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Pacific Northwest riparian forests provide important functions relating to aquatic organisms. Riparian forests alter the microclimate. The canopy of the riparian vegetation provides shade to streams, helping lower the stream’s temperature. When stream temperatures get to high Trout and salmon will die of low oxygen concentrations in the water. The temperature range for the salmon and trout is 55 degrees Fahrenheit to 70 degrees Fahrenheit. The vegetative cover also influences light levels, air temperature, and relative humidity reaching the stream. Riparian trees help maintain the overall quality of stream they are next to. The leaves/ needles and other litter the trees generate fall into the water and contribute carbon and nutrients for the organisms in the stream. The root systems of the trees make stream banks stable and affect the channel morphology. The stems of the trees provide resistance to over bank flows, this helps retain sediments on the flood plain and not allow them to be washed away when the stream levels lower. Riparian trees also provide a source of large woody debris to streams. The large wood in the streams has three functions: 1. Long- term source of nutrients to the stream 2. Fish habitat 3. Dissipation of downstream routing of coarse sediment.
Riparian forests have benefits that directly affect people as well. Riparian vegetation and soils contribute to the filtration and transformation of nutrients and pollutants. This increases water quality which reduces the effort cities spend on treating their drinking water. Another contribution these forests make is the detention of flood waters. Perhaps values most people would associate with riparian forest would be the aesthetic and recreational values. People enjoy fishing, hunting, bird watching, hiking, and many other outdoor activities. These forests provide a great place for outdoor enthusiasts to recreate.
The specific species of interest are western red cedar (Thuja plicata) and western hemlock (Tsuga heterophylla). Both of these species are adapted to growing in moist sites typical of riparian areas. Western red cedar grows on the stream bank and along the flood plain while western hemlock grows gradually farther from the stream bank, on less moist sites but still in the flood plain. These two species make up a mixed species riparian forest.
Western red cedars native range is form northern California (lat.40º 10’N) to the Sumner Straight of southeastern Alaska (lat. 56º 30’N). The annual precipitation that western red cedar receives ranges from 28in-260in depending on whether it grows along the coast or in the interior. Western red cedar can tolerate stagnate winter water tables throughout the Pacific Northwest. Western hemlock’s native range is from central California to the Kenai Peninsula in Alaska. The annual precipitation western hemlock receives is from 15in to 262in. This amount depends on where it grows along the coast or in the interior mountains. Western hemlock mainly grows on northerly aspects, moist stream bottoms or seepage sites.
Many of the riparian forests in the Pacific Northwest have been dramatically changed from their original makeup. Some of these forests have been grazed by cattle or other livestock. The heavy hooves of these animals compact the soil. This compaction doesn’t allow the water to be absorbed into the ground, so the water runs off into the stream carrying top soil along the way. The simplification of the stream itself has also had negative affects. The large woody debris in the streams has been removed to allow for easy access to the stream and for better travel in the streams themselves. But the faster moving current erodes the stream banks filling the stream with more sediment. The removal of trees on the stream banks also leads to erosion and stream degradation. Stream temperatures rise with the removal of trees because the lack of shade. The primary interest in aquatic and riparian forest ecosystems under the Northwest Forest Plan (NWFP) is the need to restore habitat for fish populations, particularly anadromous salmonids. The changes in the riparian forests have made most native salmon species endangered.
