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

Red alder (Alnus rubra Bong.) commonly dominates the overstory in managed riparian forests of the Pacific Northwest. Although it is assumed that competition with a dense shrub layer inhibits succession from hardwoods to shade-tolerant conifers within these forests, this assumption has not been explicitly tested. We hypothesized that seed availability rather than competitive interactions largely shape patterns of conifer regeneration in managed riparian forests. At nine locations in western Washington, we established paired transects in riparian sites adjacent to abundant versus few or no sources of seed to examine the importance for conifer regeneration of dispersal limitation, rooting substrate, and vegetation cover. Frequency and density of regeneration were significantly greater in transects within 100 m of remnant forest patches than in paired sites at greater distances (mean frequencies of 59 vs. 18% and densities of 0.23 vs. 0.16 trees/m(2), respectively). Where seed sources were present, regeneration was positively associated with coarse woody debris and negatively associated with fine litter. Regeneration was most abundant in plots with < 10% herb or shrub cover; however, for most species, recruitment occurred across the full range of understory plant cover. We detected no relationship between regeneration density and overstory conifer or hardwood cover. Our results suggest that, in managed forests, conifer regeneration is largely limited by seed availability and only secondarily by competitive interactions or substrate conditions. In managed landscapes, conventional strategies of vegetation control are not likely to be as effective in increasing conifer regeneration in riparian forests as managing for seed sources through green-tree retention.