Steep Loamy Foothills
Scenario model
Current ecosystem state
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Management practices/drivers
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- Transition 1B More details
- Transition 1A More details
- Transition T2A More details
- Transition T2B More details
- Transition T3A More details
- Transition T3B 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 Phase: the lower elevations of this site tend to be grass and blue oak dominated. 2) Blue oak//Birchleaf mountain mahogany-Pacific poison oak//Annual Grasses and Forbs Phase: with increasing elevation and slope, tree and shrub density increases. 3) Blue oak-Foothill pine//Pacific poison oak//Wild oat: north to northeast-facing slopes with favorable moisture conditions support increased biomass.
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 (Mayer and Laudenslayer, 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).
This state is relatively stable unless tree removal occurs. Blue oak contributes to soil productivity through increased soil nutrition and health 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 (Dalgren 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). 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.
Submodel
Description
Non-native grasses now have become naturalized in much of California. Introduced annual forbs and grasses have unique adaptations that give them a competitive advantage over native species. Some of these plant adaptations include high seed production, fast early season growth and the ability to set seed in drought years (Stromberg et al., 2007). Soil disturbance from burrowing animals and feral pigs continue to create new opportunities for exotic species invasion.
Nutrient turnover is rapid in grassland systems and is lost via leaching, gaseous exchange and soil erosion (Stromberg et al, 2007). Because most of the nitrate that accumulates during the summer and fall is moved to seeds at senescence and the remainder is removed via rains prior to initiation of growth, little is available for later absorption by growing plants. There is a higher nutrient loss from annual systems as opposed to shrub-dominated systems (Michaelides et al., 2012) and a higher percentage of “fines” transported offsite despite similar erosion rates, according to one study. Although nutrient leaching from grassland systems is variable, nutrients that are moved beyond the shallow root systems of the annual grasses are lost to leaching. Annuals use available water primarily in the top 1 foot of soil (George et al., 2001); their shallow root structures dry out quickly during rapid spring growth and evapotranspiration quickly depletes soil moisture. Water infiltration may be more rapid in grasslands than in shrub-dominated landscapes.
Submodel
Description
Upper drier backslope positions tend to be droughty and support more whiteleaf manzanita in the shrub layer. As development of the shrub community progresses after fire, inter-shrub native and non-native herbaceous vegetation decreases, and less understory vegetation is remaining. In a shrub state available water may be present later in the growing season due to decreased evaporation and shading, maintaining moisture longer than under just grasses alone (Gill and Burke, 1999). Deeply rooted shrubs may also induce hydraulic lift, transporting water to the upper soil layers (Richards and Cadewell, 1987). Nutrients are also concentrated around shrub bases from litter fall and from sediment capture via movement of soil particles.
Submodel
Mechanism
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. A moderate to severe fire that kills blue oak and foothill pine is a trigger that could result in reaching a Threshold (T1B) causing a transition to a shrub/annual grass and forb state (State 3). Some smaller blue oak may survive, but larger oaks would likely have low sprouting and reduced or eliminated re-seeding capacity. Fuel load, season of burn and fire frequency interact to affect the amount of damage and mortality response (Swiecke et al., 1997). Pacific poison oak re-sprouting is stimulated by fire (Howard, 1994). Shrub species such as whiteleaf manzanita that have seed stored in the soil has abundant germination following fire (Abrahamson, 2014). Other shrubs that may be present in lesser amounts such as scrub oak (Quercus beberidifolia), birchleaf mountain mahogany (Cercocarpus montanus var glaber) and toyon (Heteromeles arbutifolia) sprout from the root crown following cutting or fire. Competition from shrubs for moisture and growing space may also limit re-establishment of blue oak.
Mechanism
Triggers that limit blue oak reproduction and/or survival (intensive prolonged grazing) in combination with fire or mechanical removal and prolonged drought that cause mortality or unfavorable conditions for sprouting result in a Threshold (T1A), transitioning to an annual grassland state ( State 2). Natural regeneration seldom occurs further away than 100 feet from an existing oak tree canopy, and stump sprouting is likely to be very limited in mature trees in a low rainfall area (FRAP, State of California, 1998). Blue oak seedlings may be less adapted to drought conditions, 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 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 (Dalgren et al., 2003), and increase the potential for erosion due to a decline in soil porosity and increased bulk density from organic matter losses (Dalgren 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 et al., 1998).
Mechanism
Removal of oak trees and subsequent soil erosion may change runoff and infiltration rates making restoration efforts difficult without significant inputs. Planting success might be achieved, especially on the low elevation sites, through planting acorns or seedlings and tending young trees using tested methods (McCreary, 2004) and by protecting young oaks from grazing by utilizing fencing. Blue oak saplings take approximately 10 to 30 years to reach a height of about 4.5 feet (Swiecki, 1998).
Mechanism
Shrub recruitment has increased in the absence of periodic fire or grazing in some foothill environments (Duncan, 1987). Though unlikely to take over large areas with annual grasses, drier mid to high elevation sites would be most susceptible to shrub encroachment.
Mechanism
Over a period of time, blue oak and California foothill pine may initiate limited re-establishment through sprouting and from adjacent seed sources. Pioneer reproduction of oak in rangelands is uncommon (Swiecki and Bernhardt, 1998). While studies are lacking on growth of the species found on this site, mature shrubs can take 10-20 years to develop, mature pine 30 to 50 years, and blue oak 40 to 100+ years (Mayer and Laudenslayer, 1998) to reach maturity.
Mechanism
Short fire return intervals act to kill shrubs and reduce the seed bank, and promote herbaceous annuals, creating a grass/herbaceous mixture. Repeated grazing would also contribute to a reduction in shrub cover. Continued high fire frequency can convert shrubs to an annual grassland dominated by non-native and native grasses and forbs (Keeley, 2002).
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