Dynamics of self-organized vegetation patterns

Vegetation patterns are a common and well-defined characteristic of many arid and semi-arid landscapes. In this study we explore some of the physical mechanisms responsible for the establishment of self-organized, non-random vegetation patterns that arise at the hillslope scale in many areas of the world, especially in arid and semi-arid regions. In doing so we use a water and energy balance model and provide a fundamental mechanistic understanding of the dynamics of vegetation pattern formation and development. Within the modeling, reciprocal effects of vegetation on the hillslope energy balance, runoff production and run-on infiltration, root density, surface albedo and soil moisture content are analyzed. In particular, we: 1) present a physically based mechanistic description of the processes leading to vegetation pattern formation; 2) Compare simulated vegetation coverage at the hillslope scale with observations; 3) quantify the relative impact of pattern-inducing dynamics on pattern formation; and 4) describe the relationships between vegetation patterns and the climatic, hydraulic and topographic characteristic of the system. The model is validated by comparing hillslope-scale simulations with available observations for the areas of Niger near Niamey and Somalia near Garoowe, where respectively tiger bushes and banded vegetation patterns are present. The model validation includes comparison of simulated and observed vegetation coverage as well as simulated and measured water fluxes, showing both qualitative and quantitative agreement between simulations and observations. The analysis of the system suggests that the main driver of pattern establishment is climate, in terms of average annual precipitation and incoming solar radiation. In particular, decreasing precipitation or, conversely, increasing incoming radiation are responsible for the system departure from fully vegetated with indistinguishable vegetation structures to sparsely vegetated with (self-organized) distinguishable patterns. In addition, within the range of climatic conditions that promote the formation of self-organized vegetation patterns, the phenomenon is found as mainly driven by surface runoff production and run-on infiltration. On the other hand, the spatial interactions between adjacent vegetation groups and the effect of roots and surface reflectance on soil moisture redistribution have a determinant role on both the characteristics and stability of the pattern and on the total biomass that is established on the domain.