Vegetation Pattern Formation Research Articles

Role of biomass spread in vegetation pattern formation within arid ecosystems

The spatial organization of biomass resulting from plant‐water feedbacks in arid ecosystems, “patterned vegetation,” provides a macroscopic signal of nonlinear plant‐water interactions and ecosystem health. Current models that reproduce such patterning assume diffusive biomass movement but do not account for realistic transport through seed dispersal. An adaptation of an existing three‐equation model that accounts for the interactions between overland flow, subsurface flow, and biomass dynamics is used to investigate the impact of representing biomass spread with realistic dispersal kernels (a “kernel‐based method”). Model results indicate that dispersion behavior changes the spatial organization of vegetation, destabilizing the regular patterns predicted by diffusion‐based models. The kernel‐based approach provides a closer match to power spectra derived from a remotely sensed image of patterned vegetation when compared to their diffusion‐based counterpart. Potential feedbacks between the presence of spatial patterns and selection of optimal seed dispersal length scales are also investigated.

A new statistical dynamic analysis on vegetation patterns in land ecosystems

As complex self-adaptive systems, land ecosystems usually tend to produce intricate vegetation patterns under specific environmental constraints, which have ever puzzled us for decades. In this paper, a universal principle on pattern formation of land ecosystems is highlighted as: a land ecosystem always tries to find an optimal process to acquire maximized flux under a certain constraint or price. The underlying microscopic dynamic mechanism that induces complex vegetation patterns in land ecosystems is then revealed. Based on the Holdridge life zone system, evolutionary dynamic processes and structural classifications of several typical vegetation patterns are then simulated and discussed. Good agreements between theoretical analyses and numerical results are found. This paper demonstrates that the new theoretical analyses and simulation technique can be possibly applied to the whole land ecosystem, not only benefiting academic study on the formation and dynamics of vegetation patterns, but also providing implications for vegetation classification, vegetation protection and soil erosion control.

Formation of Vegetation Patterns and Hysteresis Phenomena in Arid and Semiarid Zones

Considering the spatial effect of the complicated interactions between plant and resource (soil water) and the stochastic character of the complicated ecological processes in an ecosystem, we introduce a lattice gas model to investigate the vegetation pattern formation in the arid and the semiarid ecosystems with Monte Carlo simulations. With consistency of some previous mean-field models, the model reproduces a wide range of patterns observed in the arid and the semiarid regions. The sequences of vegetation states are also investigated with scanning three parameters, i.e., the precipitation and the plant growth rate, and then the parameter ranges are identified where a hysteresis loop appears and two different stable states coexist. The simulation results denote that the complex structures occur only when the precipitation parameter decreases. Moreover, based on the model, it is found that an increase of the competition of the plant for the resource along the slope direction results in the regular stripes parallel to the hill's contours. The model provides an interesting mechanism to understand the formation of the other regular vegetation stripe in a limited resource condition.

Why do plants in resource-deprived environments form rings?

We report on a combined theoretical–experimental study of ring formation by plant species in water-limited systems. We explored, under controlled laboratory conditions, the relationships between water regime and ring formation using a small perennial grass, Poa bulbosa L., as a model species. Our experimental studies show that ring-shaped patches are formed as a result of differential biomass growth across the patches, in spite of uniform water supply. The theoretical studies were based on a mathematical model for vegetation pattern formation that captures feedback processes between biomass growth and water exploitation. Our model studies reproduce the experimental findings and predict that spots destabilize to form rings in species whose lateral root augmentation per unit biomass growth is sufficiently small. This finding explains why rings in nature are formed by dense clonal plants.

Multi-scale linkages between topographic attributes and vegetation indices in a mountainous landscape

This paper addresses a few issues that are fundamental for the understanding of vegetation–topography relations: scale dependency, seasonal variability, and importance of observing individual properties. Particularly, it uses two statistical tools – Pearson's r and Moran's I – to define relationships of several topographic attributes with the Normalized Difference Vegetation Index (NDVI), the Normalized Difference Infrared Index (NDII), and their seasonal changes (from May to July and then September) in the Mediterranean-type landscape of the Santa Monica Mountains, California. The analyses are conducted at both the original data resolution and 20 aggregated resolutions, covering a total range of 30 m to 1500 m, so that topography–vegetation relationships can be compared at different scales. Large sample sizes have supported the significance of the following main findings for this landscape. First, elevation, slope, and southness are the most relevant primary topographic attributes among the tested attributes and their importance changes across seasons. Second, NDVI, NDII, and their seasonal variations have notably different relationships (including no relationship) with topography. Third, the observed topography–vegetation correlations ( r) tend to change – typically increase – with the coarsening of spatial scale. Lastly, the spatial autocorrelation of vegetation variables and topographic attributes are often comparable, and the comparability is more apparent when topography–vegetation correlations are stronger. In all, the topography–NDVI/NDII relations defined in this paper may improve the understanding of the multi-scale and property-specific role that mountain topography plays in the formation and seasonal change of vegetation patterns.

