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A Total Forest Management Plan
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Dead wood in a forest is commonly called "coarse woody debris". It included snags, fallen logs, and broken branches or tops. Stream organic debris (sometimes called LWD or large woody debris) is the assorted group of logs, tree tops, limbs, stems, roots, leaf mats and whole trees found in streams.
Determining the amount in streams is difficult because of the variety and changing nature of the group of materials. Determining desirable or permissable amounts is difficult both because measurement is difficult and also because objectives are not clear. "Desirable for what organisms? People? Reduced sediment? Reduced risk if it should wash out?" To study organic debris in streams requires consideration of the entire aquatic ecosystem. It plays an importnat role in water and sediment routing and increases the structural complexity of streams. It comes from adjacent forests, slides from banks, and is carried by regular water flows and floods. Current stem density determines the maximum short-term input to the stream. Topography and tree lean have such variable effects that such data provides no information to influence decisions. Once debris is in streams, it provides places of attachment, reduced energy, and a substrate for aquatic and terrestrial organisms.
Determining optimum amounts of large debris in streams is difficult. The situation is complicated by the complexity and variability of the stream environment and the variety of ways resource managers view stream debris. Fisheries biologists. stream ecologists, water quality experts, and road design and maintenance personnel would all probably set different standards for stream debris concentrations. Much of the present indecisiveness about management of stream debris may stem from lack of understanding of the biological and physical functioning of debris in small mountain streams. Streams and their biota have developed through a long history of high concentrations of debris.
Organic debris greatly influences stream biology, hydrology. water quality, and sediment transport. In Oregon. concentration ranged from 0.9 ton to 26 tons per 100 ft of stream channel. In Tennessee they were 13 kg per square meter. Following logging. debris was modified from 0.60 to 3.6 times that of prelogging concentrations, depending on practices used (Swanson et al. 1976:2)
Biological consequences of debris are:
Debris causes stream channel width to be wider and more variable in forests than in open grass sod-bank streams. It has a variable effect on stability of the banks and bed. Stability is enhanced and the channel is narrowed where root systems extend through the banks and channel bottom. The channel may also be destabilized and widened above debris dams and where trees are tipped over. In drainages larger than several square miles, stream width is more uniform than in smaller basins. It has a strong influence in shaping the geometry of small streams where hydraulic capabilities of the stream are not dominant.
Large debris in streams tends to destabilize the streambed and banks. This is particularly common in gravel-rich streams where natural and human-caused hillslope instability has accelerated sediment supply to the channel. In these settings, large organic debris may slow sediment transport, cause braiding or meandering of the channel over its widened bed and lead to stream bank cutting. Increased bank erosion aggravates the stream problems by introducing more sediment to the channel and possibly causing more extensive instability of adjacent hillslopes.
Accumulations may form as a result of in-stream sorting during high streamflow. In large streams there is great opportunity for debris transport and sorting to take place; consequently, the debris in intermediate and large streams tends to be concentrated in distinct accumulations. Generally, organic debris in small streams is randomly distributed. Because of this scattering of debris and the large size of individual pieces relative to channel dimensions, it is seldom possible to identify individual accumulations in first- and second-order streams.
The quantity of debris in a stream channel at any time reflects a balance between the processes controlling the debris inputs and outputs of the stream system. Factors which control the input of large debris are age and condition of the surrounding timber stand, the stability and steepness of banks and adjacent hillslopes. and the ability of the stream to transport in new material from upstream channel areas. The export of large organic debris is determined by the ability of the stream to float debris downstream, rates of decomposition and physical breakdown of debris in channels, and the probability of more debris torrents flushing out the channel. In many instances, input and output take place in sporadic events occurring every few decades or centuries. These include major episodes of blowndown, extreme discharge events, debris torrents, and stream cleanup following logging. In most streams, however, it is a continual give and take situation, with one debris accumulation slowly growing as it traps floated material while the next accumulation along the stream may be collapsing and breaking up into small, floatable pieces after a long period of rotting or use by animals. It is therefore, important to view the status of stream debris in a historical perspective.
