Site description
This research occurred at the Fraser Experimental Forest (FEF), 137 km west of Denver, Colorado. Mean annual temperature at FEF headquarters (elevation 2725 m) is 0.5 °C, and ranges from -10 °C in January to 12.7 °C in July. Mean annual precipitation is 610 mm (range 427-902 mm; 1981-2010), nearly two-thirds of which falls as snow from October to May. Annual inorganic N deposition averaged 2.6 kg N ha-1 over the last decade (Argerich et al., 2013). The Fraser area was extensively glaciated and FEF is underlain by metamorphosed rock, most commonly biotite schist and hornblende or calc-silicate gneiss (Shroba, Bryant, Kellogg, Theobald, & Brandt, 2010). Soils are skeletal, sandy loam Dystric and Typic Cryochrepts (Alstatt & Miles, 1983) with 20-30% gravel and 30-50% cobble-sized materials. To evaluate the role of forest cover on hillslope hydrologic processes, including quantifying the timing and amount of subsurface flow, paired trenched hillslopes were installed in 1979. The paired hillslopes are adjacent to each other on a glacial moraine that is underlain at 3 m by a clay aquaclude (Reuss, Stottlemyer, & Troendle, 1997; Troendle & Reuss, 1997). Subsurface flow is collected by slotted PVC pipe installed at 4 m depth in lined trenches at the base of the hillslopes and piped to individual gauging stations (HS flumes) where water level is recorded using Druck pressure transducers and Campbell CR-1000 data loggers at 10-minute intervals. These water level data are then converted to subsurface flow rates using known relations between water depth and discharge (Bos, 1989). In December 1984, after five years of pretreatment calibration of subsurface flow rates, a 3-ha section of one of the trenched hillslopes was clear cut harvested, while the other was left intact.
Old-growth subalpine forests at FEF are comprised of equal proportions of lodgepole pine (Pinus contorta ), subalpine fir (Abies lasiocarpa ) and Englemann spruce (Picea engelmannii ), respectively 32%, 34%, and 34% of overstory basal area (Supp. Info: T1). Low-stature shrubs (Vaccinium scoparium, V. myrtillus, V. caespitosum, Shepherdia canadensis ) form a dense understory beneath the coniferous overstory (Popovich, 1993). The clear-cut hillslope has regenerated into a pine-dominated second-growth stand (82% of overstory basal area) with 9 cm mean diameter and 4 m mean height (Supp. Info: T1). Mountain pine bark beetles (Dendroctonus ponderosae ) preferentially attack larger trees (Rhoades et al., 2017), and in a recent outbreak beetles killed 18% of the old-growth basal area but did not infest the second-growth stand.