Hot desert environments

Cite as: Rewald B, Eppel A, Shelef O, Hill A, Degu A, Friedjung A, Rachmilevitch S. 2012. Hot desert environments. In: Bell EM ed. Life at extremes - Environments, organisms and strategies for survival. New York, USA, CABI Publishing.

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Hot desert environments Boris Rewald, Amir Eppel, Oren Shelef, Amber Hill, Asfaw Degu, Avital Friedjung and Shimon Rachmilevitch French Associates Institute for Agriculture and Biotechnology of Drylands, Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev. Phone: +972-86563435; Fax: +972-86596742; e-mail: [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected] 1. Hot desert environments 1.1 Formation and geographical distribution Hot deserts cover between 14 and 20% of the Earth's surface, around 19-25 million km2 (see Middleton and Thomas, 1997; Peel et al., 2007 for distribution of deserts). Most hot deserts, such as the Sahara of North Africa and the deserts of the south-western United States, Mexico and Australia, occur along the tropics in both the Northern and Southern hemispheres (between approximately 10° and 30-40° latitude). They are created as a result of global Hadley air circulation (Warner, 2004). The sun’s radiation results in hot air rising and the accumulation of moisture around the equator; as the air moves away it cools, starts to descend and at this point all of the moisture is lost as rainfall in the tropics. As the air subsides and becomes compressed it also becomes warmer, so that the relative humidity in desert air is decreased - even through the absolute amount of water vapour held may be substantial, as evidenced by dew-fall in the cold hours before dawn (Parsons and Abrahams, 2009). Because most of the water in the atmosphere is derived from evaporation from the seas, there is often an aridity gradient on large continents, i.e. the land closer to the sea often receives a larger share of this water. Regions lying deep within a continent may become drylands simply because the air currents reaching them have already traversed vast land distances and lost most moisture (Warner, 2004). Continentality is a major factor, especially for the Taklimakan Desert in Central East Asia and the Australian deserts. Further factors accounting for the distribution of deserts are a low inland relief and high coastal ranges (i.e. creating a ‘rain shadow’), the presence of cool ocean water close offshore and/or aloft of a tropical jet-stream leading to the formation of coastal deserts (Goudie and Wilkinson, 1977). A good example is the Atacama Desert, Chile where the air circulation from the sea is drying out the land. Notably, coastal deserts can be only found on the western fringes of continents and are not as hot as subtropical deserts (Warner, 2004). The current boundaries and extents of hot deserts are likely to continue to expand over the next century due to a combination of global warming and poor land management (e.g., overgrazing, indiscriminative clearing of the vegetation cover, and salinization; Laity, 2008). 1.2 Types of hot deserts A defining characteristic of a hot desert is aridity. According to the Koppen-Geiger climate classification, deserts are regions with an annual precipitation <250 mm (Peel et al., 2007). However, annual precipitation can be misleading because water loss is just as important a component of the water budget. Thus, the United Nations Environmental Program’s

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definition of desert is an annual moisture deficit under normal climatic conditions, where the potential evapotranspiration (PET) is over five-times higher than actual precipitation (Middleton and Thomas, 1997). The high PET prevails because, owing to the lack of cloud cover, incoming solar energy approaches a maximum in arid regions. Deserts can be divided into two types according to their level of aridity: hyperarid deserts have an aridity index (P/PET) of <0.05 and arid deserts have P/PET between 0.05 and 0.2. As such, deserts are distinguished from semi-arid drylands (P/PET 0.2-0.5) and dry-sub-humid drylands (P/PET 0.5-0.65). The diurnal temperature variation in deserts is very pronounced, with high daytime and low night time temperatures (Woodward, 2003). Due to the high surface temperature and temperature differences most deserts are high wind energy environments (Parsons and Abrahams, 2009). Hot deserts are stereotypically portrayed as vast seas of sand dunes with cacti and circling vultures (Fig. 1). In fact there are many different kinds of hot deserts with varying patterns of landforms, altitudes and life-forms (Parsons and Abrahams, 2009). For example, only approximately 15-20% of deserts are covered by sand dunes, instead rocky plateau or mountain desert landscapes prevail (Ward, 2009). Soil properties influence the degree of aridity and thus plant productivity. There are arid climates with >500 mm of annual rain that falls in intense events on hard soil or rock, and the water runs off horizontally or is evaporating quickly (Warner, 2004). At the other extreme there are soils in so-called edaphic deserts which are extremely porous and have such a low water holding capacity that the annual precipitation drains through them so rapidly that it is virtually unavailable to vegetation. Beneath soil properties related to water, plant productivity in deserts can be nutrient limited. For example, nitrogen is a key limiting nutrient in most deserts (West and Skujins, 1978), while phosphorous is the most limiting nutrient in Australian deserts (Ward, 2009). Soil nutrients and organic matter tend to be concentrated in the upper 2-5 cm of the soil with the greatest amounts underneath the canopies of desert shrubs or trees in so called ‘islands of fertility’ (Ward, 2009). Due to high evaporation many desert soils are also affected by accumulated salt ions. A detailed description of saline habitats appears in chapter ##. 2. Abiotic stress factors in hot deserts and mechanisms of stress resistance When one thinks of hot deserts one immediately envisages extremely hot, sunny, dry environments. Indeed, hot deserts present the organisms that live in them with a number of abiotic extremes: high levels of radiation, high temperatures, aridity, and often high salinity, strong winds and periodic dust storms. However, deserts are not sterile wasteland but host a wide diversity of organisms specially adapted to face the extreme challenges that desert dwelling presents them with (Fig. 2). One might predict natural selection to drive desert organisms along convergent paths to a common suite of adaptive traits; however, organisms inhabiting deserts may actually show a great range in growth forms and adaptations. A biological response to stress, termed stress resistance, usually involves both avoidance and tolerance mechanisms (Levitt, 1972). Stress avoidance is partially or completely excluding the stress by means of physical barrier or time of appearance, while stress tolerance is standing the stress, using either repair or prevention techniques. According to their high levels of stress resistance, many desert organisms can be classified as xerophiles (i.e. organisms adapted for growing or living in very dry surroundings) and/or thermophiles (i.e. species adapted to withstand hot air and soil temperatures). Xerohalophiles are able to survive in saline micro-environments that often occur in deserts.


