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Decreased fire frequency and increased water levels affect riparian forest dynamics in southwestern boreal Quebec, Canada

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Abstract:

The relative importance of fire and flooding on the population dynamics of eastern white-cedar (Thuja occidentalis L.) and black ash (Fraxinus nigra Marsh.) was evaluated in eight old-growth riparian stands of southwestern boreal Quebec, Canada. Rising water levels and decreasing fire frequency since the end of the Little Ice Age (ca. 1850) were expected to have favoured an inland migration of the riparian forest fringe, with the flood-tolerant black ash colonizing the lower parts of the shore terraces and eastern white-cedar the upper parts. Black ash was found to be restricted to the riparian zone (<200cm elevation), whereas eastern white-cedar trees did not occur below 100cm above lake level. Gaps of postfire eastern white-cedar recruitment were noted in stands exposed to riparian disturbances, whereas relatively continuous recruitment occurred at protected sites. Black ash, more tolerant to flooding and ice push, invaded the shore terrace sites left vacant by eastern white-cedar. The riparian forest fringe surrounding Lake Duparquet is currently migrating upland and this trend is expected to continue as water levels continue to increase and fire frequency continues to decrease during the 21st century.

L’importance relative de deux des principales perturbations de la forêt boréale, le feu et les crues, sur la dynamique des populations de thuya occidental (Thuja occidentalis L.) et de frêne noir (Fraxinus nigra Marsh.) a été évaluée dans huit peuplements riverains du sud-ouest de la forêt boréale du Québec (Canada). L’hypothèse était que l’augmentation des niveaux d’eau et la diminution de la fréquence de feux depuis la fin du Petit Âge Glaciaire (ca. 1850) auraient favorisé une migration vers l’intérieur des terres de la frange forestière riveraine, le frêne noir, tolérant aux inondations, colonisant les parties basses des terrasses et le thuya occidental les parties hautes. Le frêne noir était restreint à la zone riveraine (<200cm au-dessus du niveau d’eau), tandis qu’aucun thuya occidental mature ne se trouvait sous 100cm au-dessus du niveau d’eau. Des épisodes de faible recrutement ont été notés dans les peuplements de thuya occidental exposés aux perturbations riveraines, tandis que le recrutement était relativement continu dans les sites protégés. Le frêne noir, plus tolérant aux crues et aux poussées glacielles, a envahi les terrasses riveraines abandonnées par le thuya occidental. La frange forestière riveraine du lac Duparquet est présentement en migration vers l’intérieur des terres et la tendance devrait continuer suivant l’augmentation des niveaux d’eau et la diminution des fréquences de feu au XXIe siècle.

Document Type: Research Article

Publication date: 2008-05-01

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  • Published since 1971, this monthly journal features articles, reviews, notes and commentaries on all aspects of forest science, including biometrics and mensuration, conservation, disturbance, ecology, economics, entomology, fire, genetics, management, operations, pathology, physiology, policy, remote sensing, social science, soil, silviculture, wildlife and wood science, contributed by internationally respected scientists. It also publishes special issues dedicated to a topic of current interest.
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