Fragmentation and changes in hydrologic function of tiger bush landscapes, south-west Niger
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1. Fragmentation of tiger bush landscapes in south-west Niger between 1960 and 1992 is evidenced by a reduction of percentage woody vegetation cover, changes in the spatial attributes of vegetation patches, and an increase in the spatial heterogeneity of the landscapes. The spatial patterns and dynamics of these landscapes were effectively captured using a combination of selected patch-based landscape metrics that measured specific aspects of the spatial pattern. 2. Derived from the spatial distribution of the alternating bands of vegetation and bare ground, lacunarity curves provide a particularly effective quantitative measure of the spatial pattern and dynamics of tiger bush landscapes in terms of percentage vegetation cover, spatial heterogeneity, and the domain of scale of the landscape. Lacunarity curves can be used to characterize landscapes in areas with different climates and topographic settings, and are an effective and parsimonious indicator of the fragmentation of tiger bush. 3. The dynamics of the vegetation bands during the fragmentation process was anisotropic. A significantly larger proportion of woody vegetation reduction occurred in the downslope than upslope portions of woody patches, while the opposite was true for woody vegetation expansion. These results corroborate the hypothesis that tiger bush bands migrate upslope due to the upslope-downslope resource gradient across the vegetation band. 4. Fragmentation of the tiger bush landscapes reduced retention of water on site, significantly increasing the landscape permeability to surface flow. When vegetation bands were well connected in 1960, no transects were found that allowed surface water percolation. That is, no path travelling through the bare ground areas was found to connect the upslope edge and downslope edge of any of the 200-m long transects, regardless of their width (50, 100 or 150 m). By 1992, within the same but now severely fragmented landscapes transects of all widths allowed water to percolate across them (44% if 50 m wide to 89% if 150 m wide). This increased landscape permeability to surface flow may have reduced the water available to the remaining fragmented vegetation bands and accelerated their degradation.
