Tree colonization of sub-Mediterranean grasslands: effects of dispersal limitation and shrub facilitation

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Abstract:

Encroachment of trees into low-productivity grasslands is a serious threat to grassland biodiversity throughout Europe. Although the rate of encroachment of trees into grassland is variable and poorly understood, it is thought to result from variation in tree seedling recruitment, which is limited mainly by the availability of safe sites and the dispersal distances of propagules. In this study, we established spatially explicit models of seedling recruitment for two major sub-Mediterranean trees: Quercus pubescens Willd. and Fagus sylvatica L. We quantified the spatial distribution of Q. pubescens and F. sylvatica seedlings up to 20 years old at three grassland sites. We also quantified the spatial distribution and size of mature trees and of the two dominant species of shrubs, Buxus sempervirens L. and Juniperus communis L., at each of the three sites. Ninety-eight percent of the regeneration took place under shrub canopy. Quercus pubescens seedlings showed higher seedling production per unit of canopy area and longer mean effective dispersal distances than did F. sylvatica seedlings. Quercus pubescens seedlings also had a large advantage over F. sylvatica seedlings for establishment in open areas. Juniperus shrubs were better safe sites for the establishment of Q. pubescens and F. sylvatica seedlings than were Buxus shrubs. We calculated indices of seedling dispersal limitation and safe site availability for recruitment for four dates. In the dolomitic Causse grasslands examined in this study, the availability of safe sites for germination and survival was far more important than seedling dispersal limitation for recruitment of both tree species.

La colonisation des pâturages extensifs par les ligneux constitue une menace sérieuse pour la biodiversité des pelouses en Europe. Bien que la vitesse d'embroussaillement soit variable et mal connue, on pense qu'elle est liée à des variations de recrutement des semis ligneux, qui dépendent principalement de la présence de microsites favorables et de la distance de dispersion des propagules. Dans cette étude, nous avons établi des modèles spatialement explicites pour le recrutement de deux espèces forestières subméditerranéennes majeures : Quercus pubescens Willd. et Fagus sylvatica L. Nous avons quantifié la distribution spatiale de semis de Q. pubescens et de F. sylvatica âgés de moins de 20 ans dans trois sites de pelouse. Nous avons aussi caractérisé la distribution spatiale et la taille des semenciers matures et de deux espèces ligneuses basses, Buxus sempervirens L. et Juniperus communis L., à chacun des trois sites. Quatre-vingt-dix-huit pourcent de la régénération ligneuse se produit sous le couvert des buissons. Par rapport à F. sylvatica, Q. pubescens présente une plus forte production de semis par unité de surface de houppier, une plus longue distance de dispersion efficace et un large avantage en terme d'installation en milieu ouvert. Les buissons de Juniperus constituent des sites plus favorables que ceux de Buxus pour l'établissement de Q. pubescens et F. Sylvatica. Nous avons calculé des indices de limitation de la dispersion et de disponibilité de microsites favorables à quatre dates. Dans les pelouses des Causses calcaires étudiées ici, la disponibilité de sites favorables à la germination et à la survie des plantules était beaucoup plus importante que la limitation liée à la dispersion, pour les deux espèces d'arbres.

Document Type: Research Article

Publication date: January 1, 2006

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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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