Short-term effects of three commercial thinning treatments on diversity of understory vascular plants in white spruce plantations of northern New Brunswick

• Understory responses to modified plantation treatments were compared over 2 yrs.
• Spruce plantations were unthinned, or thinned with no, moderate, or enhanced debris.
• Thinning altered canopy and forest floor, increasing most taxa especially ruderals.
• Enhanced woody debris yields no short-term benefits for understory plant diversity.
• Reducing debris removal may limit biodiversity loss; longer-term study is needed.

Plantation forestry is increasing in frequency and extent across the landscape, but can negatively impact biodiversity by promoting stand homogeneity and early-seral species dominance. Modifying silvicultural prescriptions in plantations, particularly with respect to creating or retaining more coarse woody debris (CWD), may improve their ability to support biodiversity, by mimicking some aspects of natural disturbance. We used a modified before-after-control-impact design to examine the response of understory vascular plants to four treatments in 25-year-old white spruce plantations of northwestern New Brunswick. Treatments included commercial thinning (CT) with enhanced, moderate, or no debris, as well as an unthinned control. Understory composition was analyzed using complementary methods that summarize biological communities at different resolutions: indicator species analysis, functional group responses, ordination, and biodiversity indices. Understory plants increased in overall richness and abundance after thinning, particularly in no-debris treatments. This was driven by (1) expansion of pre-established clonal forest herbs and (2) invasion of graminoids and long-distance dispersers. Several disturbance-sensitive species were significantly more abundant in unthinned controls. Few compositional differences were observed between the moderate and enhanced debris treatments, perhaps because the effects of CWD creation require longer to detect than those of thinning and ground-layer disturbance (from debris-removal). We recommend (1) continued monitoring of changes in moderate and enhanced treatments to determine the effects of debris-modification and (2) avoiding silvicultural prescriptions that remove branches, tree-tops, and other non-merchantable wood, which appear to facilitate early-seral species, and negatively impact disturbance-sensitive species.