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‐to‐plot 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.

In many regions of the world, variable retention has replaced clear-cutlogging as the principal method of regeneration harvest. Partial retention of the overstory is thought to ensure greater continuity of the species and ecological processes that characterize older forests. Level (amount) and spatial pattern of overstory retention are two basic elements of forest structure that can be manipulated to achieve specific ecological or silvicultural objectives. However, experiments that elucidate the relative importance of retention level and pattern (or their interaction) are rare. Here we assess long-term (> 10 yr) responses of forest understories to experimental harvests of mature coniferous forests replicated at five sites in the Pacific Northwest (PNW). Treatments contrast both the level of retention (40% vs. 15% of original basal area) and its spatial distribution (dispersed vs. aggregated in 1-ha patches). For most vascular plant groups (early seral, forest generalist, and late seral), postharvest changes in cover and richness were reduced at higher levels of retention and in dispersed relative to aggregated treatments. Although retained forest patches were stable, changes in adjacent harvested (cleared) areas were significantly greater than in dispersed treatments. Late-seral herbs were highly sensitive to level and pattern of retention, with extirpations most frequent in the cleared areas of aggregated treatments and at low levels of dispersed retention. In contrast, early-seral species were most abundant in these environments. Forest-floor bryophytes exhibited large and persistent declines regardless of treatment, suggesting that threshold levels of disturbance or stress were exceeded. Our results indicate that 15% retention (the minimum standard on federal forestlands in the PNW) is insufficient to retain the abundance or diversity of species characteristic of late-seral forests. Although 1-ha aggregates provide refugia, they are susceptible to edge effects or stochastic processes; thus, smaller aggregates are unlikely to serve this function. The ability to achieve multiple ecological or silvicultural objectives with variable retention will require the spatial partitioning of habitats to include dispersed retention and larger undisturbed aggregates along with cleared areas.

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We examined first-year responses of forest-floor bryophytes to structural retention harvests at four locations in western Washington. Treatments represented a range of retention levels (100%, 75%, 40%, and 15% of original basal area) and spatial patterns (dispersed vs. aggregated in 1 ha patches). Declines in bryophyte cover and species' frequencies were comparably large at 40% and 15% retention. Retention pattern had little effect on the magnitude of decline, although declines in richness tended to be greater in aggregated treatments. Changes in cover were small within forest aggregates (comparable to controls). However, richness declined relative to controls within aggregates at 15% retention; rarer taxa in these exposed patches may be susceptible to edge effects. Declines in species' frequencies and richness were consistently greater in "clear-cut" areas of aggregated treatments than in dispersed retention; liverworts were particularly sensitive to harvest. In cut areas, bryophytes responded positively to cover of understory vegetation and negatively to logging slash. The positive correlation of richness (but not cover) to tree basal area may reflect the contribution of tree boles to persistence of rarer corticolous species. Our results suggest that conservation of bryophytes in forests managed with structural retention will require large retention patches and dispersed trees at levels considerably higher than current retention standards.