Pariente Sarah

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.

Enzyme activities along a climatic transect in the Judean Desert

Abstract Soil enzymes have an important influence on nutrient cycling. We examined spatial and temporal patterns in dehydrogenase, arylsulfatase, alkaline and acid phosphatase activities, and their relationships with organic carbon and microbial biomass nitrogen at three sites in Israel representing different climatic regions: Mediterranean (humid), mildly arid and arid. The sites were selected along a climatic transect from the Judean Mountains in the west to the Dead Sea in the east of Israel. With increasing aridity, soil organic carbon, soil microbial biomass nitrogen, dehydrogenase, phosphatase and different pools of arylsulfatase activities decreased significantly. A sharp change in enzyme activities existed between 260- and 120-mm mean annual rainfall. The arylsulfatase activity of the microbial biomass in the 0–2- and 5–10-cm soil layers usually accounted for more than 50% of the total activity, and the fraction of total activity in the 0–2-cm soil layer of the arid sites was significantly greater than that of the humid site. Dehydrogenase and total and microbial biomass arylsulfatase activities were sensitive indicators of the climatic change along the transect. At the humid and mildly arid sites, the activities of dehydrogenase were less in the winter than in the summer and spring, whereas total and microbial biomass arylsulfatase activities were less in both summer and winter. At the arid site, lower values were observed in the summer at 0–2-cm soil depth. At all sites, lower alkaline phosphatase activities at 0–2 cm were observed in the summer, but there were no significant seasonal differences in acid phosphatase activities. These different seasonal patterns of enzyme activities are attributed to the enzyme source, and specific seasonal soil moisture and temperature conditions at the studied sites. The low dehydrogenase and microbial biomass arylsulfatase activities in the winter at the humid and mildly arid sites are explained by the cold and wet soil conditions, and the low enzyme activity in the summer at the arid site is attributed to the dry and hot soil conditions.

Arylsulfatase activity of soil microbial biomass along a Mediterranean-arid transect

The relationships between arylsulfatase and microbial activity were investigated in regional and microenvironmental scales, at three study sites in Israel, that represent different climatic regions—Mediterranean (sub-humid), mildly arid and arid. Total arylsulfatase activity was divided into extracellular and intracellular (microbial biomass enzyme) activities according to the chloroform-fumigation method. The results show that with increasing aridity, Corg (soil organic carbon), Cmic (soil microbial biomass carbon), Nmic (soil microbial biomass nitrogen) and respiration rate decreased, while Cmic/Corg and metabolic quotient (qCO2) increased. Total, extracellular and microbial biomass arylsulfatase activities decreased with aridity. Expressed as percentage of total activity, the arylsulfatase activity of microbial biomass in the soil, at 0–2 cm and 5–10 cm depths, accounted for more than 50% of the total, in most measurements. This activity was significantly higher in the arid sites than that found in the Mediterranean one for the 0–2 cm soil. The results indicate the importance of the microflora as an enzyme source in soils, especially in arid climate conditions. Enzyme activity in the different study sites was found to be influenced by microenvironmental conditions. The Mediterranean site showed a much higher enzyme activity under shrubs than that under rock fragments and in bare soil. In the arid site rock fragments created a favorable microenvironment for microbial activity on soil surface, which resulted in a much higher microbial biomass and arylsulfatase activity than that in bare soil. The total, extracellular and intracellular arylsulfatase activities, were significantly correlated with Corg, Cmic, Nmic and respiration rate (p<0.05) at all study sites. The correlation coefficients between microbial biomass and arylsulfatase activity were usually higher than those between organic carbon and enzyme activity, especially in the arid sites. Close relationships between microbial biomass and arylsulfatase activities in all the studied sites supported the hypothesis that Corg content and enzyme activities should be related to each other via microbial biomass. Arylsulfatase activity was found to be a good indicator of microbial one. The regression equations between these factors can be incorporated into models of biogeochemical cycling for their easy method of analysis.

