Anton Imeson

VEGETATION PATCHES AND RUNOFF–EROSION AS INTERACTING ECOHYDROLOGICAL PROCESSES IN SEMIARID LANDSCAPES

Ecological and hydrological processes can interact strongly in landscapes, yet these processes are often studied separately. One particularly important interaction between these processes in patchy semiarid lands is how vegetation patches serve to obstruct runoff and then how this retained water increases patch growth that, in turn, provides feedbacks to the system. Such ecohydrological interactions have been mostly demonstrated for semiarid landscapes with distinctly banded vegetation patterns. In this paper, we use data from our studies and from the literature to evaluate how strongly four ecohydrological interactions apply across other patchy semiarid vegetations, and how these interactions are affected by disturbances. We specifically address four questions concerning ecohydrological interactions: (1) if vegetation patches obstruct runoff flows during rainfall events, how much more soil water is stored in these patches compared to open interpatch areas; (2) if inputs of water are higher in patches, how much stronger is the pulse of plant growth compared to interpatches; (3) if more soil water in patches promotes greater biological activity by organisms such as earthworms that create macropores, how much does this improve soil infiltrability; and (4) if vegetation patches are damaged on a hillslope, how much does this increase runoff and erosion and decrease biomass production? We used the trigger-transfer-reserve-pulse framework developed for Australian semiarid woodlands to put these four questions into a landscape context. For a variety of patchy semiarid vegetation types in Australia, Europe, and North America, we found that patches significantly stored more soil water, produced more growth and had better infiltrability than interpatches, and that runoff and erosion can markedly increase on disturbed hillslopes. However, these differences varied greatly and appeared to depend on factors such as the intensity and amount of input events (rainstorms) and type of topography, soils, and vegetation. Exper- imental and modeling studies are needed to better quantify how these factors specifically affect ecohydrological interactions. Our current findings do support the conclusion that vegetation patches and runoff-erosion processes do strongly interact in many semiarid landscapes across the globe, not just banded landscapes.

Influence of soil properties on the aggregation of some Mediterranean soils and the use of aggregate size and stability as land degradation indicators

Abstract Soil aggregation in relation to other soil properties was studied along a climatological transect in the Southeast of Spain. Three sites were selected along this transect ranging from semiarid to subhumid climatological conditions. The aggregate size distribution, the macro and microaggregate stability of the superficial soil horizon and their relations with other soil properties were analysed. Large aggregates (>10, 10–5, 5–2 mm) were present in highest proportions in the most arid of the studied areas. These large aggregates were associated with high values of water-stable microaggregates; however, they did not improve soil structure and are related to high bulk densities and low water retention. Aggregates 1–0.105 mm were positively correlated to medium, fine, very fine sand and silt fractions and to organic matter. Aggregates Water stability of microaggregates showed a positive correlation with clay content while the stability of the macroaggregates depended on the organic matter content when the organic matter content was greater than 5 or 6%. Below that threshold the carbonate content was strongly correlated with aggregate stability. A general conceptual scheme of associations between aggregate size distribution, water-stable microaggregation and textural characteristics for the area was developed. The land use history affecting soil overlaps the pattern of climatological situations and has to be taken into account. Aggregate size distribution and stability can be used as indicator of soil degradation, but not as a unique parameter.

THE IMPACT OF CLIMATE CHANGE ON GEOMORPHOLOGY AND DESERTIFICATION ALONG A MEDITERRANEAN- ARID TRANSECT

From the perspective of geomorphology, three important aspects of climate should be considered if conditions become more arid: (a) any decrease that might occur in the annual rainfall amount; (b) the duration of rainfall events; and (c) any increase in the intervals between rainfall events. These, together with increasing temperature, lead to less available water, less biomass and soil organic matter content and hence to a decrease in aggregate size and stability. As a consequence, the soil permeability decreases, soils develop surface crusts and infiltration rates decrease dramatically. Such changes in vegetation cover and soil structure lead to an increase in overland flow and in the erosion of the fertile topsoil layer. Positive feedback mechanisms may reinforce these eAects and lead to desertification. This paper considers the results of field investigations into the spatial variability of a number of ‘quick response’ variables at two scales: the regional and the plot scales. Concerning the regional scale spatial variability, results of experimental field work conducted along a climatic transect, from the Mediterranean climate to the arid zone in Israel, show that: (1) organic matter content, and aggregate size and stability decrease with aridity, while the sodium adsorption ratio and the runoA coeAcient increase; and (2) the rate of change of these variables along the climatic transect is non-linear. A steplike threshold exists at the semiarid area, which sharply separates the Mediterranean climate and arid ecogeomorphic systems. This means that only a relatively small climatic change would be needed to shift the borders between these two systems. As many regions of Mediterranean climate lie adjacent to semiarid areas, they are threatened by desertification in the event of climate change. Concerning spatial variability at the plot scale, diAerent patterns of overland flow generation and continuity characterize hillslopes under diAerent climatic conditions. While in the Mediterranean climate area infiltration is the dominant process all over the hillslope, in the arid area overland flow predominates. In contrast to the uniform distribution of processes in these two zones, a mosaic-like pattern, consisting of locally ‘arid’ water contributing and ‘moist’ water accepting patches is typical of the transitional semiarid area. Such pattern is strengthened by fires or grazing which are characteristic of this area. The development of such mosaic pattern enables most rainfall to be retained on hillslopes. Changes in the spatial pattern of contributing versus accepting waterareas can be used as an indicatorof desertification and applied to developing rehabilitation strategies. #1998 John Wiley & Sons, Ltd.

