Why fish need forests
Stream habitat can be evaluated as a product of the various input factors that come in from the surrounding watershed. These can be divided up in various ways, and one way is to segregate them as geologic input of sediments to the stream, hydrologic delivery of water, and biological or chemical delivery of wood and nutrients to the stream. It is these characters together that define instream habitat and determine fish productivity, salmon productivity, and biological characteristics within the stream. Habitat and fish are the product of these inputs from the surrounding watershed, and it is through this approach that we can evaluate how forests influence fish.
The place where these interactions occur most is in the riparian zone, forests next to the stream. Three major interactions include:
- Input of litter
- Input of shade
- Input of large woody debris
These are among the most significant when the issue of regulations is discussed. Input of litter is the delivery of the finer (smaller) types of organic matter from the forest to the stream: needles and leaves. This material serves as an important food source for invertebrates in the stream. For example, when one compares the inputs from early and late succession forests, about 60 grams per square meter per year of input occurred in the young (7 yr old) forest compared to 300 in the older forest. Some of the reduction in the young forest was compensated for by plant (algal) production in the stream channel of the young forest. Total organic matter availability in the stream of the older forest was about 80 percent greater. Forests do deliver a significant amount of organic matter to streams.
Shade and the role it plays in thermal regulation of streams is probably the riparian function that most people are familiar with. Studies have shown that presence of forests keeps the average temperature lower. Forest cover also keeps the day-to-day and day-to-night fluctuations lower as well during warm periods of the year.
Input of large wood to streams has become a very hot topic in scientific and management circles over the last 10 to 20 years. Wood has a variety of effects on stream systems:
- Impacts on channel form
- Physical characteristics of stream channels
- Wood plays a key role in regulating movement of material through stream systems
- It provides a variety of habitats for stream biota
Wood has a dramatic effect on the shape of the channel, helping to form pools. In small to moderate streams 60-80 percent of the pools can be related to coarse woody debris. Local scour around the piece of wood creates a pool with deeper water. For a long time, people thought that large woody debris did not play much of a role in determining channel form in large river systems, but in recent years we have found that wood does play a significant role in channel form in these large streams. Pieces that play such a role are invariably large trees with intact root wads, introduced into the channel from bank erosion. These trees get deposited fairly close to the channel bank, with the root wad oriented upstream and the rest of the tree trailing downstream. It begins to trap more coarse woody debris on its upstream side that creates slow flow on the downstream side and creates sites for sediment deposition and vegetation establishment. Wood influences channel form and successional dynamics in the riparian zone.
Sediment is retained by large woody debris and influences its movement through the stream system. The transport of fine and coarse sediment increases twofold in the absence of wood. Increase efficiency of transport, and mobilization of stored sediment, will both occur. Wood can also retain organic material in the stream longer than if wood is not present to trap smaller material. Not only needles and leaves, but also salmon carcasses can be retained by wood in the channel. Up to 60 percent of salmon carcasses can be associated with presence of coarse woody debris.
Wood plays a key role in providing habitat for animals. Spawning adult salmon will typically will use large pools, many of which are associated with woody debris, for holding areas prior to spawning. Many of the fish species of high interest will use similar pools while juveniles are rearing in fresh water. Comparison of cutthroat trout and coho salmon biomass (abundance X body weight) in British Columbia showed far higher biomass in sites with high amounts of large woody debris.
The entire watershed interacts with the stream channel. The delivery of water to stream channels will change, for example, with development of a forested watershed. As a result, peak flows will increase. In King County the average flow expected once every five years in a forested watershed was occurring once every year in an urbanized watershed. Biological consequences of this are increased bed scour, and reduced fish egg survival. Emerging fish are more likely to be flushed downstream, and food availability may also be less. Productivity of streams for coho salmon declines rapidly, so that in streams in King County with 10 percent impervious surfaces, salmon productivity had declined 70 percent from forested watersheds. At 40 percent to 50 percent impervious surface, these streams are essentially inconsequential in terms of salmon production.
Why forests need fish
The reason forests in the Pacific Northwest need fish is because of the kinds of fish we have here. Pacific salmon exhibit two characteristics that enable them to impact the habitats and forests of the streams they live in:
- They return to spawn in the same place, and in large numbers
- They die after they spawn
So they transport marine-derived nutrients to generally nutrient-poor headwater areas where they were born, and leave them there by dying. The kinds of effects salmon have on freshwater habitats include:
- Alteration in channel form as a result of the physical act of constructing redds, and this is particularly true for species that spawn in high densities, like chum and pink salmon.
- Changes in the characteristics of gravels, by removing large quantities of fine sediments.
- They are a food resource for wildlife as well as other fish and invertebrates in the stream.
- They are a significant nutrient source for riparian forests, too.
About 20-40 percent of the nitrogen and carbon in the body tissues of cutthroat trout, coho salmon, and steel-head are derived from marine sources. About 18 percent of the nitrogen in the tree foliage is also of marine origin, directly attributable to salmon. Subsequent work has shown that leaves of salmonberry, a riparian-oriented shrub, that nitrogen content is higher in streams that contain salmon than in streams without salmon. Increased understory cover and species diversity has been associated with streams where salmon spawn, and tree growth may also be increased significantly. This, of course, has a positive feedback in that potential pieces of large woody debris are being developed faster and fall into the stream to improve habitat quality.
We have been trying to understand fish utilization as a function of the primary land use in a watershed. The Snohomish River watershed shows that median coho salmon abundance declines significantly from forested watersheds to rural residential use to agriculture and urban areas. The only locations with consistent high utilization by salmon are the forested watersheds.
Salmon recovery should pay special attention to retaining lands in forest cover. Because of the way we currently manage these other lands, they tend to be very incapable of supporting high populations of salmon.
Robert Bilby, Ph.D.