Detection of process‐related changes in plant patterns at extended spatial scales during early dryland desertification

Abstract

Arid and semiarid shrublands occupy extensive land areas over the world, are susceptible to desertification by anthropic use and can contribute to regional climate change. These prompt the interest to monitor and evaluate these lands adequately in order to detect early stages of degradation. Evaluation topics must refer to biology‐relevant characteristics of these systems, while simultaneously satisfying sampling consistency over extended landscape areas. We present an analysis of process‐relevant parameters related to changes in the spatial arrangement of the plant canopy of shrublands inferred from high‐resolution panchromatic aerial photos and Interferometric Synthetic Aperture Radar imagery. We obtained low‐altitude images systematically located along several gradients of land‐use intensity in a Patagonian Monte shrubland in Argentina. Images were digitized to spatial resolutions ranging from 0.09 to 0.72 m (pixel size) and the average values and an‐isotropic characteristics of the plant canopy patterns were quantified by means of a Fourier metric. We used radar‐derived imagery to overlay the panchromatic images on a digital elevation model in order to study the correspondence of potential runoff patterns and the spatial arrangement of plants. We related an‐isotropic features of the plant canopy images to the prevailing wind regime. Observed trends were further interpreted on the basis of a spatial‐explicit simulation model describing the dynamics of the main functional groups in the plant community. We conclude that early stages of anthropic‐driven dryland degradation in the Patagonian Monte can be characterized by the incipient un‐coupling of spatial vegetation patterns from those of runoff at a landscape scale, and a progressive coupling to the spatial pattern of the wind regime. The method and metrics we present can be used to quantify early desertification changes in other similar drylands at extended spatial scales.

Depression storage and infiltration effects on overland flow depth-velocity-friction at desert conditions: field plot results and model

“…It builds on previous results obtained by Ares et al (2003) indicating that early desertification processes involve modifications of the overland flow patterns and its distribution among the various vegetation life forms in the Patagonian Monte (Argentina). Here, an experimental protocol is presented of the field plot experiments and coupled simulation model to estimate infiltration and overland flow parameters in the Monte environment.…”

“…Surface flow redistribution may also occur on flat ground if the presence of vegetation patches creates contrasts in infiltration rate between dry soil and water saturated areas. Changes in the pattern of surface water re-distribution occur during early stages of desertification (Ares et al, 2003). Several studies indicate that the horizontal movements of the naturally scarce rainfall water on the surface soil in arid regimes contribute to define the spatial pattern of woody and herbaceous vegetation forms as well as the extent of bare soil patches (Aguiar and Sala, 1999;Ares et al, 2003;Borgogno et al, 2009;Dunkerley, 2002).…”

Non-linear relationships between grazing pressure and conservation of soil resources in Patagonian Monte shrublands

“…To discriminate the slight differences among sites located close to watering point, more quantitative measurements of grazing disturbance such as the scale of heterogeneity of shrub patches, which is positively related to grazing disturbance, are necessary (Bisigato et al, 2005). The scale of heterogeneity is derived from the spatial arrangement of shrubs, which shows considerable less temporal variation than other variables based on seasonally fluctuating herbaceous forms, such as plant cover (Ares et al, 2003b). In addition, it is relatively easily measured in high-resolution aerial photographs, whereas plant cover measurements in the field are time consuming and, depending on the sampling protocol, subjective.…”

“…This study does not include sites located at intermediate distances from watering points, which would fill the big gap in scales of heterogeneity between ''far'' and ''close'' sites. The inclusion of these sites, which present intermediate spatial patterns between those of ''far'' and ''close'' sites (Ares et al, 2003b), would help to define a range of scales of heterogeneity where internal remobilisation of resources begins. However, this range should be close to 3 m, because the site F2 (the site with the highest scale of heterogeneity among sites located far from watering points) presents several characteristics in common with sites C1 and C3, such as autocorrelation in soil-N and covariance between shrub cover and soil mounds (Fig.…”

“…Three paddocks were selected from 12 paddocks included in a previous study where high-resolution aerial photographs were taken along grazing gradients (Ares et al, 2003b). These paddocks are representative of the typical Patagonian Monte paddocks as regards size, shape, vegetation, and grazing management.…”