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In this chapter we will (1) introduce the major abiotic constrains species face in hot deserts and discuss how organisms, primarily from the plant kingdom, have adapted to circumvent these constrains and (2) shed light on biotic interactions in desert environments. 2.1 Radiation and temperature While aspects of topography can significantly affect local temperature in deserts, radiation is the main factor. Radiation in deserts can be as high as 840 GJ km-2 per year (Laity, 2008), more than twice as much as in temperate ecosystems. This is for two main reasons: (1) deserts are located in latitudes that are relatively close to the equator, and (2) low levels of cloud cover allow large amounts of radiation to reach the Earth’s surface. The broad band extending across the Sahara and the Arabian Deserts is the largest area to receive radiation of this magnitude. Deserts are renowned for their large diurnal temperature fluctuations; low vegetation cover within deserts means that the sun heats up the area quickly, but at night heat radiates rapidly away. While the temperature of winter nights in hot deserts may fall below zero degrees, daytime air temperatures may remain as high as 40°C. The highest air temperature ever recorded was 57°C in the Libyan part of the Sahara. However, the temperature on the soil surface can be considerably higher than the air temperature, as much as 75-80°C (Ward, 2009). Both high radiation and high temperature significantly affect the physiology of organisms, especially if water is scarce. In plants and some cyanobacteria, the visible part of the radiation spectrum (photosynthetically active radiation, wavelength 400-700 nm) is captured by pigments (Chlorophylls and Carotenoids) and converts its energy, via photochemical processes, into reductive chemical products. The products are mainly used to produce sugar in a process named Calvin cycle. In hot deserts, the high level of irradiance is usually combined with high temperature and shortage of water, which restricts the activity of the Calvin cycle. Therefore high irradiance might create an over flow of excess reductive power in the chemical form of reactive oxygen species (ROS). These molecules can react and cause damage to DNA, proteins and lipid membranes, which are crucial to plant survival (Akashi et al., 2008). In higher plants, one approach to deal with ROS excess is to reduce light absorbance by the leaf which is strongly affected by leaf orientation and reflectance. While sun-tracking leaves are maximizing photon absorbance, leaves not perpendicular to the sun rays may be advantageous to desert plants by reducing irradiance and heat accumulation (Gibson, 1996). Plants with fixed vertical and very steeply angled leaves, and with azimuth east-west orientation of leaves and branches, have been observed in deserts (Nobel et al., 1993). These leaves receive most light in early mornings and late afternoon while avoiding intercepting excess levels of radiation during midday. Leaf hairs and deposition of wax on the leaf surface can also serve as a barrier that reflects excess light, resulting in the evolution of whitish, silvery and greyish leaf types (Holmes and Keiller, 2002). It has been shown that leaf hairs on the desert plant, Encelia farinose, decreased the visible light absorption by as much as 56% compared to E. californica, inhabitant of coastal ecosystems that does not have leaf hairs (Ehleringer et al., 1976). Unlike radiation in the visible part of the spectrum, UV radiation (wavelength 10-400 nm) is damaging to all living creatures by direct harmful reaction with DNA, proteins and lipids, or indirectly by producing ROS (see also Chapter ##). In order to cope with UV radiation, desert plants and cyanobacteria produce phenolic compounds, such as flavonoids, which trap the UV radiation and prevent an impact on other molecules (see Chapter ##). In higher plants these compounds are mainly distributed on the outer layers of the leaves (epidermis) thus protecting the inner layers from damage (Caldwell


