Assessment of forest dieback on the Moroccan Central Plateau using spectral vegetation indices

Abstract

Cork oak forests in Morocco are rich in resources and services thanks to their great biological diversity, playing an important ecological and socioeconomic role. Considerable degradation of the forests has been accentuated in recent years by significant human pressure and effects of climate change; hence, the health of the stands needs to be monitored. In this study, the Google Engine Earth platform was leveraged to extract the normalized difference vegetation index (NDVI) and soil-adjusted vegetation index, from Landsat 8 OLI/TIRS satellite images between 2015 and 2017 to assess the health of the Sibara Forest in Morocco. Our results highlight the importance of interannual variations in NDVI in forest monitoring; the variations had a significantly high relationship (p < 0.001) with dieback severity. NDVI was positively and negatively correlated with mean annual precipitation and mean annual temperature with respective coefficients of 0.49 and − 0.67, highlighting its ability to predict phenotypic changes in forest species. Monthly interannual variation in NDVI between 2016 and 2017 seemed to confirm field observations of cork oak dieback in 2018, with the largest decreases in NDVI (up to − 38%) in December in the most-affected plots. Analysis of the influence of ecological factors on dieback highlighted the role of substrate as a driver of dieback, with the most severely affected plots characterized by granite-granodiorite substrates.