Remote sensing of vegetation cover change in islands of the Kerguelen archipelago

Résumé En

The plant communities in the Iles Kerguelen (South Indian Ocean) have been extensively modified by human activities, particularly through the deliberate release of rabbits, and the intentional or accidental introduction of several plant species. During the 1990 and 2000s, a decrease in precipitation resulted in a drastic reduction of some native plant species and in the increase in alien taxa. To monitor at a wide spatial scale the rapid changes of vegetation cover induced by summer droughts, we developed a method combining field data and satellite image analysis. A long-term field monitoring of plant communities was initiated on five small islands in 1992, and annually continued for over 15 years on a total of 161 line transects. Among these islands, the rabbit--which was the only introduced herbivore--was eradicated on three, remained on one control island, and had never been present on a second control island. We computed a linear model to link remote sensored vegetation indexes to plant cover deduced from line transects in numerous habitat types. After testing 14 vegetation indexes, we used a model based on the normalized difference vegetation index to precisely map the vegetation cover at several dates. A map of differences and spatial statistics indicated that vegetation cover, as a whole, decreased over the 15-year period. This study provides a reliable tool for long-term monitoring of the dynamics of plant cover in relation to climate change on the Iles Kerguelen.