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Abstract. We investigated changes in the composition and abundance of understory species after fire in the southern boreal forest around Lake Duparquet, Québec. Ten plots of 100 m2 were sampled in each of eight sites varying in post‐fire age from 26 to 230 yr, with 20 1‐m2 quadrats in each of these 80 plots. Variation in the understory was described by DCA ordination and interpreted as a regeneration succession series. Thickness of the organic layers, stand age and canopy composition were all correlated with vegetational change. This change was not constant throughout succession; some old sites showed an increase in the diversity and abundance of certain pioneer species. This was partly related to openings in the canopy resulting from a major outbreak of spruce budworm, which affected sites dominated by Abies balsamea. The ordinations were performed on both the 100‐m2 plots and the 1‐m2 quadrats. Heterogeneity within sites was larger at the 1‐m2 scale and there was a great deal of overlap in the position of the quadrats in ordination space. At the smaller scale of analysis, stand age and thickness of the organic layers were not correlated with the changes observed in the understory.

Many populations of threatened American ginseng (Panax quinquefolium) and vulnerable wild leek (Allium tricoccum) have declined and gone extinct because of overharvesting in Canada. We assessed the impact of harvesting on populations of these species in stochastically varying environments and estimated their extinction thresholds and minimum viable populations. With both species we used four transition matrices taken from the literature in stochastic population projections under various harvesting regimes. For American gingseng the mean population growth rate (λ) declined with increasing harvesting rate (h) according to the number of years between harvests (tr), as −0.54 htr−0.90. When plants with more than two leaves are harvested every 5 years, a harvest rate of approximately 30% was sufficient to bring the λ below the equilibrium value of 1.0. Extinction thresholds, the minimum number of plants needed to rebuild a population, varied from 30 to 90 plants, and the minimum viable population size was estimated at 170 plants. Only a dozen populations known in Canada exceed 170 plants, so most populations could not support any harvesting without serious threats to their long‐term persistence. For wild leek, two harvesting strategies were identified from confiscated, illegal harvests from Gatineau Park (Québec): (1) “choosy” harvesters collect fewer but larger bulbs, and (2) “busy” harvesters collect numerous but smaller bulbs. These data allowed simulations of more‐realistic harvesting strategies. The rate of the decline λ along the harvest gradient was faster for wild leek than for ginseng and varied with harvesting strategies. At harvesting rates between 1 and 8% the probability that λ falls below the equilibrium value was less than 5%. The extinction threshold of wild leek was estimated at 140–480 plants and the minimum viable population at 300—1030 plants, according to the threshold chosen. Remnant wild leek populations in Québec rarely contain more than a few hundred plants, indicating the serious threat commercial harvesting represents for this species. For both species we found that the whims of a stochastically varying environment significantly reduce sustainable harvest levels.

A survey of the phytochemistry of Panax quinquefolius L. (North American ginseng) collected from wild populations in Ontario, Quebec, Maine, Vermont, and Wisconsin was undertaken. Reverse-phase HPLC was used to determine the natural variation of levels of ginsenosides Rg1, Re, Rf, Rb1, Rc, Rb2, and Rd and their total in leaf, stem, and root of authentic wild-grown material. The totals in roots varied from 1 to 16%, with the greatest number of individual samples having 4-5% total ginsenosides. The lack of ginsenoside Rf in roots of authentic wild populations confirmed its status as a phytochemical marker differentiating American and Asian ginseng. Ten geographically isolated wild populations were collected, and several showed significant variation in the levels of major ginsenosides. There was no statistical difference in mean ginsenoside content between wild and cultivated P. quinquefolius roots at 4 years of age, suggesting there is no phytochemical justification for wild crafting. Baseline data on total ginsenoside levels for authentic wild P. quinquefolius reported here provide reference levels for quality assurance programs.

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The clonal perennial herb Helianthus divaricatus and the clonal shrub Rhus aromatica reach the northern limit of their distribution in southern Que bec (Canada), where both are rare, but they are more common southwards in the adjacent province of Ontario. We tested whether peripheral populations of these species maintain themselves by having highly variable individual growth and survival rates. Their annual rates of population growth, l, would then vary more, from year to year, than those of more southern populations in neighbouring Ontario, where the species are common. 2 Demographic analyses compared, over two transition intervals, size-based transition matrices for two northern (Gatineau Park, southern Que bec) and two southern populations (Pinery Park, southern Ontario) of each species. Comparisons used log-linear analyses, bias-corrected l-values with 95% con®dence intervals, and elasticity values. 3 For both species, the northern populations showed a larger variation between transition intervals and local populations in some of their vital rates and in lvalues. For H. divaricatus, elasticity values combined with log-linear analyses indicated that dierences in the growth and survival of medium-sized ramets contributed most to the dierences in l-values between northern and southern populations. For R. aromatica, they indicated that dierences in the fate of both medium and large ramets contributed to variation in l. 4 Most of our data indicated a greater demographic variability in the northern populations of H. divaricatus and R. aromatica. At the northern periphery, these species grow in very few of the numerous habitat patches that appear to be suitable for them, whereas the two species together present a near continuous cover in all suitable habitats further south. High demographic variability therefore appears to contribute to determining the distribution of plant species, by increasing the extinction probabilities of local populations at the distribution limit.