Patterns of Riparian Forest Vegetation

David E. Hibbs, Forest Science, Oregon State University, Corvallis, OR

This talk is about setting better goals for riparian management. The silviculture to practice in riparian areas is rarely unique. But the general understanding of what vegetation goals are possible or desirable is quite limited. The basic message is that riparian areas have diverse vegetation conditions at both the small and the large scale, and that they are always changing. You have a lot of choices.

Coast Range research has shown that buffer strips are susceptible to windthrow in a defined set of locations but that they are otherwise quite biologically stable: they are going through the successional processes that one would expect in an intact forest. In the long-run, tree regeneration is often limited and so many of these riparian forest can become unforested. Tree stocking is generally low compared to upland forest except in the fog belt in both managed and unmanaged forest. There is less tree regeneration and bigleaf maple in managed than unmanaged forest. Conifer stocking is best on slopes over 15%.

At the reach scale, vegetation tends to occur in even-aged monoculture patches in the north and grades to a more tree-by-tree mix of ages and species in the southwest. Across western Oregon, species diversity increases from north (wet) to south (dry).

Historic fire patterns differed across western Oregon and contributed to the compositional and structural differences we see today. Fire control appears to be changing the developmental trajectory of these forests, most notably in the mid-elevation zone of southwest Oregon. There, the riparian areas used to be an open woodland of oaks and conifers. Today, they are a closed forest dominated by Douglas-fir. Soon, the oaks will be largely gone.

This quick review illustrates the diversity of composition and structures found and might be managed for. This review also illustrates that these areas are constantly changing and that to maintain the current or historic diversity of conditions will require active management in some places.