Fragmentation and changes in hydrologic function of tiger bush landscapes, south‐west Niger

Abstract

Summary1 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 eectively captured using a combination of selected patch-based landscape metrics that measured speci®c 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 eective 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 dierent climates and topographic settings, and are an eective and parsimonious indicator of the fragmentation of tiger bush. 3 The dynamics of the vegetation bands during the fragmentation process was anisotropic. A signi®cantly 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, signi®cantly increasing the landscape permeability to surface¯ow. 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¯ow may have reduced the water available to the remaining fragmented vegetation bands and accelerated their degradation.

Using wavelength and slope to infer the historical origin of semiarid vegetation bands

“…Similarly, downhill spread is impeded in comparison with flat ground. These findings are reflected by the empirical observation of higher seedling densities on the uphill edge of vegetation patches, and higher levels of plant death on the downhill edges (44)(45)(46). Therefore, at low levels of rainfall, both edges of a localized vegetation patch on a slope move uphill (Fig.…”

Baseline review and ecosystem services assessment of the Tana River Basin, Kenya

“…This has taken place with attendant potentially negative consequences on a host of ecosystem services, such as climate regulation and landscape hydrological function (Asner et al 2003;Wu et al 2000;Archer et al 1995). Livestock grazing is often cited as a challenge in East Africa, where overgrazing is considered a source of serious land degradation.…”

Is the patch size distribution of vegetation a suitable indicator of desertification processes?

“…Given its importance, it is not surprising that many early-warning indicators of desertification based on the use of different vegetation attributes and soil properties (e.g., vegetation cover and spatial pattern, soil resistance to penetration and texture) have been proposed over the years (Tongway 1995, Wu et al 2000, Herrick et al 2005, Veron et al 2006. Among these, the spatial structure of vegetation is receiving increased attention.…”