Suivi et caractérisation de la dynamique spatio-temporelle des paysages en milieu subantarctique par télédétection : cas des îles de Kerguelen

Le résumé fourni par la source

Sub-Antarctic environments are currently undergoing rapid transformations driven by climate change, particularly in the Kerguelen Islands (49°S, 69°E), where record air warming of +1.5 °C since the mid-20th century and a decline in precipitation have been observed. According to glaciological projections, the Cook Ice Cap is expected to disappear completely by 2100. These findings highlight the vulnerability of ecosystems and the need to develop methods capable of monitoring and interpreting environmental changes at a regional scale. In this context, landscape mapping emerges as an essential tool to identify, monitor, and interpret environmental trajectories at regional scales.This dissertation proposes a mapping and characterization approach based on the combined use of MODIS NDVI time series (2003-2022) and climate reanalyses from the MAR model. Three complementary methods were developed: (i) stratification and classification into radiometric landscapes; (ii) monitoring greening and browning trends and their relationship with climatic factors; (iii) phenological characterization through remote sensing and its link with climate variability. The concept of the radiometric landscape, defined as a homogeneous spatial unit derived from NDVI temporal signatures, serves as both the entry point of this research and the foundation for analyzing environmental trajectories.The methodology developed enabled the mapping of five landscape types: scattered coastal mosaics, intermediate altitudes, high altitudes, transitional zones, and low-relief plains. Validated locally through field data, topographic metrics, and climate reanalyses, this typology reveals significant differences and constitutes the first map of the main landscape domains of the archipelago, providing a robust spatial framework for studying sub-Antarctic ecosystems.Vegetation dynamics reveal a sharp contrast between the west, which remains generally stable, and the east of the archipelago, where changes are more heterogeneous. The central-eastern sector stands out with strong greening, particularly in high-altitude and transitional landscapes, while the plains and coastal areas of the east display a more contrasted mosaic dominated by browning trends. Air temperature emerges as the main explanatory factor, accounting for 79% of observed variations. From a phenological perspective, a widespread lengthening of the growing season—by up to 30 additional days—was detected across all five landscapes. This lengthening is accompanied by a significant increase in NDVI signal, indicating an intensification of vegetation productivity over nearly 28% of vegetated areas.By re-examining landscape trajectories, this work underscores the diversity of recent dynamics, ranging from greening to browning to stability. It demonstrates, for the first time in the Kerguelen Islands, the potential of MODIS time series to map and analyze long-term landscape trajectories across the archipelago. The methods developed, both robust and reproducible, pave the way for monitoring other remote and climate-sensitive regions. Placed within a broader context, these results reflect the profound ecological upheavals currently reshaping terrestrial environments of the Earth system, with major consequences for the functioning and survival of present-day ecosystems.

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