Landscape fragmentation and soil cover changes on south- and north-facing slopes during ecosystems recovery: an analysis from multi-date air photographs

A quantitative analysis is presented, of soil and vegetation patch patterns from air photographs of three dates (1956, 1976 and 1990). Relationships between changes in soil cover and landscape fragmentation are assessed for north- and south-facing slope units in the semi-arid zone of Israel undergoing recovery following severe overgrazing and woodcutting phases. Soil loss accompanied by an increase in rock fragment exposure, and an increase in soil erodibility due to a decrease in aggregate size and organic matter content are the two main characteristics of soil changes in this area. Changes in both of these characteristics are highly correlated with soil brightness properties as recorded from aerial photographs. The methodology developed combines image processing techniques, enabling differentiation between soil and vegetation patches, with raster Geographical Information Systems (GIS) techniques, allowing patch distribution, fragmentation and form to be analyzed. Expansion of vegetation patches and contraction of soil patches were found to follow similar lines of spatial pattern evolution when parameterized according to their size, form and density. Assessment of temporal changes in soil and vegetation patch composition indicated that there is a phase difference of 12 years between the processes taking place on the two slope units. Hypothetical descriptions of soil patch fragmentation and vegetation patch expansion processes between 1920 and 2010 were formulated following the analysis of data between 1956 and 1990. Evidence of soil brightness changes provided indications for the continuation of soil loss, mainly in large patches (a few thousands of square meters in size) undergoing relatively slow fragmentation.