Growth change of young Picea sitchensis in response to deer browsing |
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Authors: | Bruno Vila, Franck Torre, Fr d ric Guibal,Jean-Louis Martin |
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Affiliation: | a Institut Méditerranéen d’Ecologie et de Paléoécologie, CNRS UMR 6116, Faculté des Sciences et Techniques de Saint-Jérôme Université d’Aix-Marseille III, Avenue Escadrille Normandie Niemen, F-13397, Marseille Cedex 20, France b Centre d’Ecologie Fonctionnelle et Evolutive, CNRS UPR 9056, 1919 route de Mende, F-34293, Montpellier Cedex 5, France |
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Abstract: | Taking advantage of the introduction of the black-tailed deer to the Queen Charlotte Islands (British Columbia, Canada), we used dendrochronological analyses to understand the consequences of deer browsing on Sitka spruce growth. We compared shape, radial growth, height growth and age of young spruce in three sites. We identified two types of trees growing side by side: (1) stunted and heavily browsed spruce, smaller than the browsing limit and (2) escaped spruce that were taller than the browsing limit but still browsed in their lower part. The compact and heavily ramified shape in stunted spruce was the result of repeated and intense browsing. In escaped spruce this was also the case below the browsing limit (1.16 m±0.07 m), in sharp contrast with the normal shape that escaped spruce resumed above the browsing limit. We show that the release of browsing pressure, once the tree reaches the browsing limit, is characterised by an abrupt increase in radial growth. Before release, trees show a growth stagnation characterized by narrow rings (0.5 mm per year) and small annual height increments (<5 cm per year). After release, trees show a growth stabilisation characterised by wider rings (3 mm per year) and larger annual height increments (20 cm per year). We use this pattern to estimate frequency and age at release and their possible variation over time. Age differences between stunted and escaped spruce are highly significant and indicate that, despite of browsing, most if not all trees will ultimately reach the browsing limit and escape. Heavy deer pressure (30 deer per km2) delays spruce sapling recruitment by about 8 years. This delay varies in relation to site quality and seems to have increased over time, suggesting an increase in browsing pressure. |
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Keywords: | Picea sitchensis (Bong.) Carrière Deer browsing Growth pattern Growth indices Age Temporal variations |
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