Monitoring cork and holm oak woodlands with Google Earth Engine

Abstract : Oak woodlands are being documented to be declining in southern Europe. In order to verify and quantify this phenomenon, 34-year trends of the Normalized Difference Vegetation Index (NDVI) were calculated and mapped at 30-meter pixel scale for all cork oak (Quercus suber) and holm oak (Quercus ilex) areas of continental Portugal. NDVI is considered a good proxy to monitor trees health and productivity along years. Landsat 5, 7 and 8 imagery from 1984 to 2017 were used to derive a long-term NDVI time series. MODIS 250-m spatial resolution NDVI product was used to compare values for the period after 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. At country scale, a spatio-temporal trend analysis was performed on stable cover areas of holm and cork oak. Thirty percent of woodlands area presents declining trends, mainly located in Centro region, and interior and littoral of Alentejo.
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Master Thesis
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Valentine Aubard. Monitoring cork and holm oak woodlands with Google Earth Engine. Life Sciences [q-bio]. 2018. ⟨dumas-01950823⟩

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