Several lines of evidence suggest that during pre-management time, small streams have had high concentrations of debris throughout most stages of succession of the surrounding vegetation. Rotting of logs in or above streams is appreciably slower than where they are lying on the ground. Therefore, debris may have a long residence time in the stream environment, possibly remaining in channels well into a second-growth stand developed after wildfire or early agriculture. Flushing of channels appears to be controlled more by slope stability factors (debris slide and earthflow activity within the drainage) than by cycles of debris accumulation in the channel that are terminated by periodic cleaning. These observations indicate that small streams draining moderately stable watersheds have contained abundant large organic debris for much of the past few thousand years.
It is now evident that (1) large concentrations of debris in streams occur naturally; (2) debris may have residence times to more than a century; (3) debris increases the "roughness" of the channel, causing sediment and floated organic matter to be trapped and slowing the movement of these materials through the stream system; (4) a large proportion of the stream drop is in fall over debris, thereby dissipating much of the stream's energy at a few points along the channel rather than more uniformly along the channel as is the case in similar streams which are debris-free; and (5) the impact of large debris on channel morphology is complex because debris causes widening and narrowing, deepening and shallowing, and stabilization and destabilization at different points along the channel bed and banks. On larger streams which can float most of the debris which enters them, the debris plays a rather minor role in the stream environment.
In addition to stream size, the role of debris in streams also varies in response to the geology of the region. It seems likely that one day, based on the desired or tolerable bedload movement. specific recommendations can be made. In some areas organic debris may be managed to slow bedload movement and enhance fish spawning areas. In other areas it may be used to reduce aggradation and bank cutting.
Extremely high levels of debris loading result in abundant habitat for wood processing organisms but may reduce habitat opportunities for other aquatic life such as fish. Coulston (1977 :30) found that instream debris accounted for about 58% of the variation in trout biomass per hectare in streams. He noted it was especially important for larger fish. Debris removal decreased trout biomass per hectare. Trout in areas of debris accumulation are larger than those in areas without debris.
In steep headwater streams, management induced reductions in debris loading relative to typical, natural levels may occur in several ways. Cleanup operations themselves may be "over zealous." removing valuable components of habitat for fish and other organisms.
What are the long-term consequences of eliminating large organic matter from streams? Of course, it is impossible to answer with certainty; but it seems likely many small streams will undergo downcutting and become effectively "channelized" on bedrock or a stable boulder pavement. A stream which had previously flowed over a series of steps formed by debris will assume a more uniformly steep profile and experience other changes in channel geometry. There will be a resulting decrease in diversity of stream habitat as biologically productive, debris- related depositional pools are eliminated. Increased water velocity will also contribute to the accelerated transport of fine organic matter through the channel system, thereby decreasing the opportunity of stream organisms to process the material. Consequently, the removal of large debris from streams may reduce long-term biological productivity and increase the rate of sediment transfer from headwater streams to downstream areas.
Progressive design and management of logging operations and~or buffer strips will help to minimize the impacts of both long- and short-term alterations of the stream enironment. Some generalized guidelines, now practiced in many management circles, are:
The greatest return on management efforts to minimize stream damage would come from protecting the heads of relatively long, straight, torrent-prone channels.
With both moderate and excessive debris loads in streams, there is no single, simple set of rules which can be applied indiscriminately. Each site presents a different set of conditions of stream biology, channel gradient, status of stream debris, conditions of surrounding timber stand, abundance and size of bedload, and slope stability in the drainage. The great complexity of the stream environment means that each site must be inspected in the field and treated on an individual basis. Debris management problems call for a high degree of cooperation between specialists and administrative personnel.
It is anticipated that The Trevey will later include site specific prescriptions for such debris. A temporary standard, easily checked, is that stream reaches with adequate debris should be expected to have more than 10 pieces of debris (about 4-inch diameter at the small end) per 50 meters of channel.
Physical characteristics of debris in streams vary systematically through stream systems. Debris loading is highest in small steep headwater streams and generally decreases downstream. In first- and second-order streams, large debris is randomly located where it initially fell, because the streams are too small to redistribute it. Third- through fifth-order streams are large enough to redistribute debris, forming distinct accumulations which may directly affect the entire channel width. In larger rivers, large debris is generally thrown up on islands or the banks and has little influence on the channel except at high flow conditions.