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et al., 1998). The aforementioned plant adaptations to avoid excessive amounts of visible light are also helpful in dealing with high UV radiation (Holmes and Keiller, 2002). Plants’ tolerance to high radiation involves the activity of the photosynthetic process itself. One such adaptation is photorespiration; in this process the reductive power generated by the light reaction is used to reduce oxygen ions into CO2, thus getting rid of the excess energy absorbed by the plants (Kozaki and Tabeka, 1996). The use of photorespiration in photo protection has been demonstrated in desert plants; the perennial semi-shrub Reaumuria soongorica, increases its rate of photorespiration under drought conditions to deal with excess radiation (Bai et al., 2008). Photorespiration is considered an ‘energetically wasteful’ process which means it consumes oxygen and emits CO2. Consequently many plant species in hot, dry and high light environments have an alternative photosynthesis mechanism, known as the C4 photosynthetic pathway. It is designed to concentrate high levels of CO2 in the proximity of the Calvin cycle process in the chloroplast where carbon fixation occurs. This high concentration of CO2 in chloroplasts means that C4 plants can utilize excess light for photosynthesis without needing photo-respiration (Sage, 2004). A different strategy to deal with excess light is non-photochemical quenching (NPQ). NPQ is a universal mechanism that diverts the absorbed excess light into heat and not to the photochemical processes (Müller et al., 2001). While it has been shown that this process occurs in desert plants grown under high irradiance in the winter, the importance of this mechanism during the hot summers is less clear due to the unfortunate side effect of leaf heating (Barker et al., 2002). Heat resistance by photosynthetic organs requires a balance between heat influx due to radiation (solar and environmental-infrared) and heat efflux (Gibson, 1996). High temperature is, thermodynamically speaking, a high level of molecule movement in a media, resulting in increased entropy. When a leaf-cell’s temperature exceeds approximately 46°C, which is a common midday summer temperature in hot deserts, several physiological processes may begin to break down (Berry and Björkmann, 1980). For example, increased entropy causes disruption of protein structures and functions. Membrane stability is also endangered, leading to the disruption of water and solute concentration, damaging cellular homeostasis and leading to cell death (Larcher, 2001). To avoid tissue damages, heat can be dissipated via increased conduction, convection, and transpirational cooling in plants (e.g., Levitt, 1980) or evaporational cooling (sweating) in animals. In deserts high temperature is correlated with reduced water availability, thus transpiration/sweating for cooling purposes may increase dehydration. Many desert animals are relatively pale in colour, possibly preventing their bodies from absorbing too much solar radiation during the day (e.g., Hetem et al., 2009), additionally the fur of mammals is often an efficient barrier against heat gain from the environment (Schmidt-Nielsen et al., 1956). However, for desert mammals body overheating is rather a result of heavy exercise (by metabolic heat production in muscles) than the passive heat gain during resting, leading often to reduced exertion during hot days (Vaughan et al., 2010). Furthermore, in contrast to sessile plants, desert mammals, reptiles (which are exotherm and thus highly affected by the heat sources around them) and others can avoid the hottest midday sun in burrows, in the shade of plants or by turning even fully nocturnal (i.e. ‘behavioural thermoregulation’; e.g., Vaughan et al., 2010; Wilms 2011). Because the brain tissue of mammals is especially heat sensitive, some species have specialized cooling devices. For example, in some antelopes like the oryx (Fig. 3) the blood going to the brain is cooler than of most of the rest of the body by countercurrent heat exchange between veins and arteries and evaporative cooling of veins in the nasal cavity; a system called ‘carotid rete’ (Vaughan et al., 2010). In general, a classification of desert animals into heat ‘avoiders’, ‘evaporators’


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and ‘endurers’ (i.e. body temperature can increase by several degrees without tissue damage; e.g. camels) is useful although there is a lot of overlap between ‘avoiders’ and ‘evaporators’ (Ward, 2009). In plants and animals alike, conduction and convection are strongly affected by the boundary layer thickness; the boundary layer being the layer of unmixed air above the organ / body surface and its thickness is related to the organ / body shape and size and the wind speed. Desert environments often habitat smaller animals with larger extremities compared to colder climates (‘Bergman rule’), increasing the cooling rate due to reduced thermal intertia and increased surface area : body mass ratios (Willmer et al., 2004). Additionally, bare skin areas, e.g. found on some vulture heads, play an important role in thermoregulation by reducing the boundary layer compared to feathered skin areas (Ward, 2009). Perennial desert plants are either drought-deciduous or aphyllous, i.e. with green photosynthetic stems, or possess small and narrow, even microphyllous and heteroplastic leaves. This allows leaf temperature to be maintained very close to ambient temperature without high transpirational costs (Smith, 1978). However, this is not true for many succulent plants; their large size largely prevents convective heat exchange thus increasing sunlit surfaces often 10-15°C above ambient air temperature. As a result some desert cacti must be able to tolerate tissue temperatures approaching 60°C, a tolerance level unsurpassed in hydrated vascular plants and a primary cost of succulence (Smith et al., 1997). On the other hand, desert species with broad leaves, e.g. fan palms, Washingtonia spp., have to cool their leaves by high transpiration rates; explaining why these species are commonly found in areas which have accessible groundwater such as oases. Heat tolerance is achieved in plants by various cellular and physiological mechanisms. One common response to increased temperature is the expression of Heat Shock Proteins (HSP). These proteins allow processes such as protein folding to occur at higher than optimal temperature and also enhance membrane stability (Larcher, 2001). In the heat tolerant succulent Agave tequilana, which is a broad leaf species, extreme heat resistant correlated with both high levels of transpiration and the expression of HSPs (Lujan et al., 2009). At the physiological level, photosynthesis is more vulnerable to heat stress than respiration which tends to increase with temperature. Under high temperature, photorespiration is enhanced at the expense of photosynthesis, thus depleting a plant's energy budget. The C4 photosynthesis is much less affected by temperature than C3 plants; thus C4 plants are more common in deserts than in temperate zone ecosystems (Sage, 2004). In plants that possess this type of metabolic pathway there is almost no photorespiration, and thus they are able to perform photosynthesis at higher temperature than C3 plants, which do perform photorespiration (Ehrlinger and Björkman, 1977). So how do C3 plants adjust their photosynthetic activity to cope with the hot and dry desert environment? One mechanism is displayed by the evergreen desert shrub Retama raetam: in this plant the lower, shaded and cooler canopy continue photosynthesis throughout the hot dry summer period, while the upper, sun exposed and warmer canopy remains photosynthetically dormant. The upper canopy maintains a certain amount of mRNA coding for photosynthetic proteins. When water becomes available, these mRNA translated, allowing for fast recovery of photosynthetic activity in the upper canopy (Mittler et al., 2001). While a wide variety of soil mesofauna can survive exposure to very high temperatures including nematodes, tardigrades and rotifers, the only truly thermophilic organisms in deserts are soil microbes (Jeffery et al., 2010) 2.2 Water scarcity


