Timber harvest with "green-tree" retention has been adopted in many temperate and boreal forest ecosystems, reflecting growing appreciation for the ecological values of managed forests. On federal forest lands in the Pacific Northwest, standards and guidelines for green-tree retention have been adopted, but systematic assessments of ecosystem response have not been undertaken. We studied initial (1-2 yr) responses of vascular understory communities to green-tree retention at six locations (blocks) in western Oregon and Washington, using a factorial design with retention at contrasting levels (15% vs. 40% of initial basal area) and spatial patterns (trees dispersed vs. aggregated in 1-ha patches). Direction of compositional change (expressed in ordination space) was similar among treatments within each block, but the magnitude of change was consistently larger at 15% than at 40% retention; pattern of retention had little effect on compositional change. Despite major changes in vegetation structure, early-seral (ruderal) herbs contributed little to plant abundance and richness in most treatments. For many forest understory groups, declines in abundance or richness were significantly greater at 15% than at 40% retention. However, pattern of retention had surprisingly little effect on treatment-level response; although changes within forest aggregates were small, declines in adjacent areas of harvest were generally greater than those in corresponding dispersed treatments. Late seral herbs were particularly sensitive to these effects, with more frequent extirpations from plots within the harvested portions of aggregated treatments than from dispersed treatments. Plot-toplot variation in understory abundance and richness within treatments increased after harvest, but level and pattern of retention had little effect on the magnitude of this change. We suspect that the initial responses of forest understories to green-tree retention are mediated, in large part, by associated patterns of disturbance and slash accumulation that differ significantly with level and pattern of retention. Because these represent short-term responses, future sampling will be necessary to understand the broader implications of structural retention harvests. We predict that, as effects of disturbance diminish with time, effects of canopy structure will increasingly shape patterns of compositional and structural development in the understory.
Halpern, Charles B.; McKenzie, Donald; Evans, Shelley A.; Maguire, Douglas A. 2005. Initial responses of forest understories to varying levels and patterns of green-tree retention. Ecological Applications. 15(1): 175-195