Succession and resilience in boreal mixedwood plant communities 15–16 years after silvicultural site preparation

Non-native plant abundance, vascular and non-vascular diversity and plant community succession were studied 10 and 15–16 years after stand initiation at two mixedwood boreal sites in northwestern Canada. At Inga Lake, five silvicultural treatments (untreated, plowed-and-inverted, rotocleared-and-mixed, burnedwindrow, repeated vegetation control) created a gradient from pure broadleaf to pure white spruce (Picea glauca [Moench] Voss) overstories. At Iron Creek, four treatments (untreated, mounded, plowed-and-inverted, vegetation control) produced a narrower range of conifer:broadleaf ratios. Both experiments were randomized block designs with 4–5 replications. Univariate ANOVA, stepwise regression and multivariate ANOVA by canonical redundancy analysis were used to address three questions: (1) did non-native species diminish over time?; (2) how was understory diversity related to overstory composition?; and (3) did highly dissimilar plant communities diverge or converge over time? Non-native cover declined three- to six-fold between the two sampling dates. Non-vascular richness generally doubled over the 5–6-year sampling interval. Understory diversity increased with treatment severity and frequency and was positively correlated with conifer overstory abundance. At Inga Lake, there was successional convergence among all treatments. Convergence was most rapid on severely burned windrows where plants originated mainly from seed, and slowest on rotocleared-and-mixed sites with a dense turf of Calamagrostis canadensis (Michx.) Beauv. At Iron Creek, the vegetation control treatment diverged slightly from untreated communities after recent manual cutting. Our results demonstrate the strong resilience of these boreal mixedwood plant communities and suggest ways that silvicultural intervention can modify stand composition and diversity to address a variety of ecosystem management goals.