Runoff generation, sediment movement and soil water behaviour on calcareous (limestone) slopes of some mediterranean environments in southeast Spain

Abstract An interpretation of soil hydrology and erosion data obtained from limestone areas in southeast Spain is presented in the framework of recent hypotheses on runoff generation mechanisms in Mediterranean environments. The main objective is to synthesise and harmonise the data in theoretical concepts or behavioural models of hydrological functioning of Mediterranean limestone slopes. For this purpose, data were collected in a typical limestone area, with climatic characteristics ranging from subhumid to semiarid. Several sites were chosen with comparable slopes, vegetation and soils along a transect of 20 km to carry out several research projects. The data discussed in this paper consist of information on water redistribution within the soil profiles obtained from long duration (up to 5 h) rainfall simulation experiments with continuous monitoring of soil moisture at different depths. Runoff and sediment concentration data were obtained (i) from rainfall simulation experiments with different antecedent soil moisture conditions and (ii) as natural results from open Gerlach plots on runoff and sediment movement over 4 years. Three conceptual models of water redistribution during the infiltration process could be observed, two of them imply nonuniform water redistribution within the soil profile due mainly to macropore flow caused by specific soil surface characteristics. At the plot scale, runoff generated in bare patches is mainly Hortonian. In plots with previously wetted soils or soils with high infiltration capacities, the generated runoff implies the saturation of the upper soil. Both types of runoff are discontinuous through time and space. The runoff generation mechanisms at the slope scale have been synthesised into two conceptual models: a Hortonian discontinuous runoff model that takes place in the most degraded slopes or during high intensity rain events and, a mixed runoff generation model in less degraded slopes or in previously wet soils, where infiltration excess runoff as well as saturation excess runoff can happen on the same slope. In both cases, slopes behave as a patchwork of runoff and runon areas, the size of the runoff or runon patches is dependent on the climatological conditions. These control the hydrological disconnection between different parts of the slopes. Hortonian and saturation runoff can both be generated and infiltrated downslope.

The evolution and significance of soil-vegetation patterns following land abandonment and fire in Spain

Examples are presented from two locations in SE and NE Spain where patterned or banded vegetation are found on semi-natural and abandoned land or where vegetation is recovering from wildfire. In both cases patterns are being investigated as process-pattern phenomena with the aim of understanding how different kinds of environmental gradients influence pattern evolution. On abandoned land, patterns occur at different scales. At the patch scale there are areas where Plantago albicans germinates in cracks and influences the accumulation of silty material. At the slope scale these form elongated steps that create a characteristic micro-topography. At the patch and slope scale Stipa tenacissima tussocks form an hexagonal pattern on level areas where water infiltrates in and around the tussocks. On sloping areas the S. tenacissima tussocks form parallel ovoid bands. They intercept fine and coarse material being eroded on the slopes by both overland flow and the hooves of sheep and goats. This also creates a distinctive micro-topography. Rainfall simulation experiments were undertaken in combination with monitoring activities in order to investigate the effects that key-processes of sediment and water movement have on the patterns. Other methods include controlled experiments and modelling. Biologically driven erosion pro- cesses are very important as key processes. Positive feedback mechanisms are important at various stages in the evolution of the pattern. The patterns studied play an important role in creating more favourable micro-environments where vegetation recovers first after disturbances. This is particu- larly the case following wildfire. The first post-fire rain produces patterns in ash and litter around sites, concentrating these at locations where shrubby vegetation subsequently resprouts or becomes

Deriving indicators of soil degradation from soil aggregation studies in southeastern Spain and southern France

Soil degradation is perceived as a major threat in the Mediterranean region due to changes in land-use and possible future climate change. Soil aggregation parameters are used here to demonstrate their potential as a key-indicator for land degradation studies. The monitoring of these indicators offers a means of establishing the vulnerability and resilience of geo-ecosystems. Soil aggregation stability and distribution were studied on soils with an open shrubby vegetation cover, from several places in southeastern Spain and southern France, by applying drop tests and determining aggregate size distributions. Aspect and vegetation cover were incorporated in the soil sampling. Several indices were derived from these analyses to indicate the degree of soil aggregation. This was done by referencing to a base level of aggregation (bare soil aggregation). It was found that soil aggregates were more stable and were often coarser under vegetation, when compared to their immediate surrounding bare areas. A similar, slightly less clear effect was noted on N-facing exposed slopes when compared to S-facing exposed slopes. Long-term changes were found by studying cultivated land, abandoned fields and land covered by semi-natural vegetation, on comparable substrate and comparable land units. It is clear that soil aggregation and aggregate stability increases with time (years). It is argued that soil aggregation indices can be used as a key-indicator for degradation processes at a fine scale with implications for runoff and sediment generating processes at the hillslope scale.

