Oak Woodland

California has approximately 8.5 million acres of oak woodlands, with nearly a quarter of that acreage within the area bounded by Tehama, Plumas, El Dorado, and Solano Counties. It is estimated that a third of California’s oak woodlands and forests have been lost to agriculture and urban development, and 750,000 more acres are expected to be lost to development within the next 25 years. Although cattle grazing in oak woodlands has so far allowed much of the historical oak woodland habitat to remain—in the face of California’s expanding population—most mature trees are over 100 years old, and very few oak saplings escape grazing pressure to replace dying trees. Despite the significant losses of this habitat type, oak woodland remains one of California’s most iconic landscapes.

Fascinating Features

  • Oaks exhibit a phenomenon called masting, in which all of the oak trees in a region produce unusually high yields of acorns in irregular cycles. No one knows exactly what triggers these mast events, but it has been hypothesized that they are an evolutionary tactic to “overload” populations of acorn-eating wildlife, ensuring that at least in some years, enough acorns remain uneaten to germinate into seedlings.
  • In the Sacramento region, the dominant tree species in oak woodlands are the valley oak (Quercus lobata), blue oak (Q. douglasii), or interior live oak (Q. wislizeni). Blue oak and valley oak do not appear to be susceptible to Sudden Oak Death, a disease that is decimating oak trees of different species in coastal areas of California.
  • Most of an acorn’s energy stores are used to develop a fast-growing tap root that goes deep underground to seek reliable moisture. After a few years, the tree’s resources are then directed to enlarging the above-ground branches and leaves.

Habitat Values

Oak woodlands are known to support over 300 species of vertebrates and 5,000 species of invertebrates, in part because oak woodlands provide a variety of vertical and spatial habitats—open grassland for burrowing animals, fallen logs for reptiles and insectivorous mammals, shrubs for foraging herbivores, high tree canopies and cavities for roosting birds and bats, standing snags for nesting raptors, and streams for anadromous fishes.

Oak gall wasps

Oak gall wasps (family Cynipidae) manipulate the oak’s biochemistry to form galls, which are often curiously shaped or brightly colored swellings that can be seen on oak leaves and twigs.

California Scrub-Jays

An individual California Scrub-Jay (Aphelocoma californica) can transport and cache up to 7,000 acorns in a season, many of which may not be eaten and have the potential to germinate into new trees at significant distances from where they fell from the parent tree.

Wildlife habitat

Even after they die, oak trees continue to be useful to wildlife. Salamanders, worms, snails, termites, and other critters live in decomposing logs and, in turn, become food for other animals.