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, Titre français : Suivi des bois de chênes lièges et chênes verts avec Google Earth Engine Titre anglais : Monitoring cork and holm oak woodlands with Google Earth Engine

, Afin de quantifier ce phénomène, les tendances sur 34 ans de l'indice de végétation par différence normalisé (NDVI) ont été cartographiées à une précision de 30 mètres, pour l'ensemble des forêts de chênes lièges (Quercus suber) et de chênes verts (Q. ilex) du Portugal continental. L'indice NDVI est un bon proxy pour suivre la santé et la productivité des arbres au cours du temps. Les images de Landsat 5, 7 et 8 ont été utilisées pour obtenir une série temporelle de 1984 à 2017. Les NDVI de MODIS servirent de comparaison après l'an 2000. Google Earth Engine permet d'utiliser librement ces images. Les valeurs de NDVI de Landsat 5 et 8 furent ajustées à celles de Landsat 7. Pour minimiser l'impact spectral de la végétation de sous étage, seuls les NDVI de Juillet et Août furent utilisés. Les tests de Mann-Kendall (MK) et contextuel de MK ont permis d'estimer la significativité et le signe des tendances, et l'estimateur de Theil-Sen (TS) les pentes. Cette méthodologie a d'abord été testée sur six parcelles de chênes lièges. Des modèles linéaires ont montré l'effet significatif des précipitations et de l'extraction du liège sur les variations de NDVI. Un test de Pettitt révéla des ruptures de tendance significatives autour de l'an, 2000.

, All imagery were freely available and treatable on Google Earth Engine. NDVI values of Landsat 5 and 8 were adjusted to Landsat 7 values. Only July and August NDVI were used to minimize the spectral contribution of understorey vegetation and its phenological variability, thus focusing on the tree layer. Signs and significance of trends were calculated by Mann-Kendall (MK) and contextual MK tests, and their slopes using Theil-Sen (TS) estimator. The methodological approach was first tested on six cork oaks stands. Linear models revealed the effect of annual cumulated precipitation and debarking on NDVI variations. All trends were found significant. A Pettitt test showed significant change-points around the year 2000, Oak woodlands are being documented to be declining in southern Europe. In order to verify and quantify this phenomenon, 2000.