Vegetation In Semi-arid Environments Research Articles

Contribution of Acacia senegal to biomass and soil carbon in plantations of varying age in Sudan

The carbon (C) stocks of drylands, particularly in Africa, have become considerably depleted through land degradation leading to loss of soil fertility and water retention. Increasing tree cover is considered a practical means of increasing soil organic carbon (SOC) contents. However, few field studies have been carried out in the eastern part of the Sahel zone to test this hypothesis. In this study the C stocks in biomass and soil were determined in Acacia senegal plantations of varying age (7–24years) and adjacent open grasslands in two locations in Sudan. The contribution of C3 plants to SOC was estimated using a simple δ13C partitioning model. Total biomass C stocks increased with plantation age, reaching 1020gm−2. While most of this increase was due to the trees, the C stock of the ground vegetation also increased with plantation age and was greater in the oldest plantations than in the open grassland. This finding supports the hypothesis that trees facilitate ground vegetation in semi-arid environments. Plantation SOC stocks (0–50cm) ranged from 846 to 1250gm−2, increased with age and were greater than open grassland values (867–950gm−2). The contribution of C3 plants (trees and herbs) to plantation under canopies SOC stocks (0–10cm) was 47.2% and 53.4% at the two sites. SOC sequestration rates up to 19gCm−2yr−1 were achieved in the oldest plantations at one of the sites. Our findings indicate that planting trees in drylands could considerably increase SOC stocks and efforts to plant Acacia trees in the Sahel should therefore be encouraged.

Pattern selection and hysteresis in the Rietkerk model for banded vegetation in semi-arid environments.

Banded vegetation is a characteristic feature of semi-arid environments. It occurs on gentle slopes, with alternating stripes of vegetation and bare ground running parallel to the contours. A number of mathematical models have been proposed to investigate the mechanisms underlying these patterns, and how they might be affected by changes in environmental conditions. One of the most widely used models is due to Rietkerk and co-workers, and is based on a water redistribution hypothesis, with the key feedback being that the rate of rainwater infiltration into the soil is an increasing function of plant biomass. Here, for the first time, we present a detailed study of the existence and stability of pattern solutions of the Rietkerk model on slopes, using the software package wavetrain (www.ma.hw.ac.uk/wavetrain). Specifically, we calculate the region of the rainfall-migration speed parameter plane in which patterns exist, and the sub-region in which these patterns are stable as solutions of the model partial differential equations. We then perform a detailed simulation-based study of the way in which patterns evolve when the rainfall parameter is slowly varied. This reveals complex behaviour, with sudden jumps in pattern wavelength, and hysteresis; we show that these jumps occur when the contours of constant pattern wavelength leave the parameter region giving stable patterns. Finally, we extend our results to the case in which a diffusion term for surface water is added to the model equations. The parameter regions for pattern existence and stability are relatively insensitive to small or moderate levels of surface water diffusion, but larger diffusion coefficients significantly change the subdivision into stable and unstable patterns.

Mucilage secretion: an adaptive mechanism to reduce seed removal by soil erosion?

Diaspores of many plant species inhabiting open vegetation in semi-arid environments secrete mucilage after wetting (myxospermy) that glues the diaspores to the ground and prevents movement when the mucilage dries. In the present study, we test whether mucilage secretion can be considered as a selective response to soil erosion in plant species inhabiting semi-arid environments. We relate the amount and type of mucilage secretion by seeds of Helianthemum violaceum and Fumana ericifolia (Cistaceae) to the number of raindrop impacts needed to remove these seeds after gluing them with their own mucilage to the ground and also the time that these seeds resist water run-off without detaching. We also compare the amount of seed mucilage production by plants growing in habitats without erosion and plants affected by severe erosion by fitting mixed effect models. Our results show an important phenotypic variation in the amount of mucilage secretion in both species, although it is suggested that the effect of mucilage secretion in the rate of seed removal by erosion is species- and mechanism-dependent. For F. ericifolia, the amount of mucilage secreted by the seeds is directly proportional to their resistance to raindrop impacts and is positively related to the intensity of the erosive processes that the plants experience. Nevertheless, all the seeds resist the force of run-off during 60 min, irrespective of the amount of mucilage they produce. In H. violaceum, mucilage secretion per se, and not the amount of mucilage produced by the seeds, has an effect on the rate of seed removal by erosive processes. Furthermore, cellulosic fibrils were found only in the mucilage of F. ericifolia but not in H. violaceum. Overall, our results only partially support the hypothesis that a selective response to soil erosion exists. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 241–251.

History-dependent patterns of whole ecosystems

Spatial patterns at the landscape scale have been documented in a wide variety of ecosystems across many parts of the world. Mathematical models have played an important role in understanding the causes of these patterns, and their implications for ecosystem change as environmental parameters vary. Preliminary results from simulation studies suggest the possibility of hysteresis, meaning that the wavelength and other properties of the pattern will vary in a history-dependent manner. This paper presents a detailed study of this phenomenon for two established models of landscape-scale patterns: the model of Klausmeier (Science 284 (1999) 1826–1828) for banded vegetation in semi-arid environments, and the model of van de Koppel et al. (American Naturalist 165 (2005) E66–E77) for patterning in young mussel beds. In both cases, the author demonstrates history-dependent patterns. Moreover, he shows how a knowledge of pattern existence and stability enables a detailed understanding of this hysteresis.