The following steps used to help restore and maintain healthy riparian forests came from the Bureau of Land Management’s best management practices (BMPs) in the Roseburg District. The first step towards riparian forest restoration should be the establishment of riparian reserves. The second step is to restore channel complexity. The third step is to apply silvicultural treatments to restore large conifers. These three steps should help direct the ecosystem back to its pre-disturbed state. The riparian reserve is the designated width from the stream where restrictions on what can be done are placed in order to protect the functions of the land and water in that reserved area. There are three different riparian reserve widths: 1. Fish bearing stream widths are 300 feet on each side of the stream. 2. Permanently flowing non-fish bearing stream widths are 150 feet. 3. Seasonally following or intermittent stream widths are 100 feet. Some activities that are restricted or limited in the riparian reserve include: 1. Cattle grazing. 2. Mineral lease operations. 3. Chemical loading operations or similar toxic activities. 4. Disturbance of unstable banks and headwalls. 5. Operation of tracked equipment on slopes greater than 30% 6. Chemical applications 7. Timber harvest or fuel wood cutting( except for salvage operations & management of stands) 8. Road construction. The second step in riparian forest management is to restore channel complexity. The placement of large woody debris (LWD) in streams creates pools and side channels. The pools provide habitat for aquatic organisms while the side channels help alleviate flooding. The LWD also controls the routing of sedimentation. The source of the LWD should be outside of the riparian reserve when ever possible so as not to promote erosion in the riparian reserve. The exception though, is if usable trees are generated during management, then they can be used to add LWD. Any trees that naturally fall in the stream are an advantage and should be left. The third step is to apply silvicultural techniques to restore large conifers (western red cedar & western hemlock). There are three Silvicultural techniques that will help restore western red cedar and western hemlock riparian forests: 1. Site preparation 2. Seeding 3. Single tree selection The numbering of the techniques doesn’t denote the order in which these operations should begin or importance, since silviculture is a cyclical process.
The first silviculture technique to be discussed is site preparation. The role of site preparation is to modify current growing vegetative conditions making the site suitable for the desired seedlings. In this case western red cedar and western hemlock are the desired seedlings. The uses of site preparation in this case are: control competing ground vegetation, erosion control, nutrient balancing, promote decomposition of surface litter layer, and expose mineral soil. Mechanical site preparation will be difficult Because of the heavy equipment’s size, and inability to maneuver in the small spaces left by single tree selection. Prescribed burning is another method of site preparation, but will not work because the shallow roots of western hemlock would get damaged hurting the seed sources. Prescribed burning would also damage the thin bark of both western hemlock and red cedar girdling the trees. Chemical applications are restricted in the riparian reserves because of the danger of runoff or leaching of chemicals into the stream. So the methods that will be used for site preparation are: passive site preparation and manual site preparation. The passive site preparation will entail keeping the debris created by naturally falling trees where they land. Keeping the slash and smaller trees that are generated by tree selections on the ground is another passive site preparation that will work well within the riparian forest. This will supply a good rotting seed bed for both western red cedar and western hemlock. Both species also can use disturbed mineral seed beds for regeneration from seed. To obtain disturbed mineral soil in the small areas that single tree selection creates manually turning up the soil with hand tools or small tillers is the manual site preparation option.
Seeding is done following site preparation. Seeding is one way to ensure the survival of the desired species on a site. With seeding, foresters have control of genetic make up of the species and the source of the seed. Natural regeneration may be obtained because of the high numbers of annual seed crops (100,000-1 million/acre). Where annual seed production is low western red cedar can be direct seeded in the fall if the soil moisture is adequate. High numbers of seeds will be needed to reach desired stocking level. Containerized stocking also works well. In the costal ranges, 2-year-old bare-root stock seems to be most efficient. Containerized stock plantings in the spring perform better than bare-root stock in the interior. Western hemlock has a good rate of survival on a wide range of conditions. This will allow for natural regeneration on sites that have good organic or mineral soil. If the site is not suitable for natural regeneration then the use of container-grown stock should be used. Hemlock doesn’t survive well with the bare-root stock method. Both western red cedar and western hemlock are able to reproduce by some form of vegetative reproduction. Western red cedar reproduces in three ways of vegetative form; layering, rooting of fallen braches, and branch development on fallen trees. In some areas of the Cascades this form of regeneration is the most successful. Another option for the establishment of red cedar is the use of stem cuttings. Western hemlock also has vegetative reproduction capabilities. Hemlock can be propagated by layering and from cuttings.