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Deserts are ‘water-controlled ecosystems’ (Noy-Meir, 1973): available water for desert organism survival relies mainly on the balance between water contribution and water loss. Surface flow, precipitation, groundwater and atmospheric vapour are the natural sources of water for organisms in arid regions (Flanagan and Ehleringer, 1991). However, perennial or ephemeral streams are limited in availability in most (hyper-)arid environments and groundwater is often saline (Akram and Chandio, 1998). Dew accumulation may contribute up to 30% of the annual precipitation in some deserts (Zangvil 1996) while in hyperarid ecosystems the amount of condensation is scarce but might exceed precipitation. While the Chilean Atacama desert gets <10 mm of annual precipitation which is mainly formed from condensed fog, the primary source of freshwater in most deserts is rainfall falling in a relatively predictable season but as an unpredictable amount (Noy-Meir, 1973). The seasonality and the time distribution of precipitation affects the ‘bioactivity’ of water; for example, rain that falls in the cooler winter months is less likely to be evaporated and is therefore more available to organisms. The northern part of the Sahara and Arabian Deserts, including the Negev Desert, receive winter rains (50-100 mm yr-1) while the southern part of the Sahara Desert receives summer rain (sometimes <5 mm yr-1; Gutterman, 2002a, b). Deserts that receive monsoonal thunderstorms, and in particular those regions within or adjacent to mountain ranges, are subjected to sudden flash floods. Flash floods have huge discharges and velocities >2 m s-1 (Fisher and Minckley, 1978), thus, riparian species in dry riverbeds must withstand the severe physical agitation of these infrequent but very forceful natural disasters, often resulting in uprooting or partially uncovered root systems (Fig. 5). Different water harvesting systems, including collecting and storing runoff of these floods in open ponds are being used in arid ecosystems world-wide to grow grass and trees (for fruits, fodder and firewood), and for recreation (Lavee et al., 1997; Fig. 4). The coincidence of low annual rainfall with extremely high evapotranspiration rates due to the high summer temperatures, leads to the rapid depletion of water reserves (Ceballos et al., 2004). In order to cope with this a plant may employ both avoidance and tolerance mechanisms. In general, particular plant growth forms may have a selective advantage in desert environments by facilitating tradeoffs between maximal net carbon gain and tolerance to environmental stress. For example, long-lived perennial shrubs and trees have relatively low net carbon gains but are buffered against environmental stress, whereas annuals possess high rates of net carbon gain but restrict their active growth to relatively short periods when soil moisture is high (Gibson, 1996). A simple classification concept was developed for the Negev Desert; plants were described as (1) able to recover from dehydration (desiccation-tolerant), (2) inactive during the dry season (arido-passive) or (3) plants that are active during the dry season (arido-active; Evenari et al., 1971). Desiccation-tolerant desert plants have the supreme ability to withstand ‘drying without dying’. The most extreme examples for drought-tolerance are the so-called resurrection plants. Resurrection plants are mostly poikilohydrous, which means that their water content adjusts with the relative humidity in the environment. They are able to stay in a dehydrated state until water becomes available, allowing them to rehydrate and to resume full physiological activities. They can revive from an air-dried state as low as 0% (v/v) relative humidity (Gaff, 1987). Resurrection plants take immediate advantage of rainfall after dry periods: they resurrect and restore photosynthetic activities often within 24 hours after rehydration, and can subsequently grow and reproduce before other species can (Scott, 2000). Two types of resurrection plants can be distinguished as: (1) poikilochlorophyllous plants, these plants lose chlorophyll and the thylakoid membranes are at least partially degraded during water loss, and (2) homoiochlorophyllous plants, such as Craterostigma plantagineum, which retain chlorophyll and retain the intact photosynthetic structures


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although structural changes occur (Bartels, 2005). While most resurrection plants are herbaceous, the largest known resurrection plant is the woody shrub, Myrothamnus flabellifolia (Sherwin et al., 1998). Arido-passive species are classic examples of drought avoiding (‘drought escaping’) plants. They have insufficient mechanisms to combat water stress and must complete their life cycle only when ample free soil water is available. Ephemeral or annual plants are found even in the extreme deserts, their seeds may remain viable in the dry soil for many years (Gutterman, 2002a, b). Variable, delayed germination of seeds is an adaptation that allows desert ephemerals to avoid conditions when soils are too dry or temperatures are too high for survival until the reproduction stage is reached. For example, cool season ephemerals in North American deserts typically germinate in the autumn and persist in the winter as leaf rosettes; in early spring, rosettes quickly produce flowers and seeds (Whitford, 2002). The annual grass Schismus arabicus (Fig. 6) demonstrates annual periodicity as adaptation to hot and dry deserts: results by Gutterman et al. (2010) indicate that survival of Schismus seedlings depends on the germination month independently of the drought level. None of about 2500 seedlings that germinated in July survived a desiccation treatment and of those germinated in June <40% survived. In contrast, >80% of the seedlings that germinated in December and January survived desiccation. However, most desert annuals are more opportunistic and typically germinate after the first large rainfall, sometimes leading to stunning ‘wildflower blooms’ (Phillips and Comus, 2000). The most apparent group of desert plants are xerophytes, drought-adapted species which resist drought by both avoiding major dehydration by increased water uptake or reduced water loss, and are able to withstand low tissue water potentials. Typical adaptations include (1) a high root:shoot ratio to increase relative water uptake; (2) the reduction of transpiration by (2a) temporal or constant reduction of surface areas, (2b) morphological or anatomical changes or (2c) sensitive stomatal regulation and; (3) the physiological capacity to tolerate low cell water potentials by (3a) sustaining the photosynthetic activity, and by(3b) protecting the tissues (Evenari et al., 1975). Succulents, such as instantly recognisable cacti, are plants that are able to store large amount of water in organs such as stems and leaves, thereby allowing survival and growth during dry and hot periods. Cactaceae or Agavaceae are new world families of subtropical origin and their distribution is thus limited geographically to the Americas (Anderson, 2001). In the old world, specifically in Africa, members of the family Euphorbiaceae have responded to the challenges of an arid climate with similar mechanisms to those of cacti in the new world. Succulence is one such survival feature. For example, some cacti consist of almost 95% water when fully hydrated. Remarkably, they can become desiccated and survive low (approx. 20%) water content conditions (Gibson and Nobel, 1990 and references within). Cacti also can immediately develop extensive, shallow root systems that can absorb large quantities of water in short periods before the soil dries rapidly under the intense sun. Reduced number of leaves (agaves), no leaves (most cacti) and drought-deciduous leaves (e.g., elephant trees, ocotillos), as well as waxy cuticles that render them a nearly waterproof layer are water conservation mechanisms in succulents (Gibson, 1998). However, the main feature carried out by the great majority of succulents is a special photosynthetic process called Crassulacean Acid Metabolism (CAM). Water vapour is lost when stomata are opened to let carbon dioxide (CO2) in, the fuel of photosynthesis, especially when opened during the hot time of the day. Thus, in CAM photosynthesis the stomata open during the night, thus less water is lost during the cooler hours; the CO2 that enters during the nightly opening is converted to organic acids that are stored in the vacuole of the cell until used during photosynthesis during