Noise‐induced vegetation patterns in fire‐prone savannas

The high degree of spatial organization of dryland vegetation has been often explained invoking a number of different deterministic mechanisms without ever explicitly addressing the role of noise in the process of pattern formation. Noise is usually believed to act on ecosystems as a source of disorganized random fluctuations. However, noise is also known for its ability to induce ordered states in nonlinear systems. An alternative mechanism is here proposed, which explains vegetation pattern formation in mesic and subhumid savannas as the joint effect of fire randomness and fire‐vegetation feedbacks. This mechanism is purely noise‐induced and has no deterministic counterpart.

Vegetation patterns induced by random climate fluctuations

Dryland vegetation is known for its ability to exhibit high degrees of spatial organization with well‐defined patterns of vegetated and unvegetated soil patches. Deterministic dynamics are usually invoked to explain the emergence of these patterns in arid ecosystems, while the effect of noise on the process of pattern formation has seldom been investigated. Relatively strong interannual rainfall fluctuations are a typical feature of dryland climates. Rainfall variability is commonly interpreted as a source of random disturbances for arid and semiarid vegetation, while its ability to play a more fundamental role in the process of vegetation pattern formation remains poorly understood. Here we show how random interannual climate fluctuations may indeed cause the emergence of vegetation patterns through the random alternation of stressed (i.e., water limited) and unstressed conditions. Thus, patterns emerge as a noise‐induced phenomenon from a mechanism of climate‐driven random switching of deterministic dynamics. Moreover, pattern formation is observed even when the deterministic dynamics of unstressed and stressed vegetation would ‐ separately ‐ lead to uniform spatial distributions of vegetation or bare soil.

A mathematical model of plants as ecosystem engineers

Understanding the structure and dynamics of plant communities in water-limited systems often calls for the identification of ecosystem engineers—key species that modify the landscape, redistribute resources and facilitate the growth of other species. Shrubs are excellent examples; they self-organize to form patterns of mesic patches which provide habitats for herbaceous species. In this paper we present a mathematical model for studying ecosystem engineering by woody plant species in drylands. The model captures various feedbacks between biomass and water including water uptake by plants’ roots and increased water infiltration at vegetation patches. Both the uptake and the infiltration feedbacks act as mechanisms for vegetation pattern formation, but have opposite effects on the water resource; the former depletes the soil-water content under a vegetation patch, whereas the latter acts to increase it. Varying the relative strength of the two feedbacks we find a trade-off between the engineering capacity of a plant species and its resilience to disturbances. We further identify two basic soil-water distributions associated with engineering at the single patch level, hump-shaped and ring-shaped, and discuss the niches they form for herbaceous species. Finally, we study how pattern transitions at the landscape level feedback to the single patch level by affecting engineering strength.

Self-organized formation of banded vegetation patterns in semi-arid regions: A model

We propose a grid model for the vegetation in bands, with special emphasis on those comprising the tiger bush in the Sahel. The model is mechanistic and involves three basic effects, the coupling of which leads to a redistribution of rainfall water. First, plants stimulate water infiltration and soil moisture. Second, there is a cutoff value for the soil moisture, below which vegetation cannot subsist. Third, soil infiltration capacity under the canopy is more easily exceeded in locations with larger annual rainfall, thus implying a larger transfer of overland water inside vegetated areas. The presence of a slope is modeled by making the overland flow unidirectional. The combination of these three effects conveys a partition of the landscape into runoff-dominated and infiltration-dominated areas, the ratio of which is a decreasing function of annual rainfall. On the other hand, the infiltration-dominated sector collects a larger share of available water in near arid conditions than it does when rainfall is more abundant. The result is a pattern of thin, but highly productive bands under low rainfall. With increasing precipitation, bands widen but become less productive. Specific zoning of a band along a transect emerges from simulations, which are carried out under both constant and varying precipitation. The issue of the upslope migration of bands is also examined. The results are in accordance with the conclusions by Valentin and d’Herbès [Valentin, C., d’Herbès, J.M., 1999. Niger tiger bush as a natural harvesting system. Catena, 37, 231–256] on measurements taken along a precipitation gradient in Niger.

Influence of river meandering dynamics on riparian vegetation pattern formation

The present work investigates how the dynamics of meandering rivers influence the formation of riparian vegetation patterns. To this aim, a model coupling river dynamics and riparian vegetation evolution was developed. Meandering dynamics were simulated with a fluid dynamical model using shallow water equations on an erodible bed. The riparian vegetation model takes into account some of the main actions caused by the river, i.e., water table oscillations, floods, and sedimentation. A logistic law and an exponential decay were used to model the increase and decrease in the biomass, respectively, consequent to river migration. The numerical simulation by the model highlights how river dynamics are able to induce typical vegetation patterns that are similar to some real riparian landscapes (some examples are shown). The results also show the role of (1) the relevant river‐vegetation interactions and (2) the temporal scales of vegetation evolution and river migration.

