Quelles origines à l’asylvatisme des îles volcaniques australes Crozet et Saint-Paul (Terres Australes et Antarctiques Françaises, océan Indien) ?

• Published in: Cybergeo
• Main Authors: Larrue, Sébastien, Chadeyron, Julien, Faucon, Frédéric
• Format: Journal Article
• Language: English; German
• Published: Unité Mixte de Recherche 8504 Géographie-cités 19.01.2018
• Subjects: biogeography; geohistory; island; physical geography; vegetation
• ISSN: 1278-3366; 1278-3366
• DOI: 10.4000/cybergeo.28917

Remote islands of Saint-Paul and Crozet (Indian Ocean) are among the most isolated islands in the world. Neither native trees nor shrubs have been reported on these Subantarctic Islands. Both regular and high wind speed that characterize these southern latitudes are considered the main factor to explain the origin of these treeless islands. However, this explanation seems unlikely because some southern islands in the “Roaring Forties” harbor many native woody species. In this article, we studied the insular context of Crozet archipelago and Saint-Paul island by means of eight geographic factors compared to 26 other Subantarctic Islands harboring native woody species or not. Factors used as potential predictors that may contribute to the naturally treeless island included island area (km²), island elevation (km), mean daily temperature (°C) and precipitation (mm), as well as the distance from the nearest continent (km), the distance to the nearest similar island (km), the distance to the Antarctic, and index of isolation (UNEP). Spearman’s rank correlation coefficient, stepwise regression analysis and bar diagrams were used to assess the relative influence of these factors on woody native species richness. Results show that the spatial pattern of woody species richness was significantly influenced by mean daily temperature (R² = 0.237, P = 0.014) with a threshold of treeless island phenomenon below ca. 6°C. However, seed dispersal abilities of woody southern species could play an important role to explain these naturally treeless islands. Principal Component Analysis (PCA) shows that the treeless phenomenon of Saint Paul cannot be explained by a “particular insular context” or low temperature. Shrubs were observed in 1626 and 1633 on the island of Saint Paul but volcanic activity in the late 17th century has potentially eliminated native trees on Saint Paul before the first botanical surveys were carried out.