Shallow Gravelly Foothills
Scenario model
Current ecosystem state
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Management practices/drivers
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- Transition T1A 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
State 1 is the Reference State that represents the natural range of variability for this ecological site. States and Community Phases included in this document include those previously recognized by Fire Resource Assessment Program (FRAP) (State of California, CALFIRE, 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). Drought, fire, and grazing are the primary drivers of the natural disturbance regime.
The reference state for this ecological site has three community phases; landscape position, slope, aspect and past disturbance influence the vegetation dynamic between the phases. All Community Phases have some degree of blue oak overstory, however, the higher elevation, steep slopes and north and east facing sites favor California Foothill Pine and heavier shrub development (Reference and Representative Community Phase 1.1), the cooler east-facing slopes where moisture conditions are more favorable for survival of young shrub have blue oak and interior live oak with a more developed shrub understory species (Community Phase 1.2) and the drier south and southwest-facing backslopes and sideslopes (Community Phase 1.3) are primarily composed of blue oak with an annual grass and forb understory. In Phase 1.1, California foothill pine is dominant, with much less blue oak in the overstory; lack of frequent fire allows foothill pine to proliferate, and as blue oak is shade intolerant, it does not grow well under dense canopy of shrubs and foothill pine (McDonald, 1990, ucanr.edu, 2015). As community phases change over time within the Reference State, shrubs and trees become a greater component of the community. 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, fire frequency and duration all interact to affect the amount of damage and mortality response (Swiecki et al., 1997, Neary et al., 1999).
Blue oak and California foothill pine are the dominant tree species found on this ecological site and to a limited extent, interior live oak. At lower elevations the blue oak and foothill pine vegetation type intergrades with blue oak savanna. At higher elevations this type transitions to dense mixed chapparal shrub cover (USDA, 1999). Small isolated populations of California juniper (Juniperus californica) may also be found on this ecological site. Moderate shrub cover most commonly includes buckbrush, whiteleaf manzanita and/or sticky whiteleaf manzanita, Pacific poison oak, and birchleaf mountain mahogany (Cercocarpus montanus var glaber). Some areas also include hollyleaf redberry (Rhamnus ilicifolia), and the shrub form of interior live oak. Non-native annual grasses and native perennial grasses are generally a moderate component of the understory. Wild oat commonly dominates the annual understory grasses in the lower elevation community phases and the perennial grasses Sandberg bluegrass is common in small openings at the higher elevations. A variety of annual and perennial forbs and herbs are also present.
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 (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). 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 duration of vegetation successional stages varies greatly, and lacks sufficient research to gain better estimates. It has been estimated by some research that for this ecological site and other similar ecological sites, conifer development may take 30-40 years (Verner, 1988). Blue oak growth is slow and variable. Most stands of blue oak range from 80 to 100 years of age (Kertis et al., 1993). There are remnant older blue oak specimens that may range to over 450 years of age (Stahle et al., 2013) in more remote or steep locations. Mature brush development can take 10 to 15 years (Mayer and Laudenslayer, 1988).
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 soil “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
State 3
Shrub State
Description
The Shrub State has four Community Phases: 1) the Yerba santa Community Phase, 2) the Scrub Oak Mixed Chaparral Community Phase, 3) Ceanothus Mixed Chaparral Community Phase and 4) the Chamise Mixed Chaparral Community Phase. Many factors may influence the pathways between community phases; the degree of fire severity and frequency, species composition “pre-fire”, topography, slope and weather as it influences fire behavior (Fried et al., 2004).
Much of the information that exists for chaparral in northern California is from the work of Biswell (1952) and Sampson (1944), however there has been extensive research in the chaparral of central and southern California by Keeley and many others.
Subshrubs, annuals and perennial herbs sometimes dominate the early stages following fire (1 to 3 years), with shrub seedlings and sprouts (England, 1988). Most shrub species that are present following fire are either obligate seeders or sprouters. From 3 to 15 years shrub canopy continues to close until at age 35 to 50 years, when shrub canopy may be up to 80 percent of total cover. 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.