Large organic debris enters streams by a variety of mechanisms, some of which are interrelated and chain reactions. Principal mechanisms of debris input are blowdown of whole trees or tops and major limbs; debris slides, debris avalanches, and deep seated mass movements from adjacent hillslopes; undercutting of streambanks; and timber falling and yarding operations. Debris also enters stream sections from upstream channel reaches by flotation and debris torrent processes.
Flotation of large organic debris may be a problem in intermediate-sized streams (third- to fifth-order, drainage areas of about 400 to 6000 ha). These streams are wide enough to float large debris during extreme flood flows. Pre-existing debris accumulations may be moved downstream for hundreds of meters. destroying riparian vegetation and rearranging the channel along the way.
Debris torrents are very common events, particularly following clearcutting and road construction. These events transport large quantities of organic debris and sediment from small streams into intermediate-sized streams. Debris export from the intermediate- sized streams occurs under extreme flood conditions as rafts and individual pieces of large debris are floated downstream damaging riparian vegetation along the channel. There appears to be a general pattern of greatly increased debris and sediment accumulation in many intermediate-sized streams.
Although debris torrents are spectacular events of real management significance, they actually move material relatively short distances (up to several kilometers). The ultimate export of large organic debris occurs in the form of fine particulate and dissolved matter resulting from breakdown of wood by the action of decomposer organisms, invertebrates, and snails. Organic matter in a log in a stream high in the mountains will eventually pass through many organisms' gut tracks in the course of transport down river to the sea.
Large debris in streams controls channel morphology and sediment and water routing. In streams to about third-order size. debris helps form a stepped gradient. The stream bed is made up of long, low gradient sections separated by relatively short, steep falls or cascades. Therefore, much of the stream bed may have a gradient less than the overall gradient of the valley bottom because much of the stream drop, or decrease in potential energy. takes place in the short, steep reaches. This pattern of energy dissipation in short stream reaches results in less available energy for erosion of bed and banks, more sediment storage in the channel, slower routing of organic detritus, and greater habitat diversity than in straight, even gradient channels.
One way to evaluate the role of debris in sediment routing is to compare the volume of sediment stored behind debris in a channel with annual sediment export from the channel. Annual sediment yield of small forests may be only about 10 percent of sediment stored in the channel systems.
The overall storage capacity serves to buffer the sedimentation impacts on downstream areas when there are pulses of sediment input to channels. Scattered debris in channels reduces the rate of downstream sediment movement and tends to feed sediment through the stream ecosystem in a slow trickle, except in cases of catastrophic flushing events. These flushing events may scour a channel every few centuries, leaving the channel devoid of large organic debris and open to rapid transfer of bedload.
Organic debris may be the principal factor determining the characteristics of aquatic habitats. In areas where streams are characterized by "classic" meandering channel patterns, hydraulic factors are more important in regulating the distribution of aquatic habitats. The role of debris in creating habitat for fish is significant. The wood itself is a habitat or substrate for a great deal of biological activity by microbial. invertebrate, and other aquatic organisms. Much of the biological activity by detritus-processing and other consumer organisms is concentrated in the areas of wood and wood-related habitat.
In Virginia so much area has been cleared and large logs removed (or never allowed to enter streams) that the energy of streams greater than first-order have been little affected by major organic debris. This has now been true for about 200 years. Some logs will last a very long time in the anaerobic conditions of the stream. Even that has changed, since the only large wood is now that of young porous trees.
Management guide (from J. Forestry, Jan., 1999):
After a harvest and regrowth, coarse woody debris drops dramatically. There is no new source. Debris is at a minimum when the forest is at maximum growth. Debris volumes follow a U-shaped distribution. Nakamura and Swanson (1993) found that in 1st and 2nd order stream that were steep and rock constrained that there was little interaction between coarge woody debris(CWD). Channel widening, steepening, and sediment storage related to CWD was mostly in 3rd and 4th order streams. Periodic spacing for both width and gradient was regulated by CWD.
see NICHOLAS E. FUHRMAN, 2003.Variability of coarse woody debris volume and use in four ecoregions of Virginia, MS Thesis, Va Tech
References
Nakamura, F. and F.J. Swanson 1993. Effects of woody debris on morphology and sediment storage of a mountain stream system in western Oregon Earth Surface Processes and Landforms 18:43-61.
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Last revision January 17, 2000.