Boreal forest ecosystems are adapted to periodic disturbance, but there is widespread concern that conventional forest practises degrade plant communities. We examined vegetation diversity and composition after clearcut logging, mechanical and chemical site preparation in eight 5-to 12-yr old studies located in southern boreal forests of British Columbia and Quebec, Canada to fi nd useful indicators for monitoring ecosystem integrity and to provide recommendations for the development and testing of new silvicultural approaches. Community-wide and species-specifi c responses were measured across gradients of disturbance severity and the results were explained in terms of the intermediate disturbance hypothesis and a simple regeneration model based on plant life history strategies. Species richness was 30 to 35% higher 5 to 8 years after clearcut logging than in old forest. Total and vascular species diversity generally peaked on moderately severe site treatments, while non-vascular diversity declined with increasing disturbance severity. On more-or-less mesic sites, there was little evidence of diversity loss within the range of conventional silvicultural disturbances; however, there were important changes in plant community composition. Removing soil organic layers caused a shift from residual and resprouting understory species to ruderal species regenerating from seeds and spores. Severe treatments dramatically increased non-native species invasion. Two important challenges for the proposed natural dynamics-based silviculture will be 1) to fi nd ways of maintaining populations of sensitive non-vascular species and forest mycoheterotrophs, and 2) to create regeneration niches for disturbance-dependent indigenous plants without accelerating non-native species invasion.

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Mechanical site preparation (MSP) changes the distribution and character of forest floor and mineral soil and may affect soil nutrient availability, soil water content, and soil temperature. The effects of different kinds of MSP were compared to a control in the tenth growing season at two research sites in northern British Columbia. To compare MSP results with those of the natural disturbance regime, a burned windrow treatment was also included in the analysis. The bedding plow, fire, and madge treatments all had significantly greater crop-tree growth compared to the control. The bedding plow and madge treatments had significantly lower soil bulk density, higher soil temperature, and lower soil water throughout the growing season compared with that of the control. The bedding plow also resulted in significantly higher concentrations of total carbon, total nitrogen, NH 4 + , and NO 3 -than that of the control, at both the 0-10 and 10-20 cm depths. The madge rotoclear resulted in significantly greater potential mineralizable N than that of the control. Ionic resins bags, installed for one growing season, did not show any significant treatment differences in available soil nitrogen. MSP did not reduce soil fertility on these sites when compared with an untreated control, but it is difficult to say that it improved it.Résumé : La préparation mécanique de terrain modifie la distribution et les caractéristiques de la couverture morte et du sol minéral, et peut affecter la disponibilité des nutriments, la teneur en eau et la température du sol. Les effets de différents types de préparation mécanique ont été comparés à un témoin au cours de la dixième saison de croissance après leur application dans deux stations de recherche du nord de la Colombie-Britannique. Afin de comparer les résul-tats des préparations mécaniques à ceux du régime de perturbations naturelles, l'analyse a aussi tenu compte d'un traitement impliquant le brûlage d'andains. Le labour de surface, le brûlage et le rotoculteur madge ont tous significativement augmenté la croissance des arbres d'avenir par rapport au témoin. Pour toute la durée de la saison de croissance, le labour de surface et le rotoculteur madge ont significativement diminué la densité apparente, augmenté la température et diminué la teneur en eau du sol comparativement au témoin. Le labour de surface a aussi provoqué une augmentation significative des concentrations de carbone total, d'azote total, de NH 4 + et de NO 3 -comparativement au témoin et ce, à des profondeurs de 0 à 10 et 10 à 20 cm. Le rotoculteur madge a entraîné une augmentation significative de l'azote minéralisable comparativement au témoin. Des sacs de résines d'échanges ioniques, installés pendant une saison de croissance, n'ont pas permis d'observer de différences significatives dans la disponibilité en azote du sol entre les traitements. La préparation mécanique de terrain n'a pas diminué la fertilité des sols de ces stations par rapport à un té-moin non traité, mais il est difficile d'affirmer qu'elle l'a amélior...

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