Exploring the effects of rainfall variability on banded vegetation

<p><span>Vegetation self-organisation in water-limited ecosystems in semi-arid climates has been studied by means of numerical simulation using a set of reaction-diffusion-equations. The predominant approach in such studies, in particular relating to banded vegetation on slopes, has been to study the long-term steady ecohydrological tates on periodic domains forced by steady rainfall. This default modelling setup does not account for the fact that on a hillslope a net runoff loss may exist at the outlet. Moreover, such runoff loss is modulated by rainfall intensity, i.e., increasing rainfall intensity is likely to favour runoff over infiltration, and therefore affect the banded vegetation formation. Additionally, different inter-storm dry periods prompt different responses from the vegetation. One of the properties of semi-arid climates is a highly intermittent and variable precipitation regime, quite often with a few intense events and a larger number of very mild events. Additionally ecohydrological theory recognises that dryland ecosystems are in a quasi-permanent transient condition, exhibiting non-linear and far-from equilibrium responses to boundary conditions and forcings. The mismatch between the default modelling approach and the properties of rainfall in such systems calls for further complexity in the models and in the forcing.</span></p><p> </p><p><span>We explore the possible effects that particular rainfall properties can have in banded vegetation dynamics. We solve the well-known Rietkerk model together with a zero-inertia approximation of the shallow-water equations for surface flow. A non-periodic domain with an outlet, i.e., a 2D hillslope with a constant slope is used. We perform simulations forced by a set of variations of idealised temporal distributions of rainfall throughout a year. The reference distribution is a periodic signal of constant intensity storms of a single day, separated by dryspells of equal duration. The total annual rainfall was selected as 270 mm, in the range of semi-arid climates. This annual signal is repeated during the entire simulation. Non-periodic rainfall signals were generated by randomising a single rainfall property but ensuring the same annual rainfall. Randomisations of the inter-storm dryspell duration, the storm duration, and the storm intensity were explored. Although this results also in idealised rainfall signals, it allows for systematic analysis of each property. </span></p><p> </p><p><span>The banded patterns are assessed both in terms of global signatures (biomass, vegetation cover), spatial properties (number of bands, wavelength and bandwidth), and dynamics (migration velocity of the bands). Our results clearly show qualitatively and quantitatively that the simulated banded vegetation has a strong response to rainfall variability. Moreover, the results also show a high sensitivity to the particular succession of events, e.g., a succession of longer than average dryspells can throw the system away from equilibrium. High sensitivity is also observed to the timing of certain individual events. The system responds differently to events which happen early on in the development, or later, when the system is near equilibrium. The simulated response of the system are arguably too volatile, suggesting that improvements in the vegetation model parametrisation and formulation are warranted to better represent dynamics and allow for stability and resilience studies.</span></p>

Soil carbon and soil moisture dynamic redistribution in a banded ecosystem

<p>Arid and semiarid environments accounts approximately 30% of the Earth’s continental surface and are especially sensitive to degradation or loss of their ecosystem functionality. In these ecosystems, vegetation patterns (e.g. banded vegetation) can be found as the adaptive response of the system to resource redistribution (runoff and sediments) and limitation (soil moisture and nutrients). The patterns consist on alternating densely vegetated bands (or ‘groves’) and bare areas (or ‘intergroves’), and can be found in large regions of Africa, Asia, Australia and North America. Understanding the mechanism that regulate banded vegetation ecosystems is critical in order to identify the dynamic behaviour and maintain their functionality. In this work, we model the spatial distribution of soil moisture and soil organic carbon, in order to analyse how differences on the availability of resources can explain the functionality of the banded vegetation systems. We are studying a catchment in Bond Springs, 25 km north of Alice Springs, characterized by the presence of Acacia Aneura trees (Mulga) aligned in bands along the terrain. We use a new model: COPLAS, a tool that couples a Landform Evolution Model with dynamic vegetation and carbon pools modules. It tracks the carbon from the photosynthesis until it becomes soil carbon and the mobilization/redistribution due soil erosion. Results of the model were compared with fieldwork conducted in fifty-three soil samples and terrain surveying with unmanned aerial vehicle. Our results indicate good agreement between the model and the measurements. We found that soil moisture uphill the bands is around 33% more than downhill, and close to 120% more than in bare soil. This result could be explained because a portion of the runoff, generated from bare intercanopy patches, is redistributed downslope and infiltrated uphill the vegetated areas. Moreover, soil carbon is 20% more downhill than uphill the bands because of deposited alluvium and litter downhill and possible less microbial respiration and decomposition due smaller soil moisture content. Additionally, we found a tendency of higher soil carbon concentrations going downhill the catchment. Overall, these findings show the heterogeneous distribution of resources in the area that could explain the ecosystem functionality and highlight the importance of modelling and measuring arid and semiarid ecosystems in order to understand their dynamic behaviour.</p>

Existence of Periodic Traveling Waves in the Klausmeier Model of Desertification and its Modification: A Comparative Study

Self-organized and spatially periodic banded vegetation patterns have been observed in many semi-arid ecosystems. In order to understand the mechanism of these patterns, we consider a system of reaction-advection-diffusion equations in a two-variable model of desertification. This work deals with the investigation of the existence of periodic traveling waves in a one-parameter family of solutions. In addition, we investigate the existence of periodic traveling waves as a function of water transport parameter in the model. GANIT J. Bangladesh Math. Soc.Vol. 38 (2018) 27-46

A topographic mechanism for arcing of dryland vegetation bands

Banded patterns consisting of alternating bare soil and dense vegetation have been observed in water-limited ecosystems across the globe, often appearing along gently sloped terrain with the stripes aligned transverse to the elevation gradient. In many cases, these vegetation bands are arced, with field observations suggesting a link between the orientation of arcing relative to the grade and the curvature of the underlying terrain. We modify the water transport in the Klausmeier model of water–biomass interactions, originally posed on a uniform hillslope, to qualitatively capture the influence of terrain curvature on the vegetation patterns. Numerical simulations of this modified model indicate that the vegetation bands arc convex-downslope when growing on top of a ridge, and convex-upslope when growing in a valley. This behaviour is consistent with observations from remote sensing data that we present here. Model simulations show further that whether bands grow on ridges, valleys or both depends on the precipitation level. A survey of three banded vegetation sites, each with a different aridity level, indicates qualitatively similar behaviour.