Streamside buffer strips : survival, effectiveness, and design

Graduation date: 1978Stream buffer strips are an important tool for protecting the stream environment. This research documents the losses from 40 stream buffer strips, in the Western Cascades of Oregon, established 1 to 15 years before the study. Predictive equations are developed\ud which identify the major reasons for buffer strip losses. Losses from wind, sunscald, logging damage, and other factors were estimated. The effectiveness of buffer strips\ud for stream shading was quantified. Wind is the major cause of stream buffer strip\ud mortality. Damage from wind is often sudden, and catastrophic, while damage due to logging or disease and insects occurs at a slower rate. The average percent of standing timber remaining in the, stream buffer strips\ud sampled was 84 percent, ranging from 22 to 100 percent. Additional losses occured over the winter of 1975-1976, amounting to 5 percent of an initial sample of 34 buffer\ud strips. A second set of 6 buffer strips suffered a 52 percent loss. The combined array of buffer strips lost 13 percent additional volume in this relatively mild\ud winter. Topography and uncut timber stand protection are the most important factors modifying the amount of windthrow in a buffer strip. The distance to the cutting line in the direction of damaging winds was the most important single variable influencing buffer strip survival, with\ud increasing distances leading to significantly poorer survival. Two other significant protection factors were the distance and change in elevation from the buffer strip to the nearest major ridge in the direction of damaging winds. Nearby ridges and steeper slopes give better protection. Timber factors also influence stream buffer strip survival. Increasing values for the following timber factors are associated with significantly poorer survival: average stand height, average height of trees taller than 100 feet, number of trees per acre taller than 160 feet,\ud original timber volume per acre, original basal area per acre, and average volume per tree. Western red cedar (Thuja plicata) was the most windfirm tree species, followed by western hemlock (Tsuga heterophylla),\ud Douglas-fir (Pseudotsuga menziesii), and true fir (Abies EE), in decreasing order of windfirmness. Species tolerance to wet sites,. plus the timber factors described above, may help explain the windfirmness\ud canking. Wet sites increase a tree's susceptibility to windthrow. Water table measurements in two buffer strips with windthrow indicated that the water table rose high enough to reach a tree's rooting zone, while the water table in a buffer strip without windthrow did not enter the root zone. Water tables within a tree's rooting zone may result in poorer rooting and tree anchorage The above factors, combined in multiple regression equations developed in this study, account for\ud approximately 68 to 95 percent of the variation in predicting buffer strip survival. Measured buffer strip shading shows that a buffer strip 85 feet wide shades a stream as well as an average\ud undisturbed canopy, while 75 percent of the undisturbed canopy shading can be achieved with a buffer strip 52 feet wide. Width alone is not adequate for buffer strip design as topographic, timber stand, and understory factors greatly influence stream shading. Windthrow in stream buffer strips poses a difficult salvage problem, and may also damage the stream environment. Therefore, on sites very susceptible to\ud windthrow, the best stream protection alternative may be to carefully remove the streamside trees with directional falling methods