Density Management and Riparian Buffer Study

The Density Management and Riparian Buffer Study in western Oregon is the only experimental study of the combined effects of alternative forest thinning designs and riparian buffer widths in the U.S. Pacific 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.

Our pretreatment analyses have advanced the characterization of headwater stream systems and aquatic vertebrate fauna; we are monitoring these stream reaches in years 1, 2, 5, and 10 posttreatment to examine the effects of thinning and stream buffers on a variety of aquatic habitat attributes and species, and we are 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.

Key Findings

Headwater Stream Habitats

A headwater stream in western Oregon. USDA Forest Service photo by L. Ellenburg.

Spatially intermittent streams are the most frequent stream reach type in our sample. These small streams are 1–2 m 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.

We also are finding abundant down wood within our stream channels. We are tracking down-wood patterns 10-years postthinning and after a second entry of thinning. Our case study analyses have documented more down wood near headwater streams than upslope.

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 (figs. 1 and 2). These species are ordered into different assemblages that are spatially segregated by habitat type: 

• Fish and coastal giant salamanders dominate perennial streams. 
• Three species of torrent salamanders, which are all sensitive species in Oregon, dominate intermittent streams. 
• Dunn’s and western red-backed salamanders dominate streambanks. 
• 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 1. A headwater drainage with pie charts showing species assemblages. USDA Forest Service graphic by K. Ronnenberg.

Figure 2. A southern torrent salamander, Rhyacotriton variegatus. Photo courtesy of W.P. Leonard.

Figure 3. An upland thinning treatment, Green Peak study site. USDA Forest Service photo by P. Anderson.

Thinning: A Benign Disturbance

Thinning from 200–350 trees per acre (tpa, about 500–850 trees per hectare [tph]) to 80 tpa (about 200 tph) appears to be a relatively benign disturbance for 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.

Current conditions appear to affect site-specific effects of thinning in uplands, 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 (fig. 3).

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 subpopulations 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 (fig. 4).

Figure 4. Linkage area designs between headwater streams, using down-wood and green-tree retention. USDA Forest Service graphic by K. Ronnenberg.

Keywords:

Adaptive management, amphibians, Aquatic Conservation Strategy objectives, aquatic habitat, aquatic-riparian ecosystems, artificial cover objects, biodiversity, carbon mitigation, carbon sequestration, coarse woody debris, connectivity, Cottidae, cover use, debris flow, density management, density management harvest, Dicamptodon tenebrosus, dispersal, dispersal corridors, down wood, downed wood thermal profiles, drought, emergent properties, Ensatina, forest conservation, forest ecosystem management, forest management, forest thinning, ground-layer plant communities, headwater stream, headwaters, heat:moisture index, herbaceous layer linkage areas, land cover, managed headwater forests, maximum-likelihood population-effects models, MLPE, microclimate, movements, Northwest Forest Plan, NWFP, old growth, Oncorhynchus clarkii, Oregon Cascade Range, Oregon Coast Range, Plethodon, Plethodontid salamanders, refugia, Rhyacotriton, Rhyacotriton cascadae, Rhyacotriton kezeri, Rhyacotriton variegatus, riparian areas, riparian buffer zones, riparian buffers, riparian management, riparian reserves, salamanders, salmon habitat, small streams, species diversity, stream banks, stream networks, stream temperature, streamside management zones, structural equation modeling, understory vegetation, watersheds, western Oregon.