Document: ERI-3-76-3

Controls on conifer regeneration in managed riparian forests: Effects of seed source, substrate, and vegetation.

BEACH, E.W.* ¹ and C.BHALPERN ²

Weyerhaeuser Company, Tacoma, WA 98477 USA ¹
University of Washington, Seattle, WA 98195 USA ²

Abstract:
Alnus rubra is a dominant overstory component in many of the managed riparian forests of the Pacific Northwest. It is commonly assumed that competition with shrubs inhibits succession from hardwoods toward shade-tolerant conifers. However, we are not aware of any studies that empirically test this assumption. We hypothesized that seed availability rather than competitive interactions largely shape patterns of conifer regeneration in these forests. At 10 locations in western Washington, paired transects representing riparian sites with abundant vs. few seed sources were used to examine the importance of dispersal limitation, rooting substrate, and vegetation cover on conifer regeneration. Frequencies and densities of shade-tolerant conifers (Tsuga heterophylla, Thuja plicata, and Picea sitchensis) were significantly greater in transects within 100 m of remnant forest patches than in paired sites at a greater distance (frequencies of 59 vs.18% and densities of 0.227 vs. 0.163 individuals m^-2, respectively). Where sources of seed were present, regeneration was positively associated with coarse woody debris and negatively associated with fine litter. Regeneration was most abundant in plots with low (<10%) herb or shrub cover, however for most species, recruitment occurred in plots representing the full range of understory cover (0 to 90 [100%]). No relationship was observed between regeneration density and overstory conifer or hardwood tree cover. Our results suggest that conifer regeneration is controlled primarily by the availability of seed and only secondarily by possible competitive interactions. In intensively managed landscapes, conventional strategies, such as vegetation control, are not likely to be as effective in increasing conifer regeneration as managing for seed sources through green-tree retention.

Keywords: riparian; conifer regeneration; seed dispersal; managed forests

This abstract is being presented at: 11:30 AM in session:
Oral Session #20: Riparian Ecology.