The objective of this study is to characterize the status of stream habitat and populations of resident cutthroat trout within Coast Range basins logged 20-60 years ago. The basins chosen for this study were logged before the adoption of Oregon's Forest Practice Act and the first set of forest practice rules in 1972. We recognized that most forests mature enough for a second harvest within the next 30 years were logged before the implementation of the first set of forest practice rules. Current rules dictate that streamside prescriptions will be more rigorous for streams in which a game fish such as cutthroat trout is present. Because the only fish occupying many of the headwater streams in Oregon's Coast Range are cutthroat trout, their presence highly influences current management decisions on degree of stream protection in many of the small basins within the region. If a stream's future potential for supporting a cutthroat trout population is to be considered in management decisions, an assessment of the response of cutthroat trout to the first logging event becomes important. Conclusions from this study could be used to develop prescriptions for stream protection that recognize the short-term and long-term needs for maintaining resident cutthroat populations in small basins of the Coast Range.
All 16 basins sampled are westside drainages of the mid-coastal region of Oregon, with locations ranging from tributaries of the Siletz drainage to the north to Big Creek Basin (Lincoln County) to the south. Field work began in 1991 and was completed in 1993. Size of the watersheds ranged from 0.5 to 3.5 km2. Seven of the basins were logged 20-30 years ago, five were logged 40-60 years ago, and four have not been logged. The unlogged basins had post-fire stands that were 125-150 years old.
The density of cutthroat trout 1 year old or older, the canopy of trees over the stream, and the quantity of large woody debris in the stream channel varied with the history of forest harvest activity in the basins. Unlogged basins had low densities of cutthroat trout and had the lowest variability in densities within a stand age class (Figure 3). These basins had streams with a small percentage of canopy cover provided by hardwoods (Figure 4) and low amounts of large woody debris (Figure 5). Basins logged 20-30 years ago had the widest range of densities of cutthroat trout, including the lowest and highest densities among all basins sampled. These younger second-growth basins had high percentages of canopy cover provided by hardwoods and highly variable amounts of large woody debris. Basins logged 40-60 years ago had low to moderate densities of cutthroat trout and had an intermediate level of variability of densities among streams. These older second-growth basins had a moderate to high percentage of canopy cover provided by hardwoods and low amounts of large woody debris. Across all stand age classes (Table 1), density of cutthroat trout 1 year old or older was generally higher (>1.4 g/m2) in streams that had canopies with low coverage by conifers (<35 percent) and channels with high amounts of large woody debris (>15 pieces/100 m).
These preliminary results suggest that simply providing a stream with riparian buffers may not be adequate for maintaining populations of cutthroat trout in extensively harvested second-growth basins. Many of the second-growth basins within the mid-Coast Range that were harvested 20-60 years ago without provisions for riparian buffers now have riparian zones dominated by hardwoods, especially red alder. Mechanisms that may explain the differences in density of cutthroat trout across stand age classes include the differences in light and nutrient inputs afforded by deciduous versus conifer trees in the stream canopy and the degree of scour and cover afforded by large woody debris. Thus a future management plan that calls for maintaining high densities of young conifers in the riparian area but provides for harvesting of the mature conifers could limit the productivity of cutthroat trout because of the resulting high levels of shade and low nutrient input. On the other hand, a riparian management plan that does not provide potential for some regeneration of conifers in streamside areas may limit the accumulation of large woody debris, an important element of habitat for cutthroat trout.
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