Ecogeomorphic Feedbacks in Semiarid Rangelands: A Review

Abstract The ecogeomorphic processes occurring on semiarid rangelands are reviewed, with emphasis on the source-sink relations and positive feedback loops that existed between shrub patches and intershrub areas, and the way livestock presence affected these interactions. Compared with intershrub areas, the shrub patches had a higher soil porosity, infiltration capacity, water-holding capacity, hydraulic conductivity, structural stability, and organic matter content, and lower bulk density. These differences derived from a host of processes whose intensity was less in the shrub patches, including raindrop impact, mechanical crust formation, overland water flow, soil erosion, evaporative moisture loss, and flock trampling. There was also greater shading of the soil surface; soil and litter deposition; water accumulation; microbial, fungal, and mesofaunal activities in the shrub patches. The overland flow of water carried soil and litter from the intershrub areas to the shrub patches and resulted in microtopographic modifications that tended to strengthen these source-sink relations. Grazing had an impact on these processes, not only at the shrub-intershrub scale but also within the intershrub areas, through the creation of highly compacted trampling routes. The combined role of the above ecogeomorphic processes was to maintain the rangelands functionality. Without these inter-relationships, water loss, soil erosion, and nutrient depletion would occur at the hillside scale, causing degradation of the landscape.

The Use and Misuse of Climatic Gradients for Evaluating Climate Impact on Dryland Ecosystems - an Example for the Solution of Conceptual Problems

Current trends of emissions of greenhouse gases are expected to cause the global temperature to rise faster over the present and next century than during any previous period (Houghton et al., 1996, 2001; Zweirs, 2002). Climate models for the Middle East predict an increase in winter temperatures combined with changes in rainfall amounts and distribution (Ben-Gai et al., 1998; Black, 2009, Klafe and Bruins, 2009). These changes may alter ecosystem functioning, with direct effects on ecosystem, community and population processes such as plant litter decomposition, nutrient cycling, primary productivity, biodiversity, plant recruitment and survival (e.g., Aronson et al., 1993; Hobbie, 1996; Robinson et al., 1998; Sternberg et al., 1999; Chapin et al., 2000; Hughes, 2000; Sarah, 2004). Considerable research has been directed at understanding the responses of terrestrial ecosystems to global environmental change. This topic is of great societal concern in the light of the potential impacts on the natural resources on which the human population depends on (Vitousek, 1994). Nevertheless, the challenge to predict ecosystem response to climate change is based on the multi-dimensional and multi-scale nature of the problem (Osmond et al., 2004). Complex ecological interactions make it difficult to extrapolate from individuals to communities and to predict the ecosystem response when only few levels of ecosystem organization are targeted. In addition, the lack of realistic climatic scenarios at relevant scales adds further complexity to the up-scaling process (Harvey, 2000). Vast experimental research efforts have been invested in understanding the effects of global warming and CO2 atmospheric enrichment on ecosystem functioning. These processes are considered key drivers of environmental change, particularly in northern latitudes.

Mapping topsoil organic carbon in non-agricultural semi-arid and arid ecosystems of Israel.

Mapping of soil organic carbon (SOC) was accomplished with remote sensing methods to assess its spatial variability. The relationship between bi-directional reflectance measurements and SOC was investigated with respect to C.I.E. color coordinates. Empirical relationships were generated for the spectral detection of SOC of two semi-arid to arid study sites. These regression models allowed the prediction of SOC with a cross-validated r z = 0.910 (RMSE cv = 2.825) and r 2 of 0.795 (RMSE cv = 2.113), respectively. Because C.I.E. color coordinates were found to be appropriate parameters for predicting the SOC, reflectance values of Landsat TM bands were transformed into C.I.E. color coordinates. The C.I.E. based regression models were applied to a Landsat image to estimate SOC in the spatial domain. Concentrations predicted from satellite data corresponded well with concentration ranges derived from chemical analysis. Estimated concentrations reflect the geographic conditions and depend on annual rainfall, with a general trend to decreasing SOC with increasing aridity.

Livestock Modify Ground Surface Microtopography and Penetration Resistance in a Semi-Arid Shrubland

Ground surface microtopography and soil penetration resistance play a major role in the geomorphic processes that occur on the hillsides of semi-arid rangelands. Although these two features are known to be impacted by the occurrence of shrub vegetation, the role of livestock remains unclear. The purpose of this study was to determine the impact of livestock on microtopography and penetration depth associated with patches of Sarcopoterium spinosum, a dominant shrub in the northern Negev region of Israel. Soil surface slope and penetration depth were determined for five segments along shrub-centered transects on the hillside axis: under the shrub canopy; upslope (US) and downslope (DS) of the central stem cluster; immediately upslope of the shrub (TR); upper intershrub (UI; above TR); and lower intershrub (LI; below DS). The effect of livestock was determined by comparing values obtained inside and outside 10-year-old exclosures. Livestock presence reduced soil penetration depth from 14.9 to 10.6 mm at segment UI, from 16.9 to 5.9 mm at segment TR, and from 15.2 to 9.9 mm at segment LI. The presence of livestock sharpened the step-like microtopographic profile along the transect: it reduced the incline from 10.2 to 5.7° at segment TR, and from 12.8 to 9.2° at segment US, whereas it increased the incline from 17.8 to 21.2° at segment DS. Such changes increase the spatial discontinuity of the hillside surface and can be expected to influence the spatial redistribution of water and soil resources, and thereby ecosystem functioning and productivity. These results lend credence to the view that livestock can function as physical ecosystem engineers by modulating resource availability to other organisms.

