Document: DAN-3-76-19

The role of large woody debris in riparian forest regeneration in western Oregon.

SARR, D.A.*, R.E.MOMSEN and D.E.HIBBS

Oregon State University, Corvallis, OR 97331 USA ¹

Abstract:
Large woody debris is central to the structure and regeneration processes of Pacific Northwest riparian forests. To evaluate macro and microscale relationships between woody debris abundance and riparian tree establishment, we sampled woody debris and tree seedlings in four western Oregon watersheds spanning a strong climatic gradient. In each watershed, we randomly sampled sixteen 1-ha sample reaches, using eighteen (40 m²) microplots on topographic transects. Geomorphology, forest stand characteristics, disturbance, and tall shrub cover were quantified as possible covariables. We recorded 24 regenerating tree species in 4.61 ha (1.152 ha/basin), 23 of which were native, with 12 (55%) growing on downed logs in at least part of their range. Large woody debris was an important regeneration surface for conifers, but was much less important for hardwoods (50.2% vs. 3.7% of all seedlings, respectively). Seedling fidelity to woody substrates differed strongly among conifer species, and ranged from >75% for Picea sitchensis and Tsuga heterophylla, 20-35% for Thuja plicata, Taxus brevifolia, and Pseudotsuga menziesii, to 0% for Pinus spp. Among hardwood species, seedling fidelities were much lower, with 11.0 % for Castanopsis chrysophylla and less than 2.0% for all other species. Among conifers, the proportion of total regeneration on large woody debris tended to decrease from north to south (66.8%, 80.4%, 52.4%, and 1.4% in the Coast Range, central Cascades, southern Cascades, and eastern Siskiyou Mountains, respectively), following closely the seedling patterns of T. heterophylla, the most abundant species. When all species were weighted equally, proportions were 59.8%, 47.7%, 16.3%, and 0.3% for the four basins, with the decreasing pattern occurring for most species. We are presently analyzing of the roles of site water balance and ground-level competition on species-specific regeneration to explain this strong regional shift in conifer regeneration patterns.

Keywords: riparian forest ecology, tree regeneration, woody debris

This abstract is being presented at: 10:30 AM in session:
RIPARIAN ECOLOGY