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the day (Guralnick and Ting, 1987). Some desert succulents have formed their leaves in the shape of a bowl to catch the morning fog e.g. aloes and agave (Arizaga and Ezcurra, 2002). While succulents are commonly considered ‘typical’ desert plants, most plants in hot deserts are non-succulents. Shrubs and sub-shrubs are more dominant growth-forms within the desert vegetation with manifold adaptations to withstand season-long droughts (Gibson, 1996). Deciduous shrubs’ mechanisms for survival include the capacity to (1) shift growth and physiological activity in order to utilize different temporal moisture (e.g. becoming dormant during the dry season, Grayia spp.); (2) change the CO2 assimilation and use during the dry season (e.g. switch to stem-twig / stem net CO2 assimilation, Hymenoclea spp., Cercidium spp. or switch stem recycling to internal CO2 recycling, Fougquieria spp.; (3) exhibit seasonal leaf polymorphism (e.g. Encelia spp.); (4) many desert shrubs have developed photosynthetic C4 pathway. Thus in summary, most desert shrubs carry out limited physiological activity and growth during drought but compensate for some negative impacts of drought through enhanced physiological activity and growth in the season following drought (Smith et al., 1997). Despite their adaptations, desert shrubs may have minimum stem water potentials (WP), i.e. a minimum amount of internal water that they require to actively move water around their structures and thus survive on. Minimum WP between -3 and -6 MPa are typical of north-western America desert shrubs (Sperry and Hacke, 2002) and minimum WP of -4.2 MPa were measured in a dwarf-shrub, Anabasis articulata, in inter-dune corridors of the Negev Desert (Veste, 2008). Low water potential in stems, twigs and petioles can cause cavitation in xylem vessels, i.e. the occurrence of air bubbles (‘emboli’) that prevent further water conductance. Many species adapted to more xeric conditions reduce their vulnerability to cavitation, e.g. via reduced pores-sizes in pit membrane or different hydraulic strategies (Rosenthal et al., 2010). Desert shrubs which are summer active but leafless in the winter (e.g., Flourensia spp., Krameria spp.) are especially prone to hydraulic xylem failure due to the high evaporative demand but may have advantages in leaf cooling if water is available. Another example of the trade-off between adaptational mechanisms are deciduous and evergreen shrubs. A habitat with both shrub types is the Mojave Desert, California, United States, which is dominated by the evergreen Larrea tridentate on mounds and the deciduous Ambrosia dumosa in soil depressions and temporal waterways. Deciduous shrubs have twice the photosynthetic capacity of evergreens under optimal conditions, but can only ‘operate’ during the most favourable parts of the year. It was calculated that the primary cost of the deciduous ‘strategy’ is the time lag to produce new leaves after the onset of the wet season, thus evergreen shrubs are at an advantage relative to deciduous shrubs and annuals when the rainy season or water availability is generally short and unpredictable (Comstock and Ehleringer, 1986). Phreatophytes are desert species which avoid water stress by developing very deep root systems able to tap into reliable moisture sources like ground water. These species, trees or shrubs, show little tendency for water conservation and were thus termed pseudo-xerophytes, clearly distinguishing them from the xerophytes described above (Fig. 5). Arborescent (tree-like) phreatophytes like Alhagi sparsifolia, Populus euphratica and diverse Tamarix spp. can develop tap roots that can sometimes extend to a depth of >50 m (Phillips, 1963; Thomas et al., 2008). Extensive clonal growth below ground has been observed and is often the basis for the occupation of space and of vegetative regeneration once the ground water is too deep to be reached by seedlings (Bruelheide et al., 2004).


