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Forest harvest presents a potential threat to forest floor bryophyte communities primarily through alteration of the microclimate and disturbance of substrates on the forest floor. Management, including harvest, applied at the landscape scale creates patches of disturbance of differing severities at the spatial scale experienced by bryophytes. Presumably, bryophyte diversity in managed landscapes is best conserved by forest harvest techniques that minimize community change, thereby allowing disturbed communities to reassemble to approach predisturbance composition. We monitored bryophyte community reassembly by sampling quadrats established in a 54-ha management block of Acadian forest in New Brunswick, before and after harvest. Quadrats were either in unharvested areas, or experienced a range of disturbance severities from removal of some or all canopy, to forest floor disturbance with complete canopy removal. Bryophyte communities showed compositional change over 4 years, even in areas that were not harvested. Although species richness was maintained or recovered 4 years after harvest, changes in species composition were significant in all disturbance classes with greatest change related to forest floor disturbance. In particular, liverworts were lost in areas with forest floor disturbance. We suggest that the simplest method to reduce immediate species loss, and presumably promote conservation of bryophyte communities within managed forest landscapes, is to utilize techniques that reduce the area of forest floor and associated substrates that are physically disrupted.Key words: bryophyte, community change, disturbance, forest harvest, monitoring.

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The Clay Belt region of Quebec and Ontario supports a large forest resource and an important forest industry. In this region, the majority of the harvested volume allotted to forest companies is in forested peatlands and boreal forests prone to paludification. Paludification is the accumulation of organic matter over time, and is generally believed to be caused by increasing soil moisture and Sphagnum colonization. Paludification is influenced by external and internal factors; it reduces soil temperature, decomposition rates, microbial activity, and nutrient availability. As a result, paludification may lead to lower site productivity with time after disturbance. Therefore, in harvested stands with a thick organic matter layer, low soil disturbance (as opposed to fire) and water table rise may create favourable conditions for paludification that may ultimately be detrimental to timber production. Past experiences suggest several solutions to prevent or control the negative effects of paludification. Drainage and fertilization applied together are generally good techniques to control paludification and to improve tree productivity. On the other hand, we suggest that site preparation as well as prescribed burning, preceded or not by drainage, are avenues of research that deserve to be explored because they hold the potential to control or even reverse paludification, especially where peat accumulation is caused by natural succession or where lateral peat expansion has occurred. Résumé : La ceinture d'argile située au Québec et en Ontario supporte une ressource et une industrie forestières importantes. Dans cette région, la majorité du volume de coupe alloué aux compagnies forestières se trouve dans des tourbières boisées et dans des forêts sujettes à la paludification considérées comme ayant une faible productivité. La paludification est principalement influencée par des facteurs internes et externes et peut être définie par l'accumulation de matière organique causée par une augmentation de l'humidité du sol et de la colonisation par les sphaignes. La paludification a pour effet de réduire la température du sol, le taux de décomposition, l'activité microbienne et la disponibilité des éléments nutritifs. Ainsi, la paludification peut avoir pour effet de réduire la productivité des sites avec une augmentation du temps depuis la dernière perturbation. Dans des peuplements récoltés avec un horizon organique épais, une faible perturbation du sol (par opposition au feu) et une élévation de la nappe phréatique sont donc susceptibles d'être dommageables pour la production de bois. Le drainage combiné à l'utilisation de fertilisant est généralement une bonne technique pour contrôler la paludification et pour améliorer la productivité des arbres. D'un autre côté, nous suggérons que la préparation de terrain ainsi que le brûlage dirigé, précédés ou non par le drainage, sont des avenues qui mériteraient d'être explorées car elles ont le potentiel de contrôler ou même de renverser la paludification, surtout dans les endroits où...

Boreal forests have commonly been described as dominated by monospecific postfire stands that are reburnt well before other ecological process than those occurring immediately after fire can take place. Research undertaken over the last 30 years has given us a very different perspective of the complexity of Canadian boreal forests. Old-growth forests are common and their development is controlled by nonfire disturbances. Consequently, monospecific even-aged stands can develop towards more diversified uneven-aged stands with increasing time since fire. This complex disturbance regime, including both fire and nonfire disturbances, is partially responsible for a higher than expected biodiversity (e.g., understory) in these forests. The dominating forest management model in Canadian boreal forests, based on clear-cut harvesting and regeneration of short rotation even-aged stands, does not reflect the complexities of the disturbance-succession cycle observed in Canadian natural boreal forests.Résumé : Les forêts boréales sont généralement considérées comme étant une large étendue de peuplements monospécifi-ques issus de feux qui vont rebrûler bien avant que des processus écologiques autres que ceux liés aux feux ne puissent avoir cours. Les recherches réalisées au cours des 30 dernières années révèlent une perspective très différente. Les vieilles forêts sont abondantes et elles sont souvent contrôlées par des régimes de perturbations autres que les feux. À mesure que le temps s'écoule après les feux, les peuplements équiennes et monospécifiques se transforment graduellement en peuplements plus diversifiés et à structure inéquienne. Ce régime de perturbations complexe qui inclut à la fois le feu et d'autres types de perturbations crée une biodiversité (ex., sous-bois) plus élevée que généralement attendue. Le modèle d'aménage-ment actuellement préconisé dans la forêt boréale canadienne basé sur la coupe totale, et la reconduction en rotation courte de peuplements monospécifiques équiennes, s'éloigne passablement de la complexité de la dynamique naturelle observée.

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Previous empirical work has shown that feedbacks between fire severity, soil organic layer thickness, tree recruitment, and forest growth are important factors controlling carbon accumulation after fire disturbance. However, current boreal forest models inadequately simulate this feedback. We address this deficiency by updating the ED2 model to include a dynamic feedback between soil organic layer thickness, tree recruitment, and forest growth. The model is validated against observations spanning monthly to centennial time scales and ranging from Alaska to Quebec. We then quantify differences in forest development after fire disturbance resulting from changes in soil organic layer accumulation, temperature, nitrogen availability, and atmospheric CO 2 . First, we find that ED2 accurately reproduces observations when a dynamic soil organic layer is included. Second, simulations indicate that the presence of a thick soil organic layer after a mild fire disturbance decreases decomposition and productivity. The combination of the biological and physical effects increases or decreases total ecosystem carbon depending on local conditions. Third, with a 48C temperature increase, some forests transition from undergoing succession to needleleaf forests to recruiting multiple cohorts of broadleaf trees, decreasing total ecosystem carbon by $40% after 300 years. However, the presence of a thick soil organic layer due to a persistently mild fire regime can prevent this transition and mediate carbon losses even under warmer temperatures. Fourth, nitrogen availability regulates successional dynamics; broadleaf species are less competitive with needleleaf trees under low nitrogen regimes. Fifth, the boreal forest shows additional short-term capacity for carbon sequestration as atmospheric CO 2 increases.