Why the big deal about forests next to streams?

Richard R. Harris
Extension Forestry Specialist

Forests next to streams are extremely important and deserve special attention in stewardship planning.

Much of the current concern about the management of streamside forests stems from recent listings of anadromous fish, such as salmon and steelhead, under the Endangered Species Act. As we shall see, however, it is not just fish that are affected by streamside practices.

Size is important

There are many types of streamside forests. How much a streamside forest differs from the nearby upland forest depends largely on the size and location of the stream. This relationship between stream size and the associated forest are extremely important ecologically and have implications for management.

Smaller streams (say, less than 10 feet wide) in steep forested terrain often have steep rocky channels with no adjacent flat floodplains and seasonal streamflow. The width of the stream channel is small in relation to the height of trees in the surrounding forest. In such cases the streamside forest may be similar in species composition or size to the forest away from the stream; it is simply the trees closest to the stream.

As streams get larger, their potential for supporting a distinctive streamside forest increases. A larger stream (say, 20 or more feet wide) may have a gentler slope, year-round flow, and a narrow floodplain.

The floodplain is important. It provides a new type of environment, an area subjected to flooding, sedimentation and erosion, and possibly a high water table. These environmental conditions are conducive to the establishment and growth of obligate riparian or wetland plants.

Obligate means dependent on the stream; these plants could not live there if the stream or some other source of water were not available. Some examples of obligate riparian plants include sedges, Indian rhubarb, many ferns, mosses, and horsetail. One of the most common obligate riparian shrubs is willow. Trees include alders, cottonwood, mountain ash, and sycamore.

Whether or not these plants occur along a given stream depends on the availability of light as well as habitat. Since many of the obligate riparian or wetland plants require plenty of sunlight, they may be present only if the stream’s size is adequate to create a sufficiently large gap in the surrounding forest.

On larger streams (e.g., 60 or more feet wide), slope is gentle, the stream has a meandering or wandering channel, and there is a relatively broad floodplain which is only inundated at high flows. Consequently, these floodplains create fairly large openings in forests and the associated streamside forest can become much more complex, with different zones of vegetation as you move away from the channel. Next to the channel itself there may be willows or other plants that are well adapted to frequent flooding. As you move away from the stream, there tends to be a greater variety of trees and shrubs.

The role of the streamside forest in shaping the stream environment and aquatic habitats also varies in relation to stream size.

What are the general functions of streamside forests?

The functions of a streamside forest ecosystem vary according to species composition and the physical arrangement of the vegetation (structural complexity). There are three general kinds of functions performed by streamside forests: 1) they create and/or change aquatic habitat; 2) provide wildlife habitat on the land; and 3) modify erosion and sedimentation.

Streamside forests influence food, shelter, light, and water temperature in aquatic habitats. They provide food in the form of leaf litter, insects, and an occasional squirrel for aquatic insects, fish, and amphibians. Roots, stems, and overhead cover provide shelter to these same animals. Many aquatic insects spend at least some part of their life cycle in the streamside forest. The effects of streamside canopy on light and water temperature largely control photosynthesis by aquatic plants and algae (instream primary production). Water temperature is also critical to aquatic insects, fish and amphibians.

Many terrestrial animals depend at least in part on streamside forests for their habitat requirements. Some, like amphibians and reptiles, may live their entire lives in or near the streamside forest. Others (e.g. birds of prey) use the streamside forest to hunt, rest, or nest. Streamside forests provide travel corridors for large secretive animals like bear, cougar, and deer. Generally, the value of forest as a wildlife habitat increases with its width, degree of continuity along and across the stream, plant species diversity, and structural complexity.

Plants rooted next to streams or on floodplains screen sediment out of passing water, slow flood velocities, and influence erosion. All of these actions impact disturbance in the streamside zone. For example, when plants screen sediment out of the water, it causes floodplain deposition and building. Tree roots can also deflect water, leading to erosion and undercut banks.

The importance and magnitude of these general functions also vary depending on the size and complexity of the streamside forest. In medium-sized streams where the streamflow is year-round, the streamside forest can have a dominant influence and is very important in shaping aquatic habitat. Terrestrial wildlife functions tend to be more important on larger streams with larger floodplains and extensive streamside communities. The effects of the streamside forest on erosion and sedimentation are important on any size stream, although the absolute magnitude of impact by the forest may be greatest on small- to medium-sized streams.

Streamside forests and large woody debris

One very important function of a streamside forest is that it provides logs, stumps, and branches that enter the stream as large woody debris (LWD). The actual size of functional large wood should be defined operationally, in relation to the size of the stream.

