livestock are introduced to the system. However, in systems where domestic livestock grazing is already in place, unexpected negative effects may occur after permanent removal of livestock. Total cessation of grazing can severally alter the hydrology of vernal pools. In the previously cited example (Marty, 2005), a discontinuation of grazing pressure resulted in approximately 50 days less of average maximum ponding when compared with continuously grazed pools. This change in hydrology may affect several species that depend on extended periods of inundation to complete their lifecycles. The study’s main implication is that “if a site is grazed and demonstrates high diversity, it should be left grazed, unless there is a compelling, scientifically-based reason to change the management regime.” (Marty, 20005).