Abstract: Large organic debris (LOD) was inventoried in two coastal streams to assess the impacts of forest harvesting on LOD recruitment in 90-year-old, second-growth redwood and fir stands on the Jackson Demonstration State Forest in northern California. One stream, North Fork of Caspar Creek, drained a 508-ha watershed that had been 60% clear-cut, with riparian buffer strips left, four years earlier. The second stream, South Fork of Caspar Creek, drains a 424-ha catchment that 60% of the timber volume had been selectively harvested and the stream cleared of LOD twenty-five years earlier. Results from these two study reaches were compared to a LOD study in the North Fork prior to logging. LOD levels increased following harvest because residual trees were left adjacent to the stream or in streamside buffer strips. Windthrow of fir provided the largest input of LOD in these second-growth redwood and fir stands due to the stand age and structure of the residual trees adjacent to the stream. Residual old-growth LOD pieces still play a major role in streams running through a mixed second-growth redwood and fir stand, this important element of stream LOD will continue to decline and must be compensated for in the future. Stream clearing can significantly reduce LOD levels for more than twenty-five years.
Large Organic Debris (LOD, woody debris greater than 10 cm in diameter) in streams is widely recognized as an important part of the aquatic ecosystem (Swanson and Lienkaemper 1978). LOD has been recognized as a vital component of high quality habitat for anadromous fish (Bisson et al. 1987). LOD provides an organic energy source for aquatic organisms, controls the routing of sediment through stream systems, and provides structure to the streambed and banks (Swanson and Lienkaemper 1978, Bilby and Likens 1979). Previous management actions in streams focused on the removal of LOD to allow fish passage and remove potential damage to bridge and stream structures. These actions along with forest management activities have raised recent concern over depletion of LOD in coastal streams and prompted study to determine activities affecting LOD in streams.
The study areas are in the North and South Forks of Caspar Creek experimental watershed on the Jackson Demonstration State Forest, California. Both drainages support stands of mixed second-growth redwood (Sequoia sempervirens) and Douglas-fir (Pseudotsuga menziesii). Historically, both drainages were clear-cut and burned in the late 1800s (Rice et al. 1979). Splash dams to transport logs were used in both reaches of the North and South Forks during this historic logging. This flushing of the stream channel from the splash dams leads to the assumption that present day LOD has accumulated during return of the current second-growth forest (O'Connor and Ziemer 1989) and from residual stumps from historic logging. The North and South Forks of Caspar Creek have since been re-harvested. The South Fork had 60% of timber volume selectively harvested, with the stream cleared between 1967-1973. The North Fork was 60% clear-cut, with riparian buffer strips left, between 1989-1991.
Results from two studies are presented and compared: 1) one study was of LOD in pre-logged conditions of the North Fork of Caspar Creek (O'Connor and Ziemer 1989), 2) the second and most recent study was of LOD after logging in the North and South Forks of Caspar Creek. Both studies used the same methodology.
The LOD was sampled from a 1800-meter stream length along both the North and South Forks of Caspar Creek. LOD was evaluated for two stream zones: 1) effective zone, within the annual high water flow of the stream, and 2) potential zone, spanning above the stream or within 1 meter of the annual high water flow. LOD sources were windthrow, bank erosion, landslide, wind fragmentation, logging debris, and unknown. Tree species were grouped into three categories: 1) redwood, 2) fir, consisting of Douglas-fir and some grand fir and hemlock, and 3) other, all other minor species, predominately hardwoods or unidentifiable LOD. To determine if significant differences occurred between measured parameters, Fisher's least significant difference analysis of variance test was used at 95% confidence level.
LOD characteristics differed between the North and South Forks of Caspar Creek.
North Fork of Caspar Creek
Fork of Caspar Creek
Fir contributed the greatest LOD component for the North Fork of Caspar Creek (Figure 1). Redwood LOD was higher in the South Fork. However, this was due to the high amount of residual old-growth redwood stumps that stream-clearing activities related to the harvesting in 1967 did not remove. If these large residual pieces are removed from analysis, the recent accumulation of LOD in the South Fork is predominately fir. The higher amount of fir stream channel LOD is attributed to the age and relative vigor of the stand and individual tree species. The health of the 100-year-old fir is beginning to deteriorate. This deterioration of the fir element is occurring before any decline in health of the long-lived redwood trees.
Windthrow and bank erosion are the largest natural contributors of LOD in the North and South Forks of Caspar Creek (Figure 2). Logging debris is a high contributor. However, much of the logging debris is large residual stumps and logs from historic logging, not from recent harvest activities. Landslides and wind fragmented trees or snags offered little significant input of LOD.
To understand the potential for future LOD, the residual old-growth LOD created from historic logging must be removed from the analysis. After exclusion of residual old-growth LOD, fir contributed the most LOD for both study streams. Windthrow is the largest LOD source in both study streams (Table 1). Logging debris was still the second highest in the South Fork of Caspar Creek, while it was third highest in the North Fork. Bank erosion was probably not as high a contributor in the South Fork of Caspar Creek because of the stream clearing during harvest operations.
The current LOD volume for the effective zone of the South Fork of Caspar Creek is significantly lower than the post-harvest North Fork's effective zone at 95% confidence. Because much of the stream channel LOD was cleared twenty-five years before, lower LOD in the South Fork does suggest that stream clearing significantly affects LOD recruitment even twenty-five years after its application.
Pool associated and debris jam associated LOD was found to be significantly higher at 95% confidence in the post-harvest North and South Fork of Caspar Creeks than the pre-harvest North Fork. This increase may be caused by numerous factors. Increases in channel LOD that followed harvesting increases the chance of LOD providing conditions suitable for pool formation. Increased debris jams provide greater stability to otherwise mobile LOD pieces. This stability along with their aggregated size promote more channel scouring and damming.
LOD in coastal streams is affected by management activities. From the results and discussion of the LOD data for Caspar Creek, some generalizations were found which can be utilized for improvement of LOD levels in coastal streams subject to forest management activities:
Bilby, R.E.;G.E. Likens. 1979. Importance of organic debris dams in the structure and function of stream ecosystems. Ecology, 61(5). pp. 1107-1113.
Bisson, P.E.; R.E. Bilby; M.D. Bryant; and others. 1987. Large woody debris in forested streams in the Pacific Northwest: past, present and future. In: Salo, E.O.; Cundy, T.W. eds. streamside management: forestry and fishery interactions. Contribution No. 57. Seattle: College of Forest Resources, University of Washington. pp. 143-190.
O'Connor, M.D.; R.R. Ziemer. 1989. Coarse woody debris ecology in a second-growth Sequoia sempervirens forest. Gen. Tech. Report PSW-110. Arcata, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station. 7 p.
Rice, R.M.; F.B. Tilley; P.A. Datzman. 1979. A watershed's response to logging and roads: South Fork Caspar Creek, California, 1967-1976. Res. Paper PSW-146. Berkeley, CA: S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station. 12 p.
Swanson, F.J.; G.W. Lienkaemper. 1978. Physical consequences of large organic debris in Pacific Northwest streams. Gen. Tech. Report PNW-69. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station. 12 p.
We wish to thank and acknowledge the staff of the Jackson Demonstration State Forest for their contributions.