[en] 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.

[fr] Les îles volcaniques subantarctiques des Crozet et de Saint-Paul ne possèdent aucun arbre ou arbuste autochtone. Cette absence est d'autant plus étonnante que certaines îles australes situées dans la zone des 50ème hurlants possèdent des ligneux autochtones. Le "contexte insulaire" des îles Crozet et Saint-Paul serait-il à l'origine du phénomène ?Nous avons examiné huit facteurs géographiques pouvant influencer le nombre d'espèces ligneuses sur 26 îles volcaniques subantarctiques incluant les Crozet et Saint-Paul. Nos résultats montrent que la température moyenne annuelle explique 23,7% de la variance du nombre de ligneux sur les îles étudiées. L'absence d'arbre et d'arbuste sur les îles Crozet est en partie liée aux températures annuelles inférieures à 6°C. Cependant, les capacités de dispersion des espèces ligneuses australes et la difficulté des graines à atteindre ces îles nous paraissent jouer aussi un rôle fondamental. L'Analyse en Composante Principale (ACP) montre que le contexte insulaire de Saint-Paul ne diffère pas des îles possédant des ligneux indigènes. L'asylvatisme de Saint-Paul ne peut être expliqué par la spécificité de son contexte insulaire. Des arbustes y étaient observés en 1626 et 1633 mais l'activité volcanique de la fin du 17ème siècle semble avoir éliminé les ligneux autochtones sur l'île Saint-Paul avant que ne soient effectués les premiers relevés botaniques.