Landscape Archaeology in a Dry-Stream Valley near Tell es-Sâfì/Gath (Israel): Agricultural Terraces and the Origin of Fill Deposits

Abstract As part of the long-term archaeological project being conducted at Tell e-Sâfi/Gath in the semi-arid foothills of the Judean Mountains, a first order dry stream channel located in a valley east of the main site was surveyed and soil pits excavated in selected locations. A ditch, 10 m in length, was dug perpendicular to one of the agricultural terraces, showing that the small valley is filled with soil to a depth of more than 3 m above bedrock. The fill dates mainly to the Byzantine period (ca. 4th-7th cent. CE), according to the ceramic sherds. Three check-dam walls and related terraces were found across the width of the valley. Surprisingly, the base of the check-dam does not go deeper than 50 em into the uppermost part of the fill, well above bedrock or gravel layers, while covering only the upper part of the terrace step. Thus we use the term “floating terrace wall” or “floating check-dams”. Each of the terrace walls is about 0.5 m high and 50 m long. The valley is bound by two slopes: (1) a northeast facing slope characterized by Nari outcrops (a hard calcrete crust in the upper part of the chalk bedrock) and soil pockets, and (2) a southwest facing slope without Nari. The source of most valley fill material is apparently from the slope without Nari. The valley shows comparatively little accumulation during the Iron Age and very much accumulation during the Byzantine period. The main cause seems to be human-made earth movement and terrace building during the Byzantine period, rather than passive erosion and accumulation as a result of general environmental pressure by human activity.

Urban Park Soil and Vegetation： Effects of Natural and Anthropogenic Factors

Abstract Various soil surface components, such as trees, shrubs and biological crusts, and human recreational activities, e.g., barbecues and trampling by visitors, may divide the area of the urban park into smaller fragments/microenvironments, differentiated by their microenvironmental conditions, which may differ in soil and vegetation characteristics. The spatial changes in the soil and vegetation characteristics and their causes were investigated in an urban park located south of Tel Aviv-Jaffa, Israel. The area of the park is 0.5 km 2 including groves, a lake, lawns, and rest areas. Soil was sampled in nine microenvironments, of which seven were within the park: under Ceratonia siliqua trees (CsU), under Ficus sycomorus trees (FiU), rest area between tables under F. sycomorus (FiB), rest area under tables under F. sycomorus (FiT), open area with bare soil (OaS), open area with biological crust cover (OaC), and open area covered by herbaceous vegetation (OaV). Two more microenvironments, planar and sloping open areas (CoP and CoS, respectively) in the vicinity of the park, were used as the controls. Electrical conductivity, concentrations of soluble ions (Mg 2+ , Ca 2+ , Na + , K + and HCO − 3 ), pH, contents of organic carbon, calcium carbonate and moisture, and grain size distribution were determined. In addition, herbaceous vegetation cover, number and diversity of herbaceous vegetation species were measured. It was found that soil properties and herbaceous vegetation characteristics varied within the park. Soil organic carbon, electrical conductivity, soluble salts, penetration depth, and vegetation characteristics were affected by human activities, mainly in the rest area between the tables. In contrast, the above characteristics were affected by natural factors mainly in the rest of the microenvironments, which were subjected to low levels of anthropogenic intervention. The heterogeneous structure of the park, as represented by the various microenvironments, offered new habitats and promoted the preservation of natural vegetation.

A method for estimating the spatial distribution of soil moisture of arid microenvironments by close range thermal infrared imaging

The topsoil moisture content, its spatial distribution and dynamics, are important variables in understanding the response of arid eco‐geomorphic hill slope systems to rainfall. This study presents a method of measuring the soil moisture within a shrubs microenvironment by employing thermal infrared (TIR) imaging. A model for converting soil moisture (SMCM) is based on multi‐temporal TIR images. The method incorporates data from in situ measurements of soil temperature, moisture content and local meteorological variables collected simultaneously with TIR imaging. The results obtained, together with the high spatial resolution of the TIR images, demonstrated the efficacy of this method for mapping soil moisture on micro‐scale and also the potential for spatial analysis and change detection over time.