The Riparian Forests of the Lower Dungeness River

by Kathy Bush, Renee Leiter, and Bob Boekelheide

The Dungeness River is extremely dynamic.   With each flood the river cuts new channels, removing old vegetation and creating new river bottomlands.   As trees fall to floods new succession opportunities develop, adding diversity in the riparian zone.  These natural cycles, in combination with timber harvests and other human disturbances of the 20th century, encourage plant species that quickly and bountifully reproduce, enabling them to rapidly reoccupy disturbed riverside habitats. 

Despite disturbances, the Dungeness forest retains characteristics typical of forests worldwide.  First, the forest is structured in vertical “layers”   Sun-loving pioneer species dominate the canopy, the highest branches that over-top the rest of the forest.  Below the canopy is an understory comprised of smaller trees and shrubs that are usually more tolerant of lower light levels.  Finally, shade-loving species such as ferns, mosses, lichens, and small flowering plants thrive in the herb layer on the forest floor.   A mosaic of horizontal habitats also exists depending upon disturbance, soil type, and soil moisture, all changing as the river moves in and out of its channel.  Overall, riparian forests contain a diverse mix of sizes and ages of plants, from pioneering seedlings in the river channels to some historical giants that have luckily survived the meanderings of the river.

Five species of trees dominate the disturbed forests of Railroad Bridge Park: Black Cottonwood, Big-leaf Maple, Red Alder, Douglas-fir, and Western Red-cedar.  Tallest of all are the “old” Black Cottonwoods (Populus tricocarpa), whose massive gray trunks reach half their height before splitting into several gigantic limbs that stretch farther upward into the sky.  Their spade-shaped leaves flutter in the slightest breeze, yet strong windstorms cause whole trees to violently torque back and forth.  These storms occasionally cause catastrophic failures of their water-logged trunks, such as happened to the cottonwood that shattered the bridge trestle when it fell in October, 2001.  The biggest cottonwoods in Railroad Bridge Park are probably less than 100 years old, and yet, because of their rapid growth rates, they are over four feet in diameter and over 160 feet tall.  Interestingly, the loop trail north of the park entrance road contains an almost uniform stand of young cottonwoods, apparent pioneers that germinated following a flood event within the last 50 years.

In contrast to the Black Cottonwoods, the over-arching limbs of Big-leaf Maples (Acer macrophyllum) originate much closer to the ground, causing spreading branches and dense shade.  Their big maple leaves are obvious, some reaching over a foot wide.  The biggest maple trees are set well back from the present river bed, where less-disturbed conditions have granted them time to reach large sizes.  These maples can easily be separated at a distance from other trees by their huge leaves and, even in the winter, by their spreading limbs.  The oldest maples may be as wide as they are tall.

The last large deciduous tree widely found throughout Railroad Bridge Park is Red Alder (Alnus rubra).  Alders are considered “weed trees” by foresters because they are opportunistic pioneers, rapidly regenerating following disturbance and shading out other more “desirable” trees.  Red Alders in the park are consistently smaller than cottonwoods and maples, typically less than 100 feet tall and at most 2 feet in diameter.  They can easily be told throughout most of the year by their smooth grayish, blotchy bark and by the presence of male tassels and female cones on their outermost branch tips.  The female alder  cones look very much like miniature conifer cones.  Alders are also renowned for having nodules on their roots that enable them to convert atmospheric nitrogen into usable soil nitrogen.  Look for these nodules where alder roots have been exposed by bank erosion along the river.  Because of their nitrogen content, alder leaves provide substantial input of nitrogen into the ground for other plants to use when they decompose in the fall and winter.

Finally, two conifers found in abundance in the canopy of the riparian forest in Railroad Bridge Park are Douglas-Firs (Pseudosuga menziesii)  and Western Red-cedars (Thuja plicata).  Douglas-Firs, with several smaller red-cedars, are particularly dominant along the main trail west of the trestle where the trail passes between rows of towering conifers.  Douglas-Firs can easily be told by their soft yellow-green needles that point in all directions from the twig and by their unique cones showing three-pointed bracts sticking out between the scales.  Grand Firs (Abies grandis), with their needles forming flat sprays on the twig, are also found throughout the park, but not as large, dominant trees.   

Curiously, Western Hemlock (Tsuga heterophylla), another common conifer of western Washington, is rare in Railroad Bridge Park.  There is only one small hemlock of which we know.  This is undoubtedly true because it germinates and grows better in less disturbed conditions.  There is one Pacific Yew (Taxus brevifolia) that we know of, hiding out near the edge of the road on the Ridgway property.

The understory of the Dungeness riparian forest is particularly notable for the number of shrubs and small trees that provide hideaways and food sources for native wildlife.   Smaller trees and large shrubs commonly found in the park, all of which produce edible fruits or seeds eaten by birds, include Indian Plum (Osoberry), Snowberry, Red Elderberry, Ocean spray, Salmonberry, Thimbleberry, Nootka and Baldhip Roses, Red Huckleberry,  Bitter Cherry, and Red-flowering Currant.  Dense shade under the conifers promotes Oregon Grape and Sword Ferns.  New riverbottoms exposed after floods are rapidly colonized by various willows (Salix sp.) and small cottonwoods, both of which are able to germinate from twigs and roots that have washed downstream.   All of these plants have a story to tell about the wildlife that uses its fruits or flowers, such as the Indian Plum that provides the first flowers for migrating Rufous Hummingbirds in March, or the Snowberries that provides nutrition for Robins when food supplies dwindle in mid-winter.