Trend analysis of Landsat-TM and -ETM+ imagery to monitor grazing impact in a rangeland ecosystem in Northern Greece

Abstract

Mediterranean rangelands are unique marginal ecosystems, which are characterized by a highly heterogeneous structure and are often interwoven with other ecosystems. Traditionally, rangelands provided resources for livestock grazing in transhumantic rotation schemes. In recent times, there has been a trend towards semi-intensive grazing systems, which is partly connected to the European system of agricultural and infrastructural subsidies, and which effectuates both intensification and extensification. This study employed trend analysis of a remote sensing data time series for a retrospective assessment of rangeland processes, and interpreted these in the light of land-use practices and previous management interventions. We have selected a test area in Northern Greece that is representative of typical land-use transitions of the European Mediterranean. A time series of Landsat TM and ETM+ data covering the years 1984–2000 with one image per year was acquired, and for all images a geometric correction including digital elevation information and full radiative transfer modelling were carried out to attain surface reflectance data. For further analyses, proportional vegetation cover was selected as the target indicator, which was derived using Spectral Mixture Analysis. The resulting data set was used in a linear trend analysis to characterize spatio-temporal patterns of vegetation cover development. These could be interpreted based on knowledge of the local grazing regime and factors driving it, as well as using auxiliary spatial data sets. Results showed that temporal trends in the test area reflect the underlying pattern of potential livestock distribution at the per-pixel level, with a spatially differentiated pattern of both positive and negative trends in close proximity. On the other hand, no direct relation could be established between the development of vegetation cover and animal stocking rates at the community level. This suggests that this aggregation level is too coarse given the combination of highly heterogeneous landscapes with semi-intensive to intensive land tenure systems.