Steep Clayey Hills
Scenario model
Current ecosystem state
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Management practices/drivers
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- Transition 1A More details
- Restoration pathway 2A More details
- Transition 2A More details
- Restoration pathway 3A More details
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No transition or restoration pathway between the selected states has been described
Target ecosystem state
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State 1
Reference State
Description
The Reference State displays the natural range of variability and ecological potential for this ecological site. States and Community Phases included in this document include those previously recognized by Fire and Resource Assessment Program (FRAP, State of California, 1998) and other entities as a result of the use of ordination software and professional consensus (Allen-Diaz et al., 1989; Vayssieres and Plant; 1998 and George et al., 1993).
The reference state has three community phases: 1) Blue oak//Annual Grasses and Forbs Phase: the lower elevations of this site tend to be grass and blue oak dominated. 2) Blue oak//Birchleaf mountain mahogany//Annual Grasses and Forbs Phase: at mid-elevations this phase is common; with increasing elevation and slope, tree and shrub density increases. 3) Blue oak-Foothill pine//Whiteleaf manzanita//Wild oat: east-facing slopes with favorable moisture conditions support increased biomass.
The southern extent of the Dibble soils (Map unit 114) are subject to a marine influence that results in cooler temperatures and moister conditions, creating a cooler phase that supports a mixed oak vegetation community. This cool phase is part of another ESD, Cool Silty Hills.
The reference community phase is oak savanna with an understory dominated by non-native annual grasses including wild oat (Avena fatua) or slender oat (Avena barbata), a low to moderate cover of blue oak (Quercus douglasii) and California foothill pine (Pinus sabiniana). Most sites have shrub cover that includes whiteleaf amanzanita (Arctostaphylos manzanita). Other shrubs found on lower slopes positions include birchleaf mountain mahogany (Cercocarpus montanus var glaber), and occasionally toyon (Heteromeles arbutifolia). Shrub density is low to moderate, with greater cover as elevation increases.
This state is relatively stable unless tree removal occurs. Blue oak contributes to soil productivity through increased soil nutrition under trees, and removal of trees causes changes to soil cover, water and nutrient status. Research indicates that oak removal results in a rapid decline in soil quality, including a loss in soil organic matter and nitrogen (Dahlgren et al., 2003). Oaks help retain more water on site and enhance soil quality through nutrient cycling, organic matter deposition and reduced bulk density (O’Geen et al., 2010). This recycling of nutrients provides an energy source to microbes, insects and other plants present on the site, increasing its productivity. As a result, soil and herbaceous vegetation under oak canopies have higher nitrogen and carbon resources than adjacent grasslands and appear to increase rates of soil nitrogen turnover, as well as increased rates of microbial activity (Herman et al, 2003). Soils under oaks retain water into the early summer months as ground temperatures are moderated by shading. Trees increase the soil water holding capacity of savanna soils (Baldocchi et al, 2004). Findings from this study also indicated that oak savannas retain and store more energy than grasslands, due to lower reflectance and surface temperatures. Some deeply rooted trees and shrubs may also induce hydraulic lift, transporting water to the upper soil layers (Richards and Cadewell, 1987; Caldwell et al., 1998; Ishikawa and Bledsoe, 2000; Liste and White, 2008), supporting the development of neighboring plants. Nutrients are also concentrated around shrub bases from litter fall and from sediment capture via movement of soil particles.
The Blue oak - Foothill pine habitat type provides important breeding habitat for a large variety of wildlife and acorns are an important food source for a variety of birds and mammals. Animal communities associated with this reference state are provided with a variety of vegetation stages and habitats. Following secondary succession annual grassland gives way to shrubs in 2-5 years; mature shrubs develop in 10 to 15 years and mature foothill pine 30 to 40 years (Verner, 1988). Most stands of blue oak range from 80 to 100 years of age (Kertis et al., 1993), however, remnant older blue oak specimens may range to over 450 years of age (Stahle et al., 2013) in more remote or steep locations. The lack of oak regeneration and the increase in foothill pine in the understory, are of concern in the long-term existence of this habitat (Mayer and Laudenslayer, 1988).
Submodel
Description
Today species composition and productivity of the annual dominated understory grasses and forbs vary greatly within and between years and is greatly influenced by the timing and amount of precipitation and the amount of residual dry matter (George et al., 2001a). Residual mulch influences and impacts germination and organic matter (George et al., 1985).
Nutrient turnover is rapid in grassland systems and is lost via leaching, gaseous exchange and soil erosion (Stromberg et al., 2007). Much of the nitrate that accumulates during the summer and fall is moved to seeds at senescence and the remainder is removed via rains before growth begins, little is available for later absorption by plants (Stromberg et al., 2007).
Submodel
Description
The two dominant invasive species that occur in this state include medusahead (Taeniatherum caput-medusae) (Cal-IPC List 1A) and barbed goatgrass (Aegilops triuncialis) (CFDA B). These plants are commonly found in greater abundance in the lower slope positions due to the clayey nature of the soils; they are also found on backslopes but are not nearly as common. Italian plumeless thistle (Carduus pychnocephalus, Cal-IPC list 2C) may also be found in limited populations but is not as extensive as the other above-mentioned species.
