Graduate Thesis Or Dissertation
 

Amphibian communities and physical characteristics of intermittent streams in old-growth and young forest stands in western Oregon

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  • Intermittent, headwater streams recently have been recognized as important components of forest ecosystems and have been provided increased protection by the Northwest Forest Plan. However, few studies have examined their distribution, dynamics, and ecological roles, such as habitat for wildlife. My goal was to provide additional information on the ecology of intermittent streams in the Pacific Northwest. I examined and compared hydrologic, water quality, and physical characteristics of 16 intermittent streams in old-growth and young forest stands in the central Cascade Range in western Oregon. I documented amphibian communities and habitat associations in these streams during spring and summer. I used comparisons of current habitat conditions and amphibian communities between stand types to gain insight into potential impacts of timber harvesting on these stream systems. Of the streams surveyed in old-growth and young forest stands, relatively few (23%) were designated as intermittent based on my definition which included presence of a definable channel, evidence of annual scour and deposition, and lack of surface flow along at least 90% of the stream length. Intermittent streams in old-growth stands exhibited the following characteristics: (1) annual flow pattern in which streams started to dry in May and June and were mostly dry by July; (2) lengthy annual flow durations (range 6-11 months); (3) cool and stable daily stream temperatures; (4) primarily coarse substrates, such as cobbles and pebbles; (5) streamside vegetation comprised of predominantly coniferous overstories, and plant species associated with uplands or dry site conditions, such as Oregon-grape and salal, as well as riparian areas or wet site site conditions, such as Oregon-grape and salal, as well as riparian areas or wet site conditions, such as red alder, oxalis, red huckleberry, and vine maple (Steinblums et al. 1984, Bilby 1988); and (6) low to moderate densities of large wood, mostly moderately- and well-decayed. Study streams in young forest appeared to dry about one to two months later than the streams in old growth but had similar annual flow durations. They also were characterized by higher daily stream temperatures, similar diel fluctuations, finer substrates, more deciduous overstory and herbaceous understory cover, and lower densities of moderately-decayed large wood. Differences in habitat conditions between stand types may be attributed to timber harvesting as well as discrepancies in physiographic and geological factors, such as elevationgradient, and soil type. Amphibian communities in spring and summer were comprised primarily of the Cascade torrent salamander (Rhyacotriton cascadae), Dunn's salamander (Plethodon dunni), and Pacific giant salamander (Dicamptodon tenebrosus). Amphibian communities in streams in young forest stands exhibited different species composition and seasonal patterns in total density from those in old growth. Cascade torrent salamanders and Dunn's salamanders maintained similar densities and biomass between spring and summer by potentially adopting drought avoidance strategies. Species differed in their use of habitat types and associations with habitat features. In general, amphibian species were positively correlated with percent surface flow, water depth, intermediate-sized substrates and negatively associated with overstory canopy cover, elevation, and wood cover. Results of my study suggest that intermittent streams may warrant protection for their potential effects on downstream habitat and water quality and for their role as habitat for aquatic species, such as amphibians. Streamside vegetation should be maintained along intermittent channels to provide shade protection for water temperature regulation and sources of large woody debris and other allochthonous energy input, to help stabilize slopes, and to minimize erosion and sedimentation. At a minimum, intermittent stream channels should receive protection from physical disturbance during timber harvesting operations. However, since intermittent stream systems are highly variable, management should address individual site conditions and vary accordingly.
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