Density Management and Riparian Buffer Study
Investigation of aquatic-dependent vertebrates within
and along headwater streams in managed forests of
The Density Management and Riparian Buffer Study of Western Oregon is the only experimental study of the combined effects of alternative forest thinning designs and riparian buffer widths in the U.S. northwest. Several coordinated projects are investigating the utility of thinning young managed forest stands with stream buffers for: 1) acceleration of late-successional forest structure; 2) sustained wood production; 3) carbon sequestration; and 4) retention of aquatic conservation strategy objectives, including stream habitats for sensitive aquatic species. In particular, my pretreatment analyses have advanced the characterization of headwater stream systems and aquatic vertebrate fauna, I am monitoring these stream reaches in years 1, 2, 5, and 10 post-treatment to examine the effects of thinning and stream buffers on a variety of aquatic habitat attributes and species, and I am currently investigating the effects of a second-entry thinning with our original riparian buffers. New directions are examining the interaction of climate change and the thinning-and-buffer treatments on headwater forest resources.
Headwater Stream Habitats
Spatially-intermittent streams are the most frequent stream reach type in our sample. These small streams are only a meter or two in width and have discontinuous surface flow during both the spring rainy season and the summer dry season. Intermittent streams and their streamside riparian zones are not well understood, and our studies are advancing knowledge of their ecology and management in managed forests. In addition, we are finding an abundance of down wood within our stream channels, and we are tracking down-wood patterns 10-years post-thinning and after a 2nd entry of thinning. Our case study analyses have documented more down wood near headwater streams than upslope.
Figure 1. Headwater stream in western Oregon. Photo by L. Ellenburg, USFS.
Amphibians: Headwater ‘Funnels’
We have found 13 species of amphibians and 3 fish taxa occurring within and along headwater streams, although some species are only rarely detected. These species are ordered into different assemblages that are spatially segregated by habitat type: perennial streams are dominated by fishes and coastal giant salamanders; intermittent streams are dominated by torrent salamanders, which are Oregon sensitive species; Dunn’s and western red-backed salamanders dominate stream banks; and a terrestrial salamander assemblage occurs upslope, associated with habitat features such as down wood and coarse substrates. Amphibians are most abundant within 2 m of streams, suggesting that headwater riparian zones are spatially compressed. A case study revealed relatively restricted movements of salamanders in uplands, and more frequent movements of animals along streamside zones. Headwater riparian zones appear to ‘funnel’ animals along narrow near-stream corridors.
Figure 2. Headwater drainage with pie charts indicating species assemblages. Graphic by K. Ronnenberg.
Figure 3. Southern torrent salamander, Rhyacotriton variegatus. Photo by W. P. Leonard.
Thinning: A Benign Disturbance
Thinning from 200-350 trees per acre (tpa; ~500-850 trees per hectare [tph]) to 80 tpa (~200 tph) appears to be a relatively benign disturbance relative to headwater aquatic-vertebrate assemblages and stream habitats. Amphibians have persisted at sites after thinning-with-buffers, and to date, reduced abundances have been observed in only a few instances for some upland salamanders. Site-specific effects of thinning in uplands appear to be affected by current conditions, with adverse effects of thinning apparently being mitigated by the abundance of large down wood within stands, a ‘legacy’ from the initial clearcut logging that occurred over 60 years ago at some of our study sites, and the occurrence of coarse substrates.
Figure 4. Upland thinning treatment, Green Peak study site. Photo by P. Anderson, USFS.
Headwater Connectivity: Up and Over
Our headwater studies and the thinning and buffer designs that we are examining have application to management of headwater connectivity areas. Amphibians occurring within and along headwater streams have terrestrial dispersal life stages, with over-ridge dispersal needed to maintain gene flow among sub-populations in adjacent drainages. Headwater linkage area designs that extend riparian buffers and connect them up and over ridgelines to neighboring drainages may reduce fragmentation of these habitats and populations in managed forests. ‘Chains’ of connectivity can be envisioned with riparian and overland links. Green tree retention in these linkage areas and down wood placement with log orientation from ridgelines toward headwater streams may aid overland dispersal of low-mobility species, including amphibians and a variety of other ground-dwelling taxonomic groups (mollusks, lichens, bryophytes, fungi, small mammals). Although retained stands may anchor habitats in headwater linkage areas, our thinning designs with leave islands and down wood management might be considered as an effective management alternative for overland chains.
Figure 5. Linkage area designs between headwater streams, using down wood and green tree retention.
Graphic by K. Ronnenberg.
a) Down Wood Spatial and Temporal Patterns
b) Headwater stream flow and climate variation: Drought years and “Shrinking Heads?”
c) Utility of Cover Boards to Assess Plethodontid Ecology in Headwater Drainages
d) 5-year and 10-year post-treatment effects of buffers and thinning
e) Interactions of Thinning and Riparian Buffers with Water Availability and Vegetation Growth Dynamics in Riparian and Upland Areas of Headwaters