The California Invasive Plant Council (Cal IPC http://www.cal-ipc.org/) maintains an invasive plant inventory that rates invasive plants based on their ecological impacts to native flora and fauna. The California Department of Food and Agriculture (CDFA) also maintain a list of "noxious weeds" that are subject to regulation or quarantine by county agricultural departments.
Some experts have suggested that medusahead and other invasive species may gradually adapt to new sites (Rice et al., 2006). These weed species have a potentially high ecological impact on this ecological site. These weed species are active later into the growing season than other non-native forage species and may significantly reduce livestock productivity due to their effect on forage quality and quantity (Eviner et al., 2009; DiTomaso et al., 2006). The slow breakdown of plant litter of both medusahead and goatgrass may shift nutrient cycling dynamics.
Submodel
Mechanism
Triggers that limit blue oak reproduction and/or survival (intensive prolonged grazing) in combination with (fire or mechanical removal and prolonged drought) causing mortality or unfavorable conditions for sprouting result in a Threshold (T1A).
As community phases change over time within the Reference State, fire regimes may shift from low intensity fires towards more mixed severity and replacement fires due to the presence of ladder fuels and several missed fire cycles. Fuel load, season of burn and fire frequency interact to affect the amount of damage and mortality response (Swiecki et al., 1997). This ecological site has showed evidence of sprouting following harvest; oaks are likely to sprout following severe fire. Competition from shrubs for moisture and growing space may limit or slow re-establishment of blue oak. Shrub species such as whiteleaf manzanita that have seed stored in the soil have abundant germination following fire (Abrahamson, 2014). Under drought conditions blue oak seedlings may be less adapted, affecting their growth and survival (Grünzweig et al., 2008).
One study (McCreary et al., 2006) found sprout survival 19 years following harvest was 28 percent and noted that sprouting ability would be expected to decline with any subsequent harvest. Removal of blue oaks has been found to reduce soil productivity due to decreased soil nutrition from tree litter (Dahlgren et al., 2003), and increase the potential for erosion due to a decline in soil porosity and increased bulk density from organic matter losses (Dahlgren et al., 1997). Erosion losses also may reduce productivity by changing the water-holding capacity of the soil and the thickness of the root zone (Elliot, Page-Dumroese, Robichard 1998).
Mechanism
Removal of oak trees and subsequent soil erosion may change infiltration and runoff rates making restoration efforts difficult without significant inputs. Restoration might be achieved, especially on the low elevation sites through planting acorns or seedlings and tending young trees using tested methods (McCreary, 2001, McCreary, 2004) and by protecting young oaks from grazing by utilizing fencing (Philips et al., 2007, McCreary 2005). Blue oak saplings take approximately 10 to 30 years to reach a height of about 4.5 feet (Swiecki, 1998). Tree heights greater than 6.5 feet positively affect seedling survival and growth (Philips, et al., 2007) in light to moderate grazing environments.
Mechanism
This state may cross a threshold, especially on the lower footslope positions with a deeper soil profile and higher clay accumulations. Various triggers (disturbances) that reduce the site resilience and lead to an increased potential for medusahead and barbed goatgrass invasion and expansion could include a lack of grazing and an absence of fire. Either of these situations could result in this community phase reaching a threshold (T2A) where it becomes a noxious weed-dominated state. Areas with reduced plant cover and an increase in bare ground provide opportunities for invasion of noxious weeds like barbed goatgrass, and in areas where production potential is moderate to high, lack of grazing sets the stage for eventual medusahead dominance. Soil disturbance from burrowing animals and feral pigs continue to create new opportunities for exotic species invasion. A nearly continuous canopy of medusahead or barbed goatgrass and a build-up of litter will exclude most other herbaceous species.
Mechanism
Restoration methods include tilling and mowing, herbicides, and planting of desirable species. Planting of both native and introduced perennial grasses and use of herbicides has shown to be one effective method to control invasive species such as medusahead and barbed goatgrass, although there are no known restorations efforts completed on this ecological site. Restoration is expensive to implement and is generally reserved for the most productive sites.
Timing of grazing and targeting the treatment to the plant life stage are important factors in the reduction of invasive species (D’Tomaso and Smith, 2009). Multiple years of intense grazing may reduce the amount of seed of invasive annuals such as medusahead. Treatments that result in thatch reduction are critical for medusahead and barbed goatgrass control or restoration. Medusahead density may be controlled through a variety of measures, including grazing, mowing, burning, disking and chemical treatments. Increasing the density of competing grasses or legumes through planting in combination with fertilization could aid in the control of this species (California Rangelands, UCD). Careful timing of prescribed burning is essential for reduction of medusahead dominance since burning following seed dispersal is largely ineffectual (DiTomaso et al., 2005). Burning for two consecutive years is also recommended for control of barbed goatgrass. Other methods of control for barbed goatgrass include chemical control and mowing before seedhead production (Davy et al., 2008).
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