Indicators of landscape function: comparing patchiness metrics using remotely-sensed data from rangelands

In arid and semi-arid rangeland regions, landscapes that trap and retain resources, such as rain water, soil particles, and organic matter, provide more favorable habitats for vegetation and fauna, and are considered more functional than landscapes that lose, or leak, these essential resources. The cover and arrangement of perennial vegetation patches is an important indicator of whether landscapes retain or leak resources. Patchiness attributes, as descriptors of resource retention potential in landscapes, can be obtained from remotely-sensed imagery, such as aerial videography and high-resolution satellites where this imagery has been classified into perennial vegetation patch and open interpatch pixels. In this paper, we compare four landscape patchiness metrics on their ability to indicate how well landscapes potentially function to retain resources. Landscape patch attributes (e.g. patch cover and spacing) and on-ground inspection of soil and vegetation attributes were used to rate and rank four sites relative to their potential to retain resources. A directional leakiness index (DLI) that is highly sensitive to patch cover, shape, and configuration correctly and adequately ranked sites in the same order as our field ratings. The lacunarity index also correctly ranked sites, but showed little separation amongst sites with reduced potential to retain resources. The weighted mean patch size (WMPS) index and proximity index failed to correctly rank sites. The directional leakiness and lacunarity indices can be calculated for any remotely-sensed imagery that is of sufficient resolution to measure landscape patchiness at scales where processes of resource conservation are operating. For example, imagery of 0.2–1 m pixel sizes from arid and semi-arid rangelands can be classified into flow-obstructing patches and open non-obstructing interpatches. Such classified imagery and leakiness or lacunarity indicators can then be used to monitor changes in the resource retention potential of these landscapes. However, the applicability of these indicators for monitoring more humid vegetation types, and for assessing larger landscape areas (i.e. at coarser scales), needs to be evaluated.