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Herds of grazing and browsing animals often migrate to find water and food in response to seasonal and year-to-year variations in rainfall and food availability (Miller and Spoolman, 2006; Fig. 3). Animals which cannot migrate often obtain nearly all their water from plants (Brown and Ernest, 2002) or store water which is gradually depleted until the body is again rehydrated; furthermore, some insects can absorp moisture from the air. An example for drought evading animals are Australian desert frogs such as Cycloana platycephala which spend dry parts of the year (>6 month) in aestivation (a form of dormancy), inside a burrow and additionally build a cocoons to reduce water loss (Tracy et al., 2007). Another mechanism to reduce water loss is increased body temperature; for example, the body temperature of camels at rest can increase from about 34°C to >40°C in the summer. The variations in temperature are of great significance in water conservation in two ways: (1) the increase in body temperature means that heat is stored in the body instead of being dissipated by evaporation of water. At night the excess heat can be given off without expenditure of water and (2) heat gain from the hot environment is reduced because the temperature gradient is reduced (Schmidt-Nielsen et al., 1956). Insects, land snails and many reptiles and birds excrete uric acid as a major nitrogenous waste while mammals excrete water soluble urea. Because uric acid is excreted as paste, this is resulting in very little water loss (Ward, 2009). In contrast, xeric-adapted mammals have often very long loops of Henle in their kidneys to concentrate ions and maximize water conservation. In the most extreme hot deserts, like parts of the Atacama Desert, higher plants or invertebrates are virtually absent. However, cyanobacteria were found to grow beneath translucent quartz rocks (rain fall/fog >1 mm; ‘hypolithic’) or even beneath the surface of rocks (‘endolithic’). The quartz allows light to penetrate and support photosynthesis and the soils beneath the quartz contain more moisture than the exposed surface soils providing a habitat for cyanobacteria (Azúa-Bustos et al., 2011). Since organic materials are scarce inside rock some can utilise inorganic materials to produce energy (chemolithothroph), e.g. bacteria found in a rock in the Atacama Desert can oxidize inorganic Arsenite (Campos et al., 2009). 2.3 Salinity Saline soils cover 6% of the continental surface (Larcher, 2001). The excessive accumulation of salts in arid soils occurs mainly due to evaporation of water derived from precipitation, irrigation or from solute movement in shallow groundwater. Arid regions consist of high evaporation and low precipitation and are therefore especially prone to high soil salinity. Saline soil environments are associated with osmotic stress, toxic ions and ion imbalances (Lambers et al., 2008). Because water uptake is driven by a passive transport which rely upon water pressure gradient (Taiz and Zeiger, 1998), water moves into plant tissues only when water potential of fine roots is more negative than the soil solution. In saline soils water is osmotically held and becomes less accessible to the plant thus creating a ‘physiological drought’ if plants are not adapted. An excess of ions, specifically Na+ and Cl-, has negative effects on enzymes and membranes which impede various systems in the plant, including energy balance, nitrogen assimilation and protein metabolism (Larcher, 2001). There is immense diversity of organisms that can be called halophiles (see chapter ##). While all plants are endowed with certain mechanisms to tolerate salinity to some extent, halophytes (about 1% of the world’s flora) are plants that are able to complete their life cycle in elevated salinities around 200 mM NaCl or more (Flowers and Colmer, 2008). In contrast glycophytes (plants not adapted to salinity) can only withstand salinities up to 10 mM NaCl (Orcutt and Nilsen, 2000). Obligatory halophytes require saline conditions for growth, whereas


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facultative halophytes can also live under freshwater conditions (Waisel, 1972). Examples of xerohalophytes include members of the Chenopodiaceae, e.g. Atriplex halimus and Salsola spp., and Tamaricaceae, such as Tamarix spp. and Reaumuria spp. (Feinbrun-Dotan and Danin, 1998). An example of a halotropic xerohalophyte is Bassia indica which is a facultative xerohalophyte. In the Negev Desert, Israel, B. indica roots exhibit a unique salt tolerance mechanism: Bassia plants develop thick horizontal roots, originating from the main tap root, and these grow toward saline regions. This phenomenon was termed halotropism, growth cued by and towards salt (Shelef et al., 2010). The major salinity tolerance mechanisms that have evolved in desert plants are: spatial avoidance, changes in root morphogenesis, retention of toxic ions in vacuoles or granular compartments, cellular ion regulation and metabolite synthesis for osmotic adjustment. More physiological salt tolerance strategies are reviewed by Parida and Das (2005) including: (1) limitation of salt uptake by roots and transport into leaves; (2) alterations in photosynthetic pathway; (3) changes in membrane structure; (4) induction of antioxidative enzymes; and (5) induction of hormones. In addition, salt may be diluted by reverse flux from shoots to roots via the phloem (Cheeseman, 1988) and plants may possess special mechanisms for seed preservation, protection and germination. In this sense, salt dwelling is sometimes of benefit, since the hygroscopic traits of salt mean that saline soils have higher water content than soils without salts. This link is prominent in water-limited ecosystems where plants are often exposed to drought conditions combined with salinity. For desert animals, excess salts can be particularly troublesome because it can exacerbate dehydration. However, some desert-adapted animals like the spiny mouse (Acomys cahirinus) were found to tolerate a significantly higher plasma osmolality after a salt diet than similar lab mice (Dickinson et al., 2007). The efficiency of the spiny mouse kidney in filtering and excreting a higher concentration of salt resulted into a reduced water consumption. 2.4 Strong wind and dust storms Most deserts are associated with weak horizontal pressure gradients and are therefore subject to weak, large-scale winds. However, weather disturbances intruding from the tropics or mid-latitudes occasionally generate high wind speeds and the high convectional overturning of air boundary layers governs wind speeds from aloft down to the desert surface. For example, the winds in the Somali-Chalbi Desert are among the strongest and most sustained wind systems of the world; wind speeds over 14 m s-1 (>50 km h-1) prevail on more than 50 days of the year (Warner, 2004). High wind conditions initiate soil particle movement, peaking in dust storms which are an important part of desert climate (Fig. 7). Because sand grains form relatively small soil aggregates, sandy soils are especially susceptible to erosion from strong wind (Skidmore, 1989). For example, threshold wind speeds inducing dust storms were calculated as 6.7 m s-1 in the sandy Taklimakan Desert, China, contrasting with 13.8 m s-1 in the stony Gobi Desert, Mongolia (Kurosaki and Mikami, 2007). Since the 1950s, wind-blown dust storms in the Sahara desert have increased tenfold due to overgrazing and drought caused by climate change and population growth (Miller and Spoolman, 2006). Wind and/or wind-blown sand have been hypothesized to be important abiotic components shaping plant phenotypes, community composition and subsequently reducing yield in sand dune and other wind-prone ecosystems like semi-arid crop lands (Grace, 1988). Besides increasing evapotranspiration rates by reduced boundary layers on leaf and soil surfaces (Caldwell, 1970), wind exerts direct mechanical effects on plants, including increased motion as a result of both the turbulence and the drag force of the wind, uprooting of plants when the


