INITIAL RESPONSES OF FOREST UNDERSTORIES TO VARYING LEVELS AND PATTERNS OF GREEN-TREE RETENTION

Abstract

Timber harvest with ''green-tree'' retention has been adopted in many tem- perate 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 eco- system 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-to- plot variation in understory abundance and richness within treatments increased after har- vest, 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 me- diated, 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 re- sponses, future sampling will be necessary to understand the broader implications of struc- tural retention harvests. We predict that, as effects of disturbance diminish with time, effects of canopy structure will increasingly shape patterns of compositional and structural de- velopment in the understory.