In grazed areas, shrub interspaces have increased potential for erosion. Native and non-native herbaceous vegetation is decreased, and less understory grass and forbs are remaining. Periodic or reoccurring fire will maintain this state. Shrub species composition changes depending on aspect and slope position.
Submodel
Mechanism
Triggers that limit blue oak reproduction and/or survival such as intensive prolonged grazing, severe fire, mechanical removal (conversion) and prolonged drought may result in the State reaching a Threshold (T1A) leading to a potential for Community Phases 1.2 or 1.3 to transition to a Grassland State (2). Severe and more frequent fire may result in the potential loss of blue oak on some sites, especially in areas near the limits of blue oak distribution, and where competition from moderate shrub and grass cover could reduce favorable conditions for oak establishment and survival. Frequent fires and intensive grazing have been shown to have a negative effect on recruitment of blue oak saplings (Swiecki and Bernhardt, 1997).
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 and duration interact to affect the amount of damage and mortality response (Swiecki et al., 1997, Neary et al., 1999).
Removal of blue oaks has been found to reduce soil productivity due to decreased soil nutrition from tree litter (Dahlgren et al., 2003). Removal also increases 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 tree cover via conversion or severe fire in combination with prolonged drought causing mortality or unfavorable conditions for oak sprouting result in a Threshold shift to a Shrub State (3). 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.
Shrub species have several mechanisms for regenerating post-fire via sprouting or through seed. Pacific poison oak, the shrub form of interior live oak, birchleaf mountain mahogany and toyon (Heteromeles arbutifolia) sprout from the root crown following cutting or fire. Fire stimulates buckbrush and manzanita seed to germinate by scarifying the seed “banked” in the soil (Abrahamson, 2014; League, 2005). Early post-fire plant communities commonly have a native shrub, yerba santa (Eriodictyon spp.), that reproduces via rhizomes as a component. This shrub slowly diminishes in abundance as the plant community progresses.
Severe fire can cause a loss of nutrients and a reduction in microbial processes, and lead to accelerated erosion and leaching. High heat transfer to soils may give rise to a loss of soil organisms, tree and shrub roots, and changes in physical soil properties resulting in reduced water infiltration and increased surface runoff (Neary et al., 1999). In some cases severe fire heating may create a water repellant layer lasting from one year to several years (DeBano, 2000), and the resulting increases in raindrop splash and rill formation can dramatically increase soil erosion rates.
Mechanism
Pioneer establishment of blue oak into treeless areas is uncommon but sometimes occurs in favorable environments (Swiecki and Bernhardt, 1997). Blue oaks are capable of sprouting following harvest or severe fire on this ecological site, especially within this higher precipitation zone, however, competition for moisture and growing space with annual grasses may limit or greatly slow re-establishment. Smaller oaks tend to sprout more vigorously than larger oaks (McCreary et al., 2002).
A reduction in grazing duration and pressure or utilizing fencing would help protect young oaks that do get established until they are above the browse line. Blue oak saplings take approximately 10 to 30 years to reach a height of about 4.5 feet (Swiecki, 1998).
Mechanism
Browsing pressure is low and the protection from or elimination of grazing may allow brush to expand into grasslands over a period of time (Freudenburger et al., 1987). Fire suppression may also lead to brush expansion into grasslands increasing fire hazard (Johnson and Fitzhugh, 1990). The seed of other shrub species may be transported via birds or mammals. Though expansion of shrubs into grassland is limited, disturbance creates the opportunity when a seed source is present.
Mechanism
Shrub canopies may facilitate oak re-establishment by providing protective canopy from herbivores (Stromberg et al., 2006) and by providing shade. Birds may also disperse acorns in shrub areas.
Mechanism
High browse pressure and frequent fire may kill many shrubs transitioning back to a Grassland State (2). High fire frequency kills shrubs, reduces the seed bank and allows grasses to become established.
Feedbacks that include changes in soil water availability in an annual system may further limit brush re-establishment (Haidinger and Keeley, 1993). Severe fires can cause a degradation in soil function and physical soil properties, nutrition losses and increased erosion (Neary et al., 1999).
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