Riparian Plants: Valley Oak - Quercus lobata
Valley oak is commonly found in Valley oak woodlands and mixed riparian forests. Valley oak is most common in deep, rich soils of fine texture like silt and clay which tend to have a higher water holding capacity than coarse soil textures like sand. Such finer soils are typically found higher on the floodplain. The roots of Valley oaks do not have to reach the water table in order for the trees to grow. Valley oak can tolerate both long term flooding and periods of drought. Valley oak grows more densely in areas of higher water availability, such as lower on the floodplain. However, in these lower sites that are closer to the main river channel, mixed riparian forest species such as cottonwoods and willows are better adapted to the more frequent, longer duration floods and the resulting physical battering and inundation of flood waters.
Valley oak is California’s largest broad-leaved tree, reaching 15 to 35 meters in height. The usually single trunks can be up to four meters thick, with rough, grooved bark. Like most riparian plants, the structure of Valley oaks changes over time and within different plant communities. For example, oaks growing in open woodlands tend to be broadly branched with leafing branches along the trunk of the tree close to the ground. These lower branches are shaded out in riparian forests, so oaks growing in closed canopy plant communities tend only to have branches towards the top of the tree.
Depending on their growth form, Valley oak branches may provide overstory and midstory cover. The multiple large branches of Valley Oaks create a broad, tall, structure and towards the ends of each large branch are multiple small stems that during the spring and summer provide good cover for nesting birds and other wildlife such squirrels and ringtails. Snags or dead branches of Valley Oaks are used by cavity nesters and raptors will also nest in oaks. Many migratory songbirds nest in Valley oaks, but these trees are especially important nesting sites for resident birds, which tend to be more frequent in oak riparian forests and woodlands, while migratory songbirds are more frequent in mixed riparian forests. Valley Oaks can support the platform nests built by herons and egrets. Valley Oak acorns are an extremely valuable winter food source for birds and mammals. Valley Oaks are frequently used by insects, especially galling insects.
For more information about the ecological tolerances and structure of riparian plants, see Gaines 1977, Conard and others 1977, Holstein 1984, Sacchi and Price 1988, Faber and Holland 1996, Cooper and others 1999, Vaghti 2003, Fremier and Talley 2009, and Vaghti and others 2009.
Valley oak dimensions
Young valley oaks
Older valley oak