Tree canopy effects on understory species abundance in high-latitudePopulus tremuloidesstands, Yukon, Canada

Abstract

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 3 m radius 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 <0.001) indicated that greater vascular understory plant cover occurred when forest stands consisted of P. tremuloides with large canopies and large basal diameters, and lacked Picea. Maximum cover for vascular understory species declined when Picea cover exceeded 7–10%. In combination, P. tremuloides stem densities or a metric based on summed canopy areas converted to a diameter value (canopy-area diameter), and the vertical silhouette area of Picea canopies (canopy profile area), as independent linear regression variables, explained ~79% of the variance in total vascular understory plant cover. Several Picea basal area-derived metrics were strongly and positively associated with increasing H. splendens cover, but canopy profile area was more informative. Populus tremuloides canopy area and Picea canopy profile area, as indicators of shading, may be important determinants of vascular understory vegetation abundance in stands where solar radiation enters at angles of up to 52 during the summer.