The final silviculture technique to be mentioned is the single tree selection. This harvest method was chosen because it works best within the riparian ecosystem. Single tree selection is a good method to keep the western hemlock and western red cedar on the site. If the stand was left alone and the forest naturally created gaps for succession, then other species that are less tolerant than the desired could overtake the created gaps. Single tree selection will contribute minimally to erosion, still provide habitat for wildlife, be aesthetically pleasing to the eye, and follow the BMPs that are associated with riparian forests. Single tree selection gives the forest a great vertical distribution of foliage. The last reason for doing a single tree selection is that it spreads out income over longer period of time. This could help pay for any costs associated with the stand’s management. Single tree selection replicates the natural process called gap-phase. This is where a tree falls down and creates enough sunlight in the opening it made that saplings can grow and eventually penetrate the canopy. The natural gap-phase process may only open the total stand by 1 percent. Single tee selection is different from the natural process though, because the openings are created more often. Since both species are tolerant an opening of the stand by 10 percent each cutting cycle would be enough for stand to do well. An un-even aged forest is the result of periodically opening the canopy.
Since both western hemlock and western red cedar are tolerant species a recommended basal area of 180 sq. ft. would be the maximum basal area the stand could support. The q-factor for these species is 1.2 because of their tolerance. So the number of trees in the 24 inch size class would be 7 trees. A cutting cycle of 20 years is recommended to ensure the stand is following the ideal stocking curve for western red cedar and western hemlock.
Western red cedar and western hemlock are both very shade tolerant species. Western red cedar can tolerate mixed species conditions in the under story and is often over topped by species such as Douglas-fir, western white pine and western hemlock (Minore, 1990). Western hemlock responds well to release after long periods of suppression. After 50 to 60 years advanced regeneration will grow vigorously after over story removal (Packee, 1990). The single tree selection works well with these growth characteristics of both species.
Before the start of a particular management regime of a forest or ecosystem the forester must take an inventory or an ecosystem needs assessment. This will give the forester the proper information about the current conditions of the site. This way the forester will know the potential of the site and can plan silviculture techniques that will be appropriate. After the completion of each phase of the plan surveys should be done to determine the successfulness of the regeneration, harvest, cuttings and site preparation. If any of this result in failure of beginning goals then the plan should be rewritten to effect changes that will bring about desired result. The main guiding principal through this process is the question of weather the land owners objectives have been met.
Works Cited
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Dwire, Kate. “Riparian Resources.” USDA Forest Service, Rocky Mountain Research Station. Forest Service Hdbk 2509.25 page 4 of 23; section12, page 13-19 section; 13 Centennial, Wyoming. September 23, 2004.
Gray, A. N. 2000. Adaptive ecosystem management in the Pacific Northwest: a case study from coastal Oregon. Conservation Ecology 4(2): 6. [Online] URL: http://www.consecol.org/vol4/iss2/art6/htm
Minore, Don. “Silvics of North America vol1. Conifers” United States Department of Agriculture Forest Service Agriculture Hdbk.654 western red cedar. 11 February 2005 [Online] http://www.na.fs.fed.us/spfo/pubs/silvics_manual/volume_1/thuja/plicata.htm
Nyland, Ralph D. Silviculture. Boston. McGraw-Hill. Ch5 pp: 88-106; ch8 pp: 177-180; ch11 pp237-243; ch22 pp: 507-511, 518. 2002.
Packee, E.C. “Silvics of North America vol1. Conifers” United States Department of Agriculture Forest Service Agriculture Hdbk 654 western hemlock. 11 February 2005 [Online] http://www.na.fs.fed.us/spfo/pubs/silvics_manual/Volume_1/tsuga/heterophylla.htm
Southerland, Doug. “Washington Forest Health issue in 2002” Washington State Department of Natural Resources. February 2005. [Online] URL: http://www.dnr.wa.gov/htdocs/rp/forhealth/issues/2002issues.htm
Steiner, Linda. “Trout & Salmon” Pennsylvania Fishes Chapter 16. April 2005 [Online] URL: http://sites.state.pa.us/PA_Exec/Fish_Boat/pafish/fishhtms/chapter16.htm
Zielinski, Elaine. “Record of Decision of the Roseburg District Resource Management Plan” Bureau of Land Management. February 2005. [Online] URL: http://www.or.blm.gov/roseburg/rod_rmp/rod.htm