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Simard and Vyse 2006 - 2486 Trade-offs between competition...

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Trade-offs between competition and facilitation: a case study of vegetation management in the interior cedar–hemlock forests of southern British Columbia 1 Suzanne Simard and Alan Vyse Abstract: Vegetation-management practices are applied in temperate-zone forests on the assumption that changing the competitive environment between conifers and unwanted vegetation will improve conifer productivity. We review this assumption using research examining interactions between paper birch ( Betula papyrifera Marsh.) and conifers in the highly productive Interior Cedar Hemlock zone of British Columbia. We have found that both competition and facilita- tion are important in young plantations, where paper birch competes for light, reducing growth of shade-intolerant co- nifers, but having a facilitative effect on shade-tolerant conifers. This facilitative effect may result from greater ectomycorrhizal diversity, population sizes of Armillaria ostoyae (Romagn.) Herink antagonistic bacteria, and associa- tive nitrogen fixation in plantations where interior Douglas-fir ( Pseudotsuga menziesii var. glauca (Beissn.) Franco) is mixed with paper birch. Where paper birch is manually cut or girdled, conifers grow faster in diameter, but more die as a result of A. ostoyae root disease, and these responses increase with increasing weeding intensity. The weeding treatments do not affect plant community species richness but reduce paper birch dominants and increase understory structural diversity. British Columbia forest policy has been slow to respond to these findings, and we suggest that as a result, the forested landscape incurs substantial risk. We propose additional pathways for managing Interior Cedar Hemlock mixtures to ensure that the natural mix of forest types in the landscape is maintained. Résumé : Les pratiques d’aménagement de la végétation sont appliquées dans les forêts tempérées en assumant que le fait de modifier le milieu pour réduire la compétition entre les conifères et la végétation indésirable augmentera la pro- ductivité des conifères. Nous avons revu cette hypothèse à l’aide de travaux de recherche qui portent sur les interac- tions entre le bouleau à papier ( Betula papyrifera Marsh.) et les conifères dans la zone intérieure fortement productive de la Colombie-Britannique caractérisée par la présence de cèdre et de pruche. Nous avons trouvé que la compétition et la facilitation sont aussi importantes dans les jeunes plantations où le bouleau à papier est en compétition pour la lu- mière, ce qui réduit la croissance des conifères intolérants à l’ombre mais a des effets bénéfiques sur les conifères tolé- rants à l’ombre. Cet effet bénéfique est possiblement le résultat d’une plus grande diversité des ectomycorhizes, de la taille des populations de bactéries antagonistes vis-à-vis Armillaria ostoyae (Romagn.) Herink et de la fixation de l’azote par des organismes associés aux endroits où le douglas vert ( Pseudotsuga menziesii var. glauca (Beissn.)
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