About the Forest
About the Forest
Andrews Watershed 7 - WS07
Andrews Experimental Forest (HJA)
Watershed above the Watershed 7 gaging station
Bounding Coordinates (decimal degrees):
Slope (%): 33.68
Aspect (degrees): 158
1974: WS7 was shelterwood cut (approximately 60% of basal area was removed, 30 to 40 trees per acre were left as overstory), WS8 was left alone as the control. WS7 was tractor logged above the road and cable logged below the road. Tractor skidding was straight downhill in WS7. 1975: WS7 was broadcast burned just below the road. 1976: District planted all of WS7 in June of 1976. 1984: The rest of the canopy in WS7 was removed. 2001: WS7 was thinned to 14 foot spacing, which leaves about 220 trees per acre. Young Douglas-fir stand was accidently targeted and thinned to 220 trees per acre in 2001. In the summer of 1974, 60% of the total basin area was removed in a shelter- wood cut. Temporary spur roads were constructed into the middle of and down the southwest ridge of the watershed. Logs in the upper 60% of the watershed were yarded by tractor. Skid trails were not preplanned, but tractor operators were instructed for safety reasons to yard downhill. Consequently, soil compaction by tractors was limited to short segments of main skid trails immediately upslope from the upper spur road. Logs in the lower 40% were yarded partially suspended by a skyline cable system. Logging residue was broadcast burned only on the lower half of the watershed in the spring of 1975. WS#7 is adjacent to WS#8 (the control). No significant difference was found among WS 6, 7, 8 basal area pre-treatment. The Hi-15 Meteorological Station (H15MET) is centrally located just below the WS#7 gaging station. GIS estimated percent harvest: 95.7%
Soils derived from deep andesitic landslide deposits occupy about 75% of the total area in Watersheds 6, 7, and 8. Texture is generally gravelly loam to sandy gravelly loam with gravel content ranging from 5 to 20% by volume (Dyrness and Hawk, unpublished). In most locations, effective rooting depth is virtually unlimited because of very deep, unconsolidated parent material. Silt loam soils derived from andesite and associated tuffs and breccias occupy approximately 10% of the watershed area, mostly at higher elevations in Watersheds 6 and 8 (Dyrness and Hawk, unpublished). Because the texture and structure of soils in Watersheds 6, 7, and 8 are nearly identical to those of the lower-elevation watersheds, Harr et al. (1982) suggest that the soil hydraulic properties should be nearly identical as well.
The H. J. Andrews Experimental Forest is underlain exclusively by bedrock of volcanic origin. Three geologic formations have been mapped for the HJA and correspond roughly with elevation (Swanson and James 1975). Little Butte Formation bedrock (< approximately 760 m elevation), dated as Oligocene to lower Miocene, consists of massive tuffs and breccias derived from mudflows and pyroclastic flows. Sardine Formation bedrock (760 m to 1200 m), dated as middle to late Micocene, consists of two units: a lower unit containing welded and non-welded ash flows (notably less altered than underlying Little Butte rocks of similar lithology), and an upper unit containing basalt and andesite lava flows. Andesitic and basaltic lava flows (>1200 m), termed "Pliocascade" Formation, were deposited during the Pliocene and overlie Sardine Formation material. Watersheds 6, 7, and 8 range in elevation from 860 m to 1200 m and are underlain by two units of the Sardine Formation (Swanson and James 1975). Basalt and andesite lava flows underlie ridges of all three watersheds and near the outlets of Watersheds 7 and 8. Welded and non-welded ash flow tuffs underlie middle elevations of all three watersheds and the lower elevations of Watershed 6. This rock is of similar lithology as in the middle elevations of the lower watersheds, but is noticeably less weathered. No flow rock is exposed in Watersheds 6, 7, or 8.
Gauged watershed area: 15.4 ha (original surveyed value used in rating equation calculations)
Prior to cutting, the overstory was dominated by Douglas-fir (Pseudotsuga menziesii)and western hemlock(Tsuga heterophylla) trees ranging in age from 100 to 250 years. Understory species communities included vine maple-Oregon grape (Acer circinatum-Berberis nervosa), vine maple-whipplea (Acer circinatum- Whipplea modesta), and rhododendron-Oregon grape (Rhododendron macrophyllum-Berberis nervosa). Pre-treatment description: stocking density = 333 stems per ha; basal area=75 m2/ha (std dev 4.8 over 3 1-acre plots)
Typically, snow begins falling in November with peak snow water equivalent storage estimated to occur in Feb-April. Mean annual maximum is about 400-650 mm water equiv. at highest elevation.