Large woody debris affects streams in a number of ways. It directs streamflow, stores and distributes sediment, and creates pools, riffles, and waterfalls. Aquatic insects and fish occupy and use the pools and riffles as habitat. LWD intercepts organic matter traveling downstream, allowing it to be used as food by aquatic insects. Gravel bars created from sediment trapped by LWD become sites for streamside forest regeneration.

The abundance and distribution of large woody debris in a stream is strongly influenced by the size of the stream, character of the streamside forest, and the manner in which wood gets into the stream. Under natural conditions LWD loading (amount per unit area of stream water surface) is greatest in small streams in coniferous forests. The highest loadings have been recorded for small streams in the redwood region. This is because small streams lack the streamflow necessary to move logs that fall into them. The major mechanisms by which LWD enters these streams are windthrow, bank erosion, and landslides.

Logs in small streams may remain in place for hundreds of years.

Intermediate-sized streams are wide and deep enough to move and redistribute LWD. Wood may find its way to a given place on an intermediate-sized stream by floating down from upstream where it accumulates in irregularly spaced but distinct clumps at constrictions in the channel or on meander bends. The number of clumps, or jams as they are called, decreases with the size of the stream while the size of individual clumps increases with stream size.

On larger forest streams, most LWD enters through bank undercutting and tends to wind up on gravel bars or on terraces along the stream margin. This wood is frequently out of contact with the low flow channel and may have a limited effect on channel shape. On larger streams, stream width and degree of sinuosity (winding) are the main factors controlling abundance and distribution of LWD accumulations.

The manner in which wood is transported also varies with stream size. On smaller channels, wood often moves in “debris flows” during extreme runoffs. On larger streams, wood is floated downstream as individual pieces at high flows until it reaches an existing jam or constriction. Transport of wood from one part of the watershed to another is a major means of increasing LWD loading; it may be more important than the streamside forest as a source of LWD on medium-sized and larger streams.

As you would expect, the size of the stream discharge determines how big the wood has to be in order to remain in place; the larger the stream, the larger the average size of stable LWD. Logs with rootwads attached are more stable because of the increased potential for snagging. Stability is also increased if one or both ends are buried in the bank or resting above the channel.

When larger streams are constrained by bedrock, higher terraces, or other natural or manmade features (e.g., levees), there are less extensive floodplains, fewer opportunities for contributing and storing LWD, and more capacity to move LWD downstream during high flows. Wide, unconstrained streams bordered by floodplains and terraces possess abundant sites for storing logs. These areas often develop streamside forests that in turn become sources of LWD.

Management Implications

Timber harvesting in streamside forests can reduce the amount of large woody debris (LWD) entering a stream. This effect is probably most important on smaller streams which do not receive inputs of LWD from upstream transport.

In this era of buffer strips along most streams, timber harvest impacts are not as severe as they once were. However, many streamside forests lack conifers and are dominated by smaller hardwood species. Even if conifers are present, today’s streamside forest may be deficient in large trees. In second or third growth streamside forests the average diameter of trees that can provide LWD (source trees) is smaller than in old growth forests.

• Clearing streams of large woody debris, for whatever reason, can have dramatic physical and ecological effects. This was practiced in the past because the debris was thought to impair fish migration. It is still practiced to prevent damage to roads and stream crossings.
• Leaving too much wood in a stream after logging can also create problems. This tends to occur with fine, rather than coarse, debris. An overabundance of fine debris can create chemical conditions in water (lowered oxygen levels, elevated tannic acids) that adversely affect instream species.
• Landowners should work to restore and maintain the functions of large woody debris in their streams. Management decisions can be guided by the knowledge that LWD performs differently in different streams. On small, steep streams, single pieces of wood spanning the channel act as major controls on channel form and ecology. On larger streams, LWD accumulations or jams are the rule. Single pieces are generally unstable. For the largest streams, LWD plays a limited role in the channel except when extremely large jams are present. LWD on floodplains and gravel bars on the largest streams provides escape habitat for fish at high flows and promotes streamside forest development.
• Managing the streamside forest to promote recruitment of LWD is a long-term project. Just prescribing buffer strips alone may not be adequate. Most LWD derived from streamside forests comes from within 100–200 feet of the stream. Growing large trees as potential sources of LWD in that zone may require active management, such as planting conifers, controlling competition and thinning, activities that are rarely done at the present time.
• Avoid indiscriminate removal of LWD from streams or floodplains. Few people agree on how much LWD is enough but nearly all agree that most streams need more. On some streams, reintroduction or placement of logs in the channel may provide a short-term solution to maintain wood-created habitats until natural recruitment processes recover. This should not be done by the novice, however, since added logs should be structured to mimic effects of natural obstructions in streams.

In the long term, creative management of streamside forests offers the most promise for improving conditions.

See also Some functions of large woody debris