Watershed 5

WS5map.pngArea: 21.9 ha
Slope: 15.4°
Aspect: S24°E
Elevation: 488-762 m
Gage type: V-notch weir, San Dimas flume
Initial year: 1962

Description: A commercial whole-tree harvest was carried out on W5, a management practice that is regaining interest due to rising demand for biofuels. Compared to conventional stem-only harvests, whole-tree harvesting involves the removal of most of the above-ground biomass (boles and branches), raising concerns about nutrient removals and impacts on site productivity. In this application, all trees greater than 5 cm d.b.h. were cut with chain-saws and a track-mounted feller buncher equipped with hydraulic shears. Trees were harvested and removed from the watershed between October 1983 and January 1984, at which time there was a machine accident, leaving the upper one-third of the watershed uncut. Trees from most of the upper portion of the watershed were removed during the following summer. Felled trees in one small area (~12% of the watershed) were not removed until 1985, and about 3% of the watershed was inaccessible and remained uncut. In this treatment, no buffer of trees was left along the stream channel.

Treatment:

  • A whole-tree harvest was conducted during the dormant season of 1983-1984 resulting in the removal of 180 t/ha of biomass.
  • Prior to treatment, the watershed was surveyed into 360 25 x 25 meter plots to be used for research.

Objective: To assess the ecosystem response to whole-tree harvest.

Resulting Observations:

  • An increase in temperature (as much as 6°C) at the soil surface and in streams, unless streamside buffers of trees are left.
  • An increase in moisture content of the soil.
  • A maximum increase in streamflow of approximately 40 percent, and an increase in summer peak flows averaging 20 percent.
  • An increase in nitrification.
  • An increase of nutrients, especially nitrate, in soil solution subject to leaching loss or uptake by plants and microorganisms.
  • No appreciable increase in erosion and sedimentation.
  • Rapid decomposition and fragmentation of slash (75 to 80 percent breakdown in the first 14 years).
  • Rapid growth of pin cherry and raspberry from seed which had remained viable in the soil for decades; these pioneer plants conserve nutrients that otherwise might be leached from the site.
  • Baseline and Initial Results

Continued research/treatment: Monitor changes in water yield and stream chemistry.

Datasets:

  Daily streamflow
  Streamwater chemistry
  Sediment yield in weir basin
  Phenology measurements
  W5 continuous revegetation survey
  Fine litterfall
  Coarse litterfall
  Throughfall
  W5 quantitative pit soil C and N