SPATIAL AND SHORT-TERM TEMPORAL VARIATIONS IN RUNOFF, SOIL AGGREGATION AND OTHER SOIL PROPERTIES ALONG A MEDITERRANEAN CLIMATOLOGICAL GRADIENT

Abstract Physical and chemical soil properties were measured along a mountainous climatological gradient in the province of Alicante (Spain). The objective was to evaluate how the climate affects certain soil properties at different temporal and spatial scales. These properties include infiltration, runoff and sediment concentrations resulting from rainfall simulation experiments performed in winter and in summer. Chemical soil properties like carbonate content, organic matter content and CEC were analysed in reference soil profiles along the gradient. Physical soil properties like soil moisture content, macroaggregation and waterstable microaggregation were measured at monthly intervals during a year. The comparison of the results was done at different spatial (site, slope and patch) and temporal (monthly and seasonal) scales by means of some statistical tests. It can be concluded that there are some soil properties positively related to the gradient, like organic matter, clay content and CEC which increase with the annual rainfall. However, runoff coefficients and erosion are higher when the climatic annual rainfall. However, runoff coefficients and erosion are higher when the climatic conditions become more arid. Aggregation and infiltration capacity are higher on north-facing slopes and in vegetated patches than in south-facing slopes and in bare patches.

Soil erosion and climate change: the transect approach and the influence of scale

Abstract Studies of geo-ecological processes are being made along climatological transects on similar limestone rocks at different locations across the Mediterranean. The main objectives of the research are firstly, to gain insight into the influence of climate on key-geomorphological process–pattern relationships that characterise different locations along the transect; and secondly, to obtain a better insight into the possible impact of climate change on ecosystem degradation. The paper begins by considering the possible methodological approaches that can be applied to investigate the impact of climate change on ecosystem degradation in complex ecosystems and explains why a nonlinear evolutionary modelling framework was chosen for the transect studies. The conceptual basis of the methodology is presented and the transect approach described. The research methodology takes into account the influence of climate at four different scales (landscape, slope, response unit and patch). Field research at these scales is oriented towards identifying and understanding the key processes and to identifying key parameters that can be monitored to establish change. Some results are presented from the Judean Desert transect in Israel to show how a key indicator, in this case aggregate stability, varies with temperature and to show how process–pattern features vary along the limestone transect from the area having a Mediterranean climate to the desert. Both conceptual and practical models of soil erosion need to address scale issues. In particular, as time and spatial scales change, so also does the relative significance of different processes. It is concluded that process–pattern phenomena are useful in this context and that they can be applied to the problem of up-scaling.

The erosional response of Calcareous soils along a climatological gradient in Southeast Spain.

Abstract The erosional response of calcareous soils along a climatological gradient in Alicante, SE Spain was studied. The erosional response was defined in terms of indicators obtained from field rainfall simulation experiments and laboratory studies of soil aggregation. Measurements were made at seven sites on limestones and marls between Benidorm with an annual average rainfall of 400 mm yr −1 and Coll de Rates (annual rainfall 900 mm yr −1 ). The sites were on matorral vegetation affected to varying intensities by grazing and fire. At the seven locations, soil aggregation was studied by sieving, by the single water drop test and by a Microscan particle-sizer. Rainfall simulation experiments were made on circular 55-cm diameter target area plots using the Calvo simulator at an intensities between 35 and 59 mm h −1 . On limestone, it was found that on bare surfaces, the coarse soil aggregation increases with depth at the highest sites and decreases at the lower sites. The Benidorm surface soil has the highest erodibility. The least stable soils are at the lower intermediate sites. On marls, the aggregate stability is generally high in contrast to those under cultivation. Simulated runoff from the wetter limestone sites is very low. The driest areas have the highest runoff and sediment yields. Trends with climate suggest that the erosional responses of the soils indeed increase with aridity and temperature but local factors have a large effect, particularly at intermediate altitudes. A clear relationship like that found in Israel by Lavee on a similar lithology does not exist.

Investigation of relationships between measured field indicators and erosion processes on badland surfaces at Petrer, Spain

Abstract Indicators were obtained to measure and predict rill erosion and mass wasting in biancana and calanchi-like badland areas. Rainfall simulation experiments were used to find a relationship between soil moisture change and sediment concentration behaviour. This relationship appeared to be able to show, in its role as indicator, differences between materials affected by various degrees of rill erosion or mass wasting. Indicators explaining this relationship are surface characteristics, runoff and infiltration characteristics and chemical and mineralogical soil properties. From the experimental study it was concluded that the high infiltration capacity in materials affected by mass wasting resulted in an unstable soil and a high degree of erodibility. The materials which were more affected by rill erosion or had a continuous crust at the surface were saturated earlier during the experiment than the other materials. This saturation caused an increase in runoff. The mechanism is explained as the system crossing a threshold after which flow hydraulics at the surface are more important for erosion processes than dynamic soil properties and infiltration characteristics.