Mixed Riparian Forests
Lower on the floodplain and closer to the main channel, the Valley Oak Woodlands transition into mixed riparian forests. In mixed riparian forests, very tall oaks are less common, and the frequency of sapling oaks is higher. A midstory canopy layer is present in mixed riparian forests, composed of medium sized trees and tall shrubs such as sycamores and box elder. The understory contains a greater proportion of smaller shrubs than is present in Valley oak woodlands. Mixed riparian forests may be dominated by tall (>30m) cottonwoods and medium sized arroyo willows (Salix lasiolepis) and black willows (Salix gooddingii). Where there are openings, dense patches of California mugwort (Artemisia douglasiana) may form, and aggressive vines such as blackberry and grape can produce huge thickets in the understory. There may be openings where trees and shrubs are almost completely engulfed in grape, or dense walls of blackberry that has climbed up trees and shrubs. Mixed riparian forests include dense, closed canopy forests interspersed with openings, which adds to their complexity and potential resources for wildlife.
The structural complexity of mixed riparian forests, along with the size and diversity of plants, may explain why these forests attract such a great diversity of wildlife. These forests are particularly important to migratory birds. Multiple fruit bearing plants like blackberry, elderberry, rose and grape are a good food source for fledgling birds but also mammals, reptiles and invertebrates. The dense cover at multiple heights allows wildlife to build nests on the ground or in trees. Such cover also provides refuge during migration or dispersal.
For more information about Riparian Plant Communities see Conard and others 1977, Gaines 1980, Holstein 1984, Holland 1986, and CWHR 1988.