Tree canopy effects on understory species abundance in high-latitude Populus tremuloides stands, Yukon, Canada

To determine the effect of tree canopy composition on understory species abundance, three-hundred 2 m × 2-m quadrats from 30 high-latitude boreal forest stands were sampled. In addition, all trees within a 3mradius of each quadrat center and ≥1 m tall were also measured for height, basal diameter, and canopy width (n = 3130). Stands were 33–178 years old, with canopies of Populus tremuloides (trembling aspen) and Picea spp. (spruce) in varying proportions. Arctostaphylos uva-ursi, Calamagrostis purpurascens, Chamerion angustifolium, Shepherdia canadensis, and Hylocomium splendens were the most frequent understory species among quadrats. Scatterplots of P. tremuloides and individual vascular understory species cover values lacked bivariate trends, but the understory species had distinct maxima that ranged from 20 to 90%. A moderately strong correlation (r = 0.52, P <0.001) occurred between P. tremuloides canopy and total vascular understory plant covers, but weak individual species correlations (r = 0.22–0.35, P <0.001), suggested understory species variation was primarily determined by factors other than the amount of immediately overhead canopy cover. Canonical correlation analysis (R = 0.82, P