Estimates of dryland degradation in Argentina with Fourier signatures from low-altitude monochromatic images with high spatial resolution Jorge O. Ares*, Mónica B. Bertiller and Alejandro Bisigato National Patagonic Center, CONICET, Puerto Madryn, 9120, Argentina; *Author for correspondence (e-mail: joares@arnet.com.ar) Received 3 September 2001; accepted in revised form 30 April 2002 Key words: Image signatures, Land degradation, Patch structure change Abstract Most world drylands are used as graziny lands and undergo degradation of their vegetation cover. The plant cover is typically structured in patchy arrangements, inducing fertility islands critical to maintenance of ecosys- tem properties. The characteristics of patch structure (size of patches, connectivity-continuity of patch units, etc.) are indicators of the degree of dryland deterioration. We characterized changes in patch structure induced by sheep grazing at a landscape scale using monochromatic low-altitude imagery digitized to a spatial resolution of about 1 m with standard techniques of harmonic analysis applied to develop Fourier signatures. The signatures developed on image line transects were tested with ground samples and mathematical models of plant cover in several dryland fields where spatial deterioration gradients existed. The sensitivity and errors associated to long- wave noise introduced by the geometry of the camera-field-sun spatial arrangement and to high frequency noise introduced by the digitizing process were evaluated by applying suitable filters in the frequency domain. Fourier signatures developed on monochromatic low-altitude imagery proved to be indicative of changes in the patching arrangements of plant cover. We concluded that adequately filtered, high spatial resolution monochromatic im- ages can be used to evaluate the degree of deterioration of dryland landscapes through the computation of se- lected Fourier signatures in their frequency domain. At comparable cost, aerial photography allows inspecting the landscape at higher spatial resolutions than those attainable with satellite imagery. Also, aerial photos of many areas are available for earlier dates than images from remote sensors, which would allow better inspection of long-term ecosystem changes. Introduction Most world drylands, where the annual potential evaporation exceeds the total annual precipitation (Bailey 1998), are used as natural grazing lands (rangelands) and undergo degradation of their vege- tation cover. Changes in drylands occur slowly in time, and dryland range is a typically extensive activ- ity, occurring over large areas of the world and in ex- tended management units. Monitoring the results of dryland management requires prospecting techniques that would allow the characterization of long term changes occurring over large areas, albeit at costs commensurate with the intensity of the range activity. The vegetation of drylands is typically structured in patches. This phenomenon has been identified as aerial photographic interpretation proceeded at a number of locations. From the air, the structure of vegetation is clearly composed of regularly spaced densely vegetated areas interspersed with bare or less densely vegetated areas in complex banded, fuzzy or spotted spatial patterns (d’Herbes et al. 2001). In the classic conception of Watt (1947), the spatial arrange- ment of patches results from processes that contrib- ute to maintain key community functions and the patch structure itself. Whittaker and Levin (1977) ex- panded the concept by relating intra-community pat- terns to microsite differences and successional mosa- ics to responses following disturbance. A central point 51 Landscape Ecology 18: 51–63, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.