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wind’s force exceeds the stem or root/soil strength and physical leaf damage arising from leaf tearing, stripping and abrasion. A plant’s resistance to breakage or overturning in windy climates depends largely on structural modifications for mechanical strength. Plant growth responses to wind movement, termed ‘thigmomorphogenesis’, include changes in branch and foliar development, stem shape and biomass allocation (Jaffe, 1973). For example, a Tamarix species growing in the Taklimakan Desert was found to increase the rigidity and length of roots and increase the thickness of stems on the leeward side. These measures were suggested to prevent plants from overturning in the windy habitat (Liu et al., 2008). Adaptations above ground to reduce wind susceptibility further include a reduced plant height, increased numbers of lignified cells, and decreased leaf numbers or leaf areas (Grace, 1988; Eugster, 2008). Another factor is sand movement which is a characteristic of dunes. Several studies concluded that the difference in the tolerance to sand movement determines the distribution of plant species in areas where accumulation or erosion of soil frequently occurs (Avis and Lubke, 1985). Many psammophilic (‘sand-loving’) species are tolerant to sand movement or their growth and vigour are even stimulated by it, contrasting species preferentially growing on rocks or clay soils. Mechanisms to tolerate sand movement include high shoot growth rates (Gilbert and Ripley, 2010). Leaf stripping and obvious tears, abrasions, lesions or breakages of laminae seems to be less of a threat in desert species with microphyllous leaves which tend to have few or no petioles, additionally reducing torsional shear stresses on laminae (Niklas, 1992). However, wind rubbing of adjacent branches and leaves against each other and soil particles carried by wind (‘sand-blasting’) can cause surface damage to plant tissues. For example, Thompson (1974) grew the grass, Festuca arundinacea, under relatively low wind speeds (~3 m s-1) but reported microscopic damage to the epidermis. The damage included rupture of epidermal cells, cracking of the cuticle and smoothing and redistribution of the wax deposits which cover the surface. In a parallel experiment it was demonstrated that leaves which had been damaged in this way lost much of their capacity to control water loss (Grace, 1974). Macroscopic damage features can be more evident when the wind carries particles of sand (Armbrust, 1982). In addition to having direct effects on plant growth and survival, sand abrasion significantly increases susceptibility to bacterial infections (Pohronezny et al., 1992). The extent to which a given species is affected by abrasion depends inter alia on the structure of the cuticle (Yura and Ogura, 2006). While desert plants are known to have thicker cuticles to reduce transpiration, long and bending leaves growing approximately horizontally are thought to reduce the direct collision of wind-borne sand particles (Yura and Ogura, 2006). Animals will likely try to reduce the impact of dust storms by behavioural changes, e.g. hiding although physiological adaptations exist (e.g., a camel can close its nostrils to keep sand out of its nose). However, although further studies are required on the toxicological role of sand storms on mammals, dust storms were found to lead to oxidative damage, mediated by pro-oxidant/antioxidant imbalance or excess free radicals, of different degrees in lungs, hearts and livers of rats (Meng and Zhang, 2006). 3. Biotic interactions in deserts Abiotic factors have long been considered more important than biotic variables in structuring desert plant communities (Grime, 1977) because constituent populations often exhibit episodic germination, recruitment and mortality as a direct result of fluctuations in environmental variables (Fowler, 1986). Some have even argued that environmental


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fluctuations prevent desert plant populations from ever reaching equilibrium with resource availability, thereby minimizing the importance of competition (Fowler, 1986 and references therein). However, research in recent decades has found evidence that biotic interactions are potentially of major functional importance in desert communities. In deserts, distributions suggestive of positive interactions (i.e. facilitation) between species have generally been attributed to the amelioration of harsh environmental factors by one species, thereby benefiting the germination or survival of another; whereas distributions suggestive of negative species interactions (i.e. competition) have been attributed to the intensification of environmental stresses by neighbouring plants by resource depletion (Fonteyn and Mahall, 1981). The presence or absence of neighboring plants can be a structuring force of desert plant communities (Allen et al., 2008). Earlier research concluded that in the desert, (1) plant cover decreases with a concomitant increase in lateral root growth, until almost all of the available space is fully occupied, and (2) regular spacing in areas of low rainfall is caused and maintained by root competition for available water (e.g. Walter, 1962). However, spatial arrangement and occurrence of plant species is recognized to be additionally influenced by factors like habitat heterogeneity, disturbance by animals (e.g. herbivores) and direct plant-plant competition via allelochemicals (Friedman, 1987). Plant cover in water limited ecosystems is usually less than 60%, most of it is a two-phase mosaic complex (Aguiar and Sala, 1999). The mosaic is often composed of woody vegetation patches and inter-shrub patches. Woody vegetation patches are characterized by perennial shrubs or trees and the open patches are characterized by crusted soil with microphytic communities (Boeken and Shachak, 1994). Shrubs in drylands modulate the flow of resources by modifying the impacts of wind and runoff water, increasing water infiltration and deposition of sediments (Eldridge et al., 2000), trapping organic matter and nutrients (Thompson et al., 2006), and ameliorating microclimatic conditions, which in turn increase animal and microbial activity (Zaady et al., 1996). ‘Soil mounds’ with loose soil are formed under the shrubs and alter soil properties in the shrub patch. The soil beneath shrub in drylands tends to be enriched with water, carbon, nitrogen and phosphate (Thompson et al., 2006). As a consequence, shrub patches in drylands are considered ‘islands of fertility’ creating improved microhabitat conditions for seedling, sub-shrubs, and annuals and other organisms (Boeken, 2008). However, xerohalophytes may increase the soil salinity under the canopy due to litter decomposition (Zygophyllum spp.; Sarig and Steinberger, 1994) or excrete excess salt from special salt glands (Tamarix spp.; Thomson and Liu, 1967), thus altering soil properties and creating microhabitat that accommodates specialized species assemblages (Sarig and Steinberger, 1994). Interactions are not limited to plants; desert plants and animals interact in ways that have strongly influenced their respective evolutionary trajectories (Ward, 2009).It has been argued (Oksanen et al. 1981; Ayal et al. 2005) that herbivores are not a key factor in deserts, due to low levels of primary production. However, herbivory does play an important role in many aspects of the design and function of desert ecosystems. Herbivores remove valuable tissues from plants whose productivity is already limited by the harsh environment with potential consequences for the plants’ reproductive capacity. An estimated 2-10% of leaf and stem biomass is consumed by herbivores in deserts (Mares, 1999). In desert plants two defence mechanisms are common: (1) production of chemical compounds and (2) mechanical protection by a thick outer skin, spines or thorns. Herbaceous desert plants commonly contain alkaloids while higher concentrations of terpenes, amines or resins can be found in woody desert plants. The possession of spines or thorns may act to deter vertebrate herbivores or at least slow their feeding rate (Young, 1987). Desert plants may also minimize grazing damage via the timing of their growth (mostly ephemeral), or hiding. The curious ‘living stones’,


