Self‐organized vegetation patterning as a fingerprint of climate and human impact on semi‐arid ecosystems

Abstract

Summary1 Spatially periodic vegetation patterns are well known in arid and semi-arid regions around the world. 2 Mathematical models have been developed that attribute this phenomenon to a symmetry-breaking instability. Such models are based on the interplay between competitive and facilitative influences that the vegetation exerts on its own dynamics when it is constrained by arid conditions, but evidence for these predictions is still lacking. Moreover, not all models can account for the development of regularly spaced spots of bare ground in the absence of a soil prepattern. 3 We applied Fourier analysis to high-resolution, remotely sensed data taken at either end of a 40-year interval in southern Niger. Statistical comparisons based on this textural characterization gave us broad-scale evidence that the decrease in rainfall over recent decades in the sub-Saharan Sahel has been accompanied by a detectable shift from homogeneous vegetation cover to spotted patterns marked by a spatial frequency of about 20 cycles km − 1 . 4 Wood cutting and grazing by domestic animals have led to a much more marked transition in unprotected areas than in a protected reserve. 5 Field measurements demonstrated that the dominant spatial frequency was endogenous rather than reflecting the spatial variation of any pre-existing heterogeneity in soil properties. 6 All these results support the use of models that can account for periodic vegetation patterns without invoking substrate heterogeneity or anisotropy, and provide new elements for further developments, refinements and tests. 7 This study underlines the potential of studying vegetation pattern properties for monitoring climatic and human impacts on the extensive fragile areas bordering hot deserts. Explicit consideration of vegetation self-patterning may also improve our understanding of vegetation and climate interactions in arid areas.

Periodic versus scale-free patterns in dryland vegetation

“…An important example is related to the suggestion that transitions from scale-free patterns to patterns with characteristic length scales may serve as warning signals for imminent desertification (Kéfi et al 2007a), and that such transitions can be identified by monitoring changes in patch statistics (Manor & Shnerb 2008a). As the time scale of such changes can be quite long (tens of years in the case of woody vegetation; Barbier et al 2006), faster indicators are needed. Estimates of the likelihood of a given region to support scale-free patterns, based on soil properties and species traits, can be made in a relatively short time, and may provide such indicators.…”

“…Vegetation patchiness affects the distribution of limiting resources and seeds, modifies species distribution and diversity, and may contain information about imminent catastrophic shifts such as desertification (Kéfi et al 2007a;Shachak et al 2008). Field observations have revealed two contrasting types of vegetation landscapes: nearly periodic vegetation patterns with characteristic length scales, such as bands on hill slopes or spotted patterns (Valentin et al 1999;Barbier et al 2006), and scale-free patterns that lack characteristic length scales and follow broad, power-lawlike patch-size distributions (Kéfi et al 2007a;Scanlon et al 2007).…”

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

“…Such mechanisms can generate any unstable uniform state, and thence patterns. However, for semiarid vegetation a uniform state can only lose stability via changes in environmental parameters such as rainfall (35,36), which are inherently gradual. Therefore, marginally stable uniform states can occur, but those that are fully unstable cannot.…”

Deeply gapped vegetation patterns: On crown/root allometry, criticality and desertification

“…Fig. 1a) is symmetry-breaking in the strict sense (Barbier, 2006;Barbier et al, 2006Barbier et al, , 2008. As such, this instability can occur in the absence of slope-induced water redistribution, a mechanism that has long been thought to be primordial for vegetation pattern formation (Tongway et al, 2001).…”