Very Shallow Steep Foothills
Scenario model
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- Transition T1A More details
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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 (State of California, FRAP, 1998) and other entities, as a result of the use of ordination software and professional consensus (Allen-Diaz et al., 1989, Sawyer et al., 2009, 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 two community phases, with elevation, slope, aspect and past disturbance influencing the vegetation dynamic between the phases. Community Phase 1.1, the Representative Community Phase, has blue oak and California foothill pine with a sparse shrub understory, and Community Phase 1.2 is dominated by California foothill pine and a buckbrush understory. Blue oak is rarely present.
Shrub cover most commonly includes buckbrush (Ceanothus cuneatus) but some areas also include hollyleaf redberry (Rhamnus ilicifolia) and whiteleaf manzanita (Arctostaphylos manzanita). Non-native annual grasses and perennial and annual forbs are generally a moderate component of the understory. Red brome (Bromus rubens) commonly dominates the annual understory grasses. A variety of annual forbs and herbs are also present, dominantly redstem stork’s bill (Erodium cicutarium).
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.
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).
The duration of vegetation successional stages varies greatly, and lacks sufficient research to gain better estimates. Annual grasslands give way to shrubs within 2 to 5 years with mature shrub development taking approximately 10 to 15 years (Mayer and Laudenslayer, 1988). Some research estimates that for this ecological site and other similar ecological sites, conifers may take 30 to 40 years to develop, and blue oak may take at least 50 years (Mayer and Laudenslayer, 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) however 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. California foothill pine may range to over 200 years in age (Howard, 1992).
The California foothill pine - blue oak type provides important breeding and foraging habitat during winter and spring and a vital food source for birds and mammals.
Submodel
Description
Non-native grasses and forbs 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.
Very low production and low vegetative cover, result in high amounts of exposed soil in this State. This very shallow exposed soil dries out quickly and is subjected to a high degree of erosion, especially on steep slopes. 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 that study.
Nutrient turnover is rapid in grassland systems and is lost via leaching, gaseous exchange and soil erosion (Stromberg et al., 2007); 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. 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.
Submodel
State 3
Shrub State
Description
The Shrub State has three Community Phases recognized: 1) the Yerba santa Community Phase, 2) the Mixed Chaparral Community Phase, and 3) the Chamise Community phase. There are many factors that 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 usually dominate the early stages following fire, with shrub seedlings and sprouts (England, 1988). Most shrub species that are present following fire are either obligate seeders or sprouters. 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
Loss of blue oak and foothill pine could result in a Community Phase 1.1 transition to grassland. Indicators of a change in structure and function on this site would include a lack of reproduction in shrubs and trees and a reduction in cover.
Triggers such as severe fire, mechanical removal and intensive prolonged grazing or prolonged drought may result in the community reaching a Threshold (T1A) causing tree mortality or unfavorable conditions for sprouting. Drought conditions may also affect blue oak seedlings, affecting their growth and survival (Grünzweig et al., 2008). Intensive grazing and frequent fire are not likely to favor blue oak recruitment (Swiecki and Bernhardt, 1993). If young seedlings and saplings are not recruited into the next age class as older trees die or before they are removed, oak populations may decline and areas converted to grassland (McCreary, 2001). Frequent fire return intervals kill new shrub seedlings and may eventually deplete the shrub seed bank (League, 2005). Grazing following fire also reduces new seedling vigor and may lower or eliminate seed production prior to the next fire (League, 2005). Mechanical treatments such as cutting or removal in combination with grazing may also achieve this result.
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, Page-Dumroese, Robichard 1998).
Insect predators and high temperatures resulted in significant buckbrush seed mortality and reduction of the seedbank in one study (O’Neil and Parker, 2005). Seed of buckbrush is also highly preferred by small mammals (League, 2005).
Mechanism
Removal of tree cover via conversion or severe fire, in combination with prolonged drought are triggers that causes mortality or unfavorable conditions for oak sprouting. 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 California foothill pine is a trigger that could result in reaching a Threshold (T1B) causing an irreversible transition to Shrub State.
Fire stimulates buckbrush seed to germinate by scarifying the seed “banked” in the soil (League, 2005). While buckbrush may have very limited natural regeneration without fire, fire-stimulated regeneration is more abundant. The “feedback” of sediment transport of fines from grasslands to shrub lands may eventually contribute to limited expansion of shrub communities (Briske et al., 2006). Low levels of browsing or complete protection from herbivory would also favor shrubs. Shrub species have several mechanisms for regenerating post-fire via sprouting or through seed. Shrub species such as buckbrush (Ceanothus cuneatus) and whiteleaf manzanita that have seed stored in the soil have abundant germination following fire (Abrahamson, 2014, League, 2005). Hollyleaf redberry (Rhamnus ilicifolia) reproduces via seed also. Birchleaf mountain mahogany (Cercocarpus montanus var glaber) sprouts from the root crown following cutting or fire.
Moderate to severe fire can result in a loss of nutrients and microbial processes and lead to accelerated erosion and leaching; high heat transfer to soils may give rise to a loss of soil organisms, destroy tree and shrub roots, and lead to 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 erosion rates.
Early post-fire plant communities commonly have a native shrub, California yerba santa (Eriodictyon californicum), that reproduces via rhizomes as a component. This shrub slowly diminishes in abundance as the plant community progresses.
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). The “feedback” of sediment transport of fines from grasslands to shrub lands may eventually contribute to limited expansion of shrub communities (Briske et al., 2006).
Moderate to severe fire is the trigger that stimulates buckbrush (and other shrubs) seed to germinate by scarifying the seed “banked” in the soil (League, 2005). While buckbrush may have very limited natural regeneration without fire, fire-stimulated regeneration is more abundant; historic photos of the area indicate that shrub patches were previously of greater extent with more frequent fire than under current conditions. 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
The prolonged absence of fire (>20-30 years) causes a shift in species composition as scattered blue oak and /or California foothill pine seedlings become established (Duncan et al., 1987). Protection from fire and grazing results in a gradual increase in trees and shrubs and contributes to increased ladder fuels and higher fuel loads.
Seeds are spread by animals, gravity and water (Howard, 1992). Shrub canopies may facilitate oak re-establishment by providing protective canopy from herbivores (Stromberg et al., 2007) and by providing shade. Birds may also disperse acorns in shrub areas.
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