GEOG/ES&P 330
California Ecosystems
Woodlands
Woodlands are vegetation associations visually dominated by the tree life form. Unlike a forest, woodland canopies are open, allowing sunlight to reach the woodland floor and support the growth of smaller plants in between the trees and, sometimes, below them. The trees can be evenly spaced or, more commonly, grouped into groves. In California, oaks are the most commonly dominant trees, but there are also woodlands dominated by pines and walnuts. In each locale, there is a particular mix of common tree species, but that mix changes as you move around in California, varying with latitude, distance from the moderating influence of the coast, elevation, slope aspect (with respect to sunlight and with respect to prevailing winds and storm tracks), and evolutionary history. Tree species diversity is typically low, perhaps one to half a dozen species of tree.Oaks can be subdivided by life form (most are trees, a few are shrubs found in chaparral) and by deciduosity (if that's a word!).
Scrub oaks are the oaks that grow as shrubs. There are quite a few of them, and some of those that normally grow as trees can adopt a shrub form, depending on circumstances. These species are not the dominants of oak woodlands, however; rather they are found as shrubs in shrub-dominated formations, such as chaparral or CSS, or as shrubs within woodlands visually dominated by trees, including other oaks. So, you find them in live oak woodlands and piñon-jupiter and joshua tree woodlands and conifer woodlands and forests.
Oak-dominated woodlands
- Quercus vaccinifolia (Huckleberry oak)
- Live scrub oak of northern and eastern California, conifer forests, tolerant of serpentinite soils
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- Quercus turbinella (shrub live oak)
- Quercus sadleriana (deer oak)
- Live scrub oak of northwesternmost California, higher elevations, conifer forest
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- Quercus palmeri (Palmer's oak)
- Deciduous scrub oak of sorthwesternmost California, middle elevations, chaparral
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- Quercus pacifica (Channel Islands scrub oak)
- Channel Islands, also in San Luis Obispo and Santa Barbara county borderlands, in chaparral and CSS, mostly evergreen but capable of facultative deciduousness due to drought or extreme cold
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- Quercus john-tuckeri (Tucker's oak)
- Live oak shrub in east-facing slopes of the Central Coast, Transverse, and Peninsular ranges of Central and Southern California, where it is associated with foothill oak woodlands and chaparral
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- Quercus durata (leather oak)
- Live scrub oak of low to middle elevations in chaparral and foothill woodlands
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- Quercus dumosa (Nuttal's scrub oak)
- Live scrub oak of low to middle elevations in Southern California chaparral and foothill woodlands
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- Quercus cornelius-mulleri (Muller's scrub oak)
- Middle elevations in Southern California, mainly in low desert foothill locations
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- Quercus berberidifolia (inland scrub oak)
- Low to middle elevations through most of coastal and Sierra foothills California, in chaparral and CSS
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- Live oaks
Deciduous oaks
- Quercus agrifolia (coastal live oak)
- Low to middle elevations through most of coastal California, dominating foothill oak woodland with grassland in between oak groves or sometimes merging with chaparral or CSS
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- Quercus wislizeni (interior live oak)
- Quercus chrysolepis (canyon live oak)
- Low to high elevations through most of California, inland in both Northern and Southern California but also found along the coast in Northern California, dominating foothill oak woodland with grassland in between oak groves or sometimes merging with chaparral or mixed woodlands with conifers, usually in canyons
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- Quercus tomentella (island live oak)
- Quercus parvula (coast oak or Santa Cruz Island oak)
- Notholithocarpus densiflorus (tanoak)
- Quercus engelmanni (coast oak or Santa Cruz Island oak)
- Deciduous oak in low to middle elevations inland and just inland of the coast of California from the Santa Monicas, through the Central Ranges and the Great Central Valley, in foothill woodlands with grass between oak groves and interpenetrating with chaparral, often in riparian situations. There are a few isolated occurrences in the Great Central Valley.