13

genus Lithops, in the Namib Desert disguise themselves as pebbles to avoid being eaten by herbivores (Fig. 8). The most common herbivores are mammals and insects. The Desert Locust, Schistocerca gregaria, was documented as early as in the Bible, and forms large swarms that can consume more than 1000 tons of plant matter a day (Mares, 1999). However, desert animals do not only have negative impacts on vegetation but can be integral parts of the ecosystem, e.g. by facilitating pollination, seed dispersal or water infiltration rates. Often mutualistic interactions between species can be recognized, e.g. plants provide shade and food whilst the animals disperse their seeds. A classic example of this is the highly complex three–way interaction between bruchid beetles (Bruchidae), Acacia spp. and large mammalian herbivores, such as the Dorcas gazelles (Gazella dorcas) and ibex (Capra ibex). These mammals are both predators and dispersers of Acacia seeds; while some seeds are destroyed, others are defecated unharmed. Ingestion by large herbivores facilitates germination by scarification of the seed coat. While infestation by bruchid beetles reduces Acacia germination, herbivores may reduce bruchid infestation by (1) stomach acids, by (2) the herbivore's teeth, or by (3) removing seeds prior to (re-)infestation (Or and Ward, 2003). Another example are Fat Sand Rats (Psammomys obesus; Fig. 9) that have developed a digestive mechanism which allows them to feed solely on desert plants of the family Chenopodiaceae which contain large amounts of salts in the leaves (Palgi et al., 2005). Apart from their trophic effect, shrubs modulate environmental parameters for animals: Woody vegetation in drylands provides shade, cool refuge in the hot days and warmer temperature in cool nights. In this manner they change movement behaviour (Shelef and Groner, 2010) and distribution of ground dwelling insects (Mazia et al., 2006) which in turn affects the behaviour and spatial distribution of their predators. Shrubs may also provide defence from bird predation for small animals (Groner and Ayal, 2001). Although, conversely, shrubs may attract predators that seek their prey mainly in shrub patches. However, for the predator, the risk of injury during contact with a shrub is an important trade-off factor (Berger-Tal et al., 2009) tipping the advantage in favour of their small, sheltering prey. References Aguiar, M.R. and Sala, O.E. (1999) Patch structure, dynamics and implications for the

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Figure captions Figure 1. Heterogenic desert landscape contains diversity of landforms, soils, and various rock cover. Kasui dune, Negev hills, Israel. Photo by Oren Shelef. Figure 2. Sternbergia clusiana bloom in the autumn in the Negev highlands, Israel. Adaptations for lack of water evolved in desert plants to enable survival and completion of life cycle in dry conditions. S. clusiana is a geophyte, relying on bulb that allows blooming before the rainy season. Photo by Oren Shelef. Figure 3. Herd of Oryx antelopes (Oryx gazella) in the Central Namib Desert (SW Africa). Photo by Elsita Maria Kiekebusch. Figure 4. Desert trees count on developed root systems, that allow them to utilize water from deep soil layers or, like this Pistacia atlantica (upper photo) - exploit protected water gripped in crevices. Developed roots of Tamarix aphylla were excavated by strong seasonal streams. Negev highlands, Israel. Photos by Oren Shelef. Figure 5. A liman (low dam) with planted Acacia spp. trees to collect run-off water from asmall catchment during rare desert rain events. Photo by Oren Shelef. Figure 6. Schismus arabicus is an annual grass, of the most common pasture plants in the deserts of Israel and Asia. Photo by Tanya Gendler. Figure 7. Strong wind may elevate heavy dust to several dozen meters height causing dust storms. This photo was taken from an aircraft during a dust storm in the Negev Desert, Israel. Figure 8. The curious ‘living stones’, genus Lithops, are succulents that exhibit similarity to pebbles, which helps avoiding herbivory (Central Namib Desert, SW Africa). The translucent cover of the leaves allows photosynthesis with excellent camouflage. Photo by Elsita Maria Kiekebusch. Figure 9. Fat sand rat


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Figure 1. Desert landscape.


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Figure 2. Sternbergia clusiana


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Figure 3 Herd of oryx.

Figure 4. A Liman, low dam for run-off water from small catchment


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Figure 5. Schismus arabicus


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Figure 6. Developed roots of desert trees


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Figure 7. A dust storm


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Figure 8. "Living stones", Lithops


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Figure 9. Fat sand rat.

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