Detecting forest cover changes in the cork oak forests surrounding the strait of Gibraltar using the Enhanced Vegetation Index (EVI)

Abstract

Information based on remote sensing was used to quantify percentage changes in the Enhanced Vegetation Index (EVI) values in cork oak forests (Quercus suber) surrounding the strait of Gibraltar, between the years 2000 and 2018, with EVI values serving as an indirect measure of forest health. The observed changes enable the detection of degradation of ecosystems within this area of high biodiversity, which also provides important habitat to migratory species passing through the strait of Gibraltar. Changes in vegetation cover were analysed for a range of different structural types of cork oak forest (dense forest, scattered forest, and forest mixed with scrubland and grassland). Percentages changes of between +1.11% and +2.42% were observed when comparing the time period of 2009-2018 to that of 2000-2009, depending on the type of forest structure analysed. These results show an overall increase in forest cover within the study area, however, by locating plots with negative EVI values, sparse cork oak forest with grassland was identified as having the greatest losses in forest cover. Cartography with EVI change values per 250 meter pixel was obtained for the protected areas surrounding the Strait of Gibraltar. This information was then matched against historic PNOA (Plan Nacional de Ortofotografía Aérea) images, revealing the presence of desertification. This article covers some of examples of such processes.