Edge-related responses of understory plants to aggregated retention harvest in the Pacific Northwest

Aggregated retention of overstory trees is now a standard component of timber harvest prescriptions on federal lands in the Pacific Northwest. Patches of remnant forest retained during harvest are thought to enhance the structural and biological diversity of managed forests, but the extent to which they maintain components of the original understory or promote recovery in adjacent harvest areas has not been tested. We examined short-term (1- and 2-yr) responses of understory plants to disturbance and creation of edges in structural retention harvest units at two sites in the western Cascade Range of Washington. Pre- and post-treatment abundance of vascular plants was measured in four (two at each site) 1-ha aggregates (patches of intact forest) and in surrounding harvest areas along sixteen 81 m long transects placed perpendicular to the edges of these aggregates. Two years after treatment, aggregates had gained an average of two forest species (vs. a loss of two in adjacent areas of harvest) and less than one early-seral species (vs. a gain of nine in adjacent areas of harvest). Aggregates supported populations of late-seral species that disappeared from or declined substantially in harvested areas. However, aggregates showed edge-related changes in plant abundance: one third of common understory herbs declined significantly in cover toward the edge, and changes in community composition were distinctly higher within 5 m of the edge than in the aggregate center. Early-seral species established infrequently within the aggregates, and only within 10 m of the edge. Herbaceous species generally showed larger declines in abundance with proximity to edge than did shrubs, with declines becoming more prominent over time. Our results suggest that, over short time frames, forest aggregates of one or more hectares may play an important role in maintaining plant species richness and composition in forests managed for timber harvest. Assessing the longer term stability of forest aggregates and the degree to which they influence recovery in adjacent areas of harvest will require continued observation.