Pattern Solutions of the Klausmeier Model for Banded Vegetation in Semiarid Environments V: The Transition from Patterns to Desert

Vegetation in semideserts often self-organizes into spatial patterns. On gentle slopes, these typically consist of stripes of vegetation running parallel to the contours, separated by stripes of bare ground. The Klausmeier model is one of the oldest and most established of a number of mathematical models for this “banded vegetation.” The model is a system of reaction-diffusion-advection equations. Under the standard nondimensionalization, one of its dimensionless parameters ($\nu$) reflects the relative rates of water flow downhill and plant dispersal and is therefore very large. This paper is the fifth and last in a series in which the author provides a detailed analytical understanding of the existence and form of pattern solutions (periodic travelling waves) of the Klausmeier model, to leading order as $\nu\rightarrow\infty$. The problem is a very rich one because the underlying mathematics depends fundamentally on the way in which the migration speed $c$ scales with $\nu$. This paper concerns the case $1=O(c)$ and $c=o(\nu^{1/2})$ as $\nu\rightarrow\infty$. The author derives leading order expressions for the curves bounding the parameter region giving patterns, and for the pattern forms in this region. An important consequence of this is leading order formulae for the maximum and minimum rainfall levels for which patterns exist. The author demonstrates via numerical simulations that a decrease in rainfall through the minimum level for patterns causes a transition to full-blown desert that cannot be reversed by increasing the rainfall again.

Pattern Solutions of the Klausmeier Model for Banded Vegetation in Semiarid Environments IV: Slowly Moving Patterns and Their Stability

Banded vegetation is a characteristic feature of semiarid environments, comprising stripes of vegetation running parallel to the contours on hillsides, separated by stripes of bare ground. Mathematical modelling plays an important role in the study of this phenomenon, and the Klausmeier model is one of the oldest and most established, consisting of coupled reaction-diffusion-advection equations for plant biomass and water density. In this model the dimensionless parameter corresponding to slope gradient is much larger than the other parameters, and this paper is part of a series investigating the asymptotic form of pattern solutions for large values of the slope parameter. The pattern solutions move uphill with a constant speed, $c$ say, and the focus of this paper is $c=O(1)$. I begin by deriving the leading order form of the patterns, and the region of parameter space in which they occur, for $c=o(1)$. Using this, I show that all patterns with $c=o(1)$ are unstable as model solutions for sufficiently large values of the slope parameter. I then consider patterns with $c=O_s(1)$, showing that this region of parameter space contains both stable and unstable low amplitude patterns. I conclude by discussing the ecological implications of my results.

Pattern solutions of the Klausmeier model for banded vegetation in semi-arid environments III: The transition between homoclinic solutions

Self-organised patterns of vegetation are a characteristic feature of semi-deserts. On hillsides, these typically comprise vegetation bands running parallel to the contours, separated by regions of bare ground (“tiger bush”). The present study concerns the Klausmeier mathematical model for this phenomenon [C.A. Klausmeier, Regular and irregular patterns in semiarid vegetation, Science 284 (1999) 1826–1828], which is one of the earliest and most influential of the various theoretical models for banded vegetation. The model is a system of reaction–diffusion–advection equations, and after rescaling it contains three dimensionless parameters, one of which (the slope parameter) is much larger than the other two. The present study is the third in a series of papers in which the author exploits the large value of the slope parameter to obtain leading order approximations to the parameter regions in which patterns exist, and to the form of these patterns. The boundary of the parameter region giving patterns consists in part of two loci of homoclinic solutions, that are homoclinic to different steady states. The present paper concerns behaviour for parameters close to the intersection point of these loci. The author shows that this part of parameter space divides naturally into three regions, with a different solution structure in each. In one region, the solution corresponds to a limit cycle of a reduced system of ordinary differential equations; the other two regions involve multiple matched layers. As part of the analysis, the author derives formulae for the homoclinic solution loci, and for the location of their intersection. All of the results are valid to leading order for large values of the slope parameter. The author presents a detailed numerical verification of his analytical results. The paper concludes with discussions of the ecological implications of the results, and the main outstanding mathematical questions.

Pattern solutions of the Klausmeier model for banded vegetation in semi-arid environments II: patterns with the largest possible propagation speeds

Pattern formation at the ecosystem level is a rapidly growing area of spatial ecology. The best studied example is vegetation stripes running along contours in semi-arid regions. Theoretical models are a widely used tool for studying these banded vegetation patterns, and one important model is the system of advection–diffusion equations proposed by Klausmeier. The present study is part of a series of papers whose objective is a comprehensive understanding of patterned solutions of the Klausmeier model. The author focuses on the region of parameter space in which the propagation speed of the patterns is close to its maximum possible value. Exploiting the large value of one of the model parameters, a leading order approximation is obtained for the maximum propagation speed, and the author undertakes a detailed investigation of the parameter region in which there are patterns with speeds close to this maximum.