The influence of soil properties on the formation of unstable vegetation patterns on hillsides of semiarid catchments

On hillsides of semiarid catchments regular bands of vegetation have been observed to form under low rainfall conditions. Many authors have observed the existence of a slope gradient threshold below which no banded patterns were observed; this increases with the mean annual rainfall. A simple model for soil moisture balance and vegetation growth, explicitly accounting for basic soil physics, is demonstrated and discussed. The influence of relevant soil characteristics on vegetation patterns and their patchiness is addressed by linear stability analysis. The results confirm the experimental evidence of a threshold slope gradient depending on the mean annual net water supply, and demonstrate the influence of the soil properties (saturated conductivity and capillary rise) on the stability condition and on the threshold slope. Nevertheless, the concluding remark concerns the oversimplified models for vegetation patterns, such as the one discussed here, that require properly defined effective soil parameters in order to become predictive tools.

Reactive glass and vegetation patterns.

The formation of vegetation patterns in the arid and the semiarid climatic zones is studied. Threshold for the biomass of the perennial flora is shown to be a relevant factor, leading to a frozen disordered pattern in the arid zone. In this "glassy" state, vegetation appears as singular plant spots separated by irregular distances, and an indirect repulsive interaction among shrubs is induced by the competition for water. At higher precipitation rates, the diminishing of hydrological losses in the presence of flora becomes important and yields spatial attraction and clustering of biomass. Turing patterns with characteristic length scale emerge from the disordered structure due to this positive-feedback instability.

Self‐Organization of Vegetation in Arid Ecosystems

Scientists are still searching for possible unifying mechanisms to explain this range of spatial patterns (Tongway and Ludwig 2001), and an important question of this research is whether this range is the result of preexisting environmental heterogeneity, the result of spatial selforganization, or both (Klausmeier 1999; Couteron and Lejeune 2001; HilleRisLambers et al. 2001; Von Hardenberg et al. 2001). Here, we contribute to the ongoing debate about vegetation pattern formation in arid ecosystems by presenting novel, spatially explicit model analyses and results, extending on the work of HilleRisLambers et al. (2001). Our results show that these different vegetation patterns observed in arid ecosystems might all be the result of spatial self-organization, caused by one single mechanism: water infiltrates faster into vegetated ground than into bare soil, leading to net displacement of surface water to vegetated patches. This model differs from earlier model results (Klausmeier 1999; Couteron and Lejeune 2001; HilleRisLambers et al. 2001; Von Hardenberg et al. 2001) primarily in two ways: it is fully mechanistic, and it treats the lateral flow of water above and below the soil as separate, not independent, variables. Although the current model greatly simplifies the biophysics of arid systems, it can reproduce the whole range of distinctive vegetation patterns as observed in arid ecosystems, indicating that the proposed mechanism might be generally applicable. We further show that self-organized vegetation patterns can persist far into regions of high aridity, where plants would become extinct if homogeneously distributed, pointing to the importance of this mechanism for maintaining productivity of arid ecosystems (Noy-Meir 1973). Our analyses are based on the model first developed in HilleRisLambers et al. (2001)

Groundwater flow in spring-fed wetland and the formation of vegetation patterns in Hiroshima Prefecture, southwestern Japan

Groundwater flow in spring-fed wetland and the formation of vegetation patterns in Hiroshima Prefecture, southwestern Japan

VEGETATION PATTERN FORMATION IN SEMI-ARID GRAZING SYSTEMS

Hypotheses about the origin of vegetation pattern formation in semi-arid areas around the world almost all include a common feature of semi-arid areas: the presence of a positive feedback between plant density and water infiltration. We investigate whether this positive feedback and the spatial redistribution of runoff water are sufficient to explain vegetation pattern formation. For this purpose, we analyze a spatially explicit model consisting of partial differential equations using a method for demonstrating pattern formation (Turing analysis). Our analysis reveals that pattern formation can occur in semi-arid areas given only the positive feedback between plant density and local water infiltration coupled with the spatial redistribution of runoff water. Thus, slope and underlying heterogeneity are not essential conditions. Other factors in the model, such as herbivory, plant dispersal, rainfall, and drought tolerance of plants, appear to determine under what conditions pattern formation is likely but are not the primary factors that generate the patterns. The model is in agreement with field observations and indicates the conditions for which vegetation pattern formation can be expected in arid and semi-arid grazing systems.

Vegetation Pattern Formation in Semi-Arid Grazing Systems

Vegetation Pattern Formation in Semi-Arid Grazing Systems

Fire, Soil Heating, and the Formation of Vegetation Patterns in Chaparral

Fire, Soil Heating, and the Formation of Vegetation Patterns in Chaparral

Short range co-operativity competing with long range inhibition explains vegetation patterns

We model the non-local dynamics of vegetation communities and interpret the formation of vegetation patterns as a spatial instability of intrinsic origin: the wavelength of the patterns predicted within the framework of this approach is determined by the parameters governing the dynamics rather than by boundary conditions and/or geometrical constraints. The spatial periodicity results from an interplay between short-range co-operative interactions and long-range self-inhibitory interactions inside the vegetation community. The influence of environmental anisotropies on pattern symmetry and orientation is discussed. As a case study, the approach is applied to a system of vegetation bands situated in the north-west of Burkina Faso. The parameters describing the co-operative and inhibitory interactions at the origin of the patterns are evaluated.