Land degradation assessment using landscape unit approach and normalized difference vegetation index in Northwest of Tunisia

Abstract

Land managers and decision makers need sound, evidence-based information about land degradation patterns and about the effectiveness of their management practices. Obtaining such information is particularly difficult in Mediterranean lands, where for long time a combination of anthropogenic pressure, high topographical and climatic variability, and frequent disturbances created a complicated and unstable environment. Likewise, the normalized difference vegetation index (NDVI) has gained particular recognition in the scientific community as proxy of vegetation and degradation – given the importance of the vegetation cover in preventing land degradation in drylands. This study aims at designing a methodology to provide land managers in a representative site in Northwest of Tunisia with spatially explicit, up-to-date information on the state of their land, the pressures driving land degradation, and the effectiveness of their management efforts using remotely sensed NDVI data. To apply the NDVI index in the process of land degradation assessment, the variance of the annual average NDVI was analysed within different landscape units, which they were identified based on land cover, aspect, and slope steepness parameters. After calibrating and validating the land degradation mapping methodology using field observations, the obtained land degradation patterns were related with spatial information about overuse and monoculture, as well as fallow and agroforestry practices. Therefore, the main objective of this study is to create land degradation maps that show the state of semi-natural dry ecosystems and highlight man - made land degradation, then to assess the relation between patterns shown by the degradation map and the main pressure drivers affecting the study site. More specifically, three hypotheses derived from information provided by experts and stakeholders has to be proven: (1) Ecosystems are more sensitive to pressure in steep and south-facing terrain than in flat and north-facing areas. (2) Inappropriate management mainly the monoculture not led only to a physical soil deterioration but also the decline of soil fertility, particularly if they occur repeatedly in continuous way, especially in hilly south terrains, which are more exposed to solar radiation and erosion by wind and rain. (3) Third, we aimed to obtain evidence of the impact of specific land management practices that had been identified as promising.