Pattern solutions of the Klausmeier Model for banded vegetation in semi-arid environments I

In many semi-arid environments, vegetation cover is sparse, and is self-organized into large-scale spatial patterns. In particular, banded vegetation is typical on hillsides. Mathematical modelling is widely used to study these banded patterns, and many models are effectively extensions of a coupled reaction–diffusion–advection system proposed by Klausmeier (1999 Science 284 1826–8). However, there is currently very little mathematical theory on pattern solutions of these equations. This paper is the first in a series whose aim is a comprehensive understanding of these solutions, which can act as a springboard both for future simulation-based studies of the Klausmeier model, and for analysis of model extensions. The author focusses on a particular part of parameter space, and derives expressions for the boundaries of the parameter region in which patterns occur. The calculations are valid to leading order at large values of the ‘slope parameter’, which reflects a comparison of the rate of water flow downhill with the rate of vegetation dispersal. The form of the corresponding patterns is also studied, and the author shows that the leading order equations change close to one boundary of the parameter region in which there are patterns, leading to a homoclinic solution. Conclusions are drawn on the way in which changes in mean annual rainfall affect pattern properties, including overall biomass productivity.

Integrated Ecohydrologic Research and Hydro‐Informatics

The purpose of this paper is to offer our view of where ecohydrologic research will be going in the next 20 years and suggest how enabling technologies from hydro-informatics will support this research. Two decades ago Klemes (1986) suggested that the hydrologist’s “efforts expended on the fitting of flood and drought frequency curves would be better spent in acquiring deeper knowledge of climatology, meteorology, geology, and ecology.” Klemes was of course calling for interdisciplinary hydrology. Recently, a number of community reports have proposed a more interdisciplinary approach to hydrology, including the development of community infrastructure such as large scale hydrologic observatories with integrated, multi-scale monitoring and advanced informatics tools to enable this research (Band et al. 2002, Gupta et al. 1999, Hornberger et al. 2000, Maidment 2008). Specific calls were included to integrate the more physically or statistically oriented approaches in hydrology with ecosystem sciences including biogeochemical cycling and population ecology. The emergence of ecohydrologic research is one example of how hydrologic science has begun to move in this direction. Ecohydrologic research seeks to understand how hydrological processes affect biological communities, and in turn how such communities affect water cycling (Newman et al. 2006, Rodriguez-Iturbe 2000). With this marriage of ecology and hydrology new avenues of research are opening up, and with these come new scientific and technical challenges. Some of the scientific challenges relate to the long-term memory in biological and geomorphic systems, complex feedbacks on water cycling, and the continuum of such interactions across space. Technical challenges include building more sophisticated simulation models to deal with such spatial dynamics, acquiring and managing the data needed to support these models, improving geo-visualization of spatial predictions and errors, and quantifying uncertainty associated with model structure and parameterization. Another interdisciplinary subfield of hydrology, hydroinformatics, emphasizes the development of information technology to help meet these challenges. Newman et al. (2006) identified a number of research challenges for ecohydrologic research in semi-arid regions, including dealing with spatial and temporal heterogeneity, scaling up to regional and global extent, improving understanding of subsurface processes, and addressing long-term processes. They argue for a greater emphasis on place-based research where long-term data sets are being compiled. Efforts aimed at addressing these problems are underway, albeit with a focus on vegetation in semi-arid environments, equilibrium models with stochastic inputs, and knowledge obtained in traditional plots or stands. We suggest that for ecohydrologic research to be globally relevant it must embrace the full spectrum of environments, including non-water limited regions and wetland-rich regions. Over the next two decades, ecohydrologic research will explore more deeply the nature of transient system evolution and elucidate characteristic timescales of processes, such as those associated with ecosystem aggradation and degradation. It will move toward developing predictive capability that builds from an understanding of processes along spatial gradients, including adaptations of nutrient cycling and plant hydraulics at wetland-upland transitions. Moreover, as cyber-infrastructure improves these activities will transcend individual study sites by utilizing combinations of data sets 16

Factors regulating spatial distribution of soil water content in small semiarid catchments

The factors which control soil moisture patterns in a semiarid area are determined in two different locations: a burnt zone with little vegetation and an unburnt zone with a greater vegetal cover. In the burnt area, the factors affecting the spatial variability of the soil water content are those considered as local controls such as soil texture and slope. These factors are able to explain a significant part of the spatial distribution of soil moisture in this zone independently of the soil moisture state. In the unburnt area, the factors affecting soil moisture were those related with the presence or absence of vegetation in semiarid environments. The upslope contributing area, aspect, soil profile curvature and soil depth best explained the spatial variability of the soil moisture content in the vegetated zone. The actual influence of these factors showed marked seasonal variations due to changes in the physiological activity of the vegetal cover.These factors, which are in part responsible for the distribution of soil moisture, are not commonly integrated in traditional topographic wetness indices. The topographic index of Beven and Kirkby (Beven, K.J., Kirkby, M., 1979. Hydrological Sciences Bulletin 24, 43–69) is extended to account for the interstorm soil water losses. The new indices incorporating the hillslope aspect improve the prediction power of soil water content patterns in semiarid areas.