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- Quercus lobata (California white oak or valley oak)
- Deciduous oak in low to middle elevations inland and along the coast of Southern California, in foothill woodlands with grass between oak groves. There are a few occurrences in the Channel Islands.the Great Central Valley.
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- Quercus kelloggii (California black oak)
- Deciduous oak in low to high elevations inland and near the coast south of Morro Bay, in foothill woodlands with grass between oak groves, mixed evergreen woodlands, and in conifer woodlands. Foliage reddens in fall.
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- Quercus douglasii (California blue oak)
- Deciduous oak in low to middle elevations inland in valleys and foothills surrounding the Great Central Valley, in foothill woodlands with grass between oak groves
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- Quercus garryana (Oregon white oak)
Threats to oaks in California
Oaks are in serious decline in many places in California. Factors behind the decline vary from species to species, but some common culprits include:
- Ongoing real-estate development. Oaks have been cleared to make parcels ready for housing construction and other land uses. Many developers really do try to save individual oaks because of the look they give to their projects, but the trees are challenged by:
- Alteration of the permeability and weight of the soil covering around them, such as driveways, parking lots, and streets.
- Alteration of the plant cover around them, taking out shelter for their acorns and seedlings, changes in the near-surface microclimates, and probably disruptions of surface and subsurface symbioses.
- Alteration of the hydrology around the plants, due to changes in permeability and plant cover and well-meaning homeowners overwatering the trees while caring for their lawns and landscaping (oaks are highly subject to root rot in wet soil)
- Feral hogs descended from from those brought in during the mission, rancho, and early American agriculture era and imports of wild European boar in the 1920s have naturalized in California, where they have become an invasive species. Urban Californians have no idea how many feral hogs are out there in rural areas, and the animals have been especially devastating to oaks, hunting for acorns and truffles (underground fruiting bodies of certain species of fungus that form symbiotic associations with certain tree species' roots, incuding oaks), insects and grubs that live among oak and other species' roots, selectively grazing oak seedlings, and dramatically tearing up the soil as they do so.
- A variety of fungal infections:
- Foamy bark canker disease caused by Geosmithia pallida fungus is decimating Quercus agrifolia in Southern California. It is introduced by western oak bark beetle, Pseudopityophthorus pubipennis, and then uses the tunnels made by the beetle to access the interior vasculature of the tree. It is creating total oak kills in several areas in several counties of Southern California.
- Sudden oak death is decimating several oak and tanoak species in Northern California, north of about Paso Robles. It has killed millions of trees since 1995! http://www.suddenoakdeath.org/wp-content/uploads/2010/07/Tjosvold-tanoaks.jpg. It is caused by Phytophthora ramorum, a pathogen once thought to be a fungus but which recent analysis suggests is actually more closely related to kelp algæ, a kind of water mold. No-one knows where this pathogen came from: Is it a native species that mutated into such virulence? Is it an exotic species? It hit Europe about the same time, so it could be some kind of introduced critter. It is possible that Southern California's drier climate has kept this pathogen in check here. To learn more, here is the Oak Mortality Task Force web page: http://www.suddenoakdeath.org/.
- Diseases like this strike trees that are under stress: Their ability to fight back is compromised by stress. Trees all over the West are in extreme stress due to the very persistent droughts of the last couple of decades. Drought has hit before, but there is a shift in the overall climate system toward greater temperatures (which alone would cause water stress) and greater variability both in temperature and in precipitation. Oaks aren't the only trees in trouble: Conifers are being decimated by various pine bark beetle species that act much like oak bark beetles. Ornamental plantings of native and introduced species in Southern California cities are being attacked by invasive shot-hole borers https://bof.fire.ca.gov/media/10212/full-15_file-2_ishb-zone-of-infestation-proposal-final-11-4-20_ada.pdf.
- Between the hogs and these diseases, oaks are having trouble reproducing. What may look like a healthy oak woodland looks that way because of the condition of adult trees, but, below them, future generations aren't coming up.
