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Chapter I. The arid environments
1. Meaning of aridity
2. Causes of aridity
3. Arid zone climate
4. Rainfall
5. Temperature
6. Atmospheric humidity
7. Wind
8. Arid zone soils and importance of soil properties
9. Arid zone vegetation
10. Classification of vegetation
11. Highlights of section
Arid environments are extremely diverse in terms of their land forms, soils, fauna, flora, water balances, and human activities. Because of this diversity, no practical definition of arid environments can be derived. However, the one binding element to all arid regions is aridity.
Aridity is usually expressed as a function of rainfall and temperature. A useful "representation" of aridity is the following climatic aridity index: p/ETP
where
P =
precipitation
ETP = potential
evapotranspiration, calculated by method of Penman, taking into
account atmospheric humidity, solar radiation, and wind.
Three arid zones can be delineated by this index: namely, hyper-arid, arid and semi-arid. Of the total land area of the world, the hyper-arid zone covers 4.2 percent, the arid zone 14.6 percent, and the semiarid zone 12.2 percent. Therefore, almost one-third of the total area of the world is arid land.
The hyper-arid zone (arid index 0.03) comprises dryland areas without vegetation, with the exception of a few scattered shrubs. True nomadic pastoralism is frequently practiced. Annual rainfall is low, rarely exceeding 100 millimeters. The rains are infrequent and irregular, sometimes with no rain during long periods of several years.
The arid zone (arid index 0.03-0.20) is characterized by pastoralism and no farming except with irrigation. For the most part, the native vegetation is sparse, being comprised of annual and perennial grasses and other herbaceous vegetation, and shrubs and small trees. There is high rainfall variability, with annual amounts ranging between 100 and 300 millimeters.
The semi-arid zone (arid index 0.20-0.50) can support rain-fed agriculture with more or less sustained levels of production. Sedentary livestock production also occurs. Native vegetation is represented by a variety of species, such as grasses and grass-like plants, fortes and half-shrubs, and shrubs and trees. Annual precipitation varies from 300-600 to 700-800 millimeters, with summer rains, and from 200-250 to 450-500 millimeters with winter rains.
Arid conditions also are found in the sub-humid zone (arid index 0.50-0.75). The term "arid zone" is used here to collectively represent the hyper-arid, arid, semi-arid, and sub-humid zones.
Aridity results from the presence of dry, descending air. Therefore, aridity is found mostly in places where anticyclonic conditions are persistent, as is the case in the regions lying under the anticyclones of the subtropics.
The influence of subtropical anticyclones on rainfall increases with the presence of cool surfaces. Arid conditions also occur in the lee of major mountain ranges that disrupt the structure of cyclones passing over them, creating "rain shadow" effects. Rainfall is also hindered by the presence of greatly heated land surfaces; as a consequence, large areas of dry climate exist far from the sea.
The arid zone is characterized by excessive heat and inadequate, variable precipitation; however, contrasts in climate occur. In general, these climatic contrasts result from differences in temperature, the season in which rain falls, and in the degree of aridity. Three major types of climate are distinguished when describing the arid zone: the Mediterranean climate, the tropical climate and the continental climate.
In the Mediterranean climate, the rainy season is during autumn and winter. Summers are hot with no rains; winter temperatures are mild. Figure 1.1 illustrates the Mediterranean climate, with a wet season starting in October and ending in April or May, followed by 5 months of dry season.
In the tropical climate, rainfall occurs during the summer. The greater the distance from the Equator, the shorter the rainy season. Winters are long and dry. In Sennar, Sudan, an area that is typical of the tropical climate, the wet season extends from the middle of June until the end of September, followed by a dry season of almost 9 months (Figure 1.2).
In the continental climate, the rainfall is distributed evenly throughout the year, although there is a tendency toward greater summer precipitation. In Alice Springs, Australia, each monthly precipitation is less than twice the corresponding mean monthly temperature; hence, the dry season extends over the whole year (Figure 1.3).
Figure 1.1 Annual precipitation and temperature in Rabat, Morocco.
Figure 1.2 Annual precipitation and temperature in Sennar, Sudan.
Figure 1.3 Annual precipitation and temperature in Alice Springs, Australia.
The rainfall that falls from the atmosphere at a particular location is either intercepted by trees, shrubs, and other vegetation, or it strikes the ground surface and becomes overland flow, subsurface flow, and groundwater flow. Regardless of its deposition, much of the rainfall eventually is returned to the atmosphere by evapotranspiration processes from the vegetation or by evaporation from streams and other bodies of water into which overland, subsurface, and groundwater flow move, as illustrated by the hydrologic cycle in Figure 1,4. The relative dynamics of the hydrologic cycle in an area are determined, in large part, by the spatial and temporal nature of the rainfall patterns, temperature and atmospheric humidity regimes, soil and topographic features, and vegetative characteristics of the area.
Unlike conditions in temperate regions, the rainfall distribution in arid zones varies between summer and winter. For example, Rabat, Morocco, receives rain during the cold winter period, while the warm summer months are almost devoid of rainfall. On the other hand, Sennar, Sudan, has a long dry season during the winter, while the rains fall during the summer months. Although Rabat and Sennar receive about the same amount of rainfall, the variation in rainfall is considerable. Winter rains in Rabat can penetrate the soil to underground storage, while the summer rains in Sennar fall on a hot soil surface and are lost to evaporation, particularly when rain falls in the form of light showers. Therefore, the effective rainfall available to plants is higher in Rabat than in Sennar.
This example shows that more annual precipitation is required in summer rainfall areas than in cooler winter rainfall climates to obtain the same amount of water available to the plants. However, where plants are dormant during the winter, they cannot completely use the available water during that period.
Rainfall also varies from one year to another in arid zones; this can easily be confirmed by looking at rainfall statistics over time for a particular place. The difference between the lowest and highest rainfall recorded in different years can be substantial, although it is usually within a range of ± 50 per cent of the mean annual rainfall. The variation in monthly rainfall is even greater.
In most instances, the expected rainfall in a given place is not the same as the mean annual rainfall recorded over a number of years. Variation in rainfall is important to forestry activities, because when the rains fail, newly-established forest plantations suffer. The selection of a planting date to coincide with rainfall is of paramount importance to the success of a forest plantation.
Figure 1.4 The hydrologic cycle.
Rainfall intensity is another parameter which must be considered. Because the soil may not be able to absorb all the water during a heavy rainfall, water may be lost by runoff. Likewise, the water from a rain of low intensity can be lost due to evaporation, particularly if it falls on a dry surface. Rainfall intensity can be measured as the number of rainy days or, more preferably, as the amount of rain per hour or per day.
Rainfall intensity also relates to the risk of soil erosion. It is known that individual raindrops carry energy capable of removing soil, particularly topsoil. The erosion caused by falling drops of water, called splash erosion, also can degrade or destroy the soil structure. It has been found that, as the rainfall intensity approaches 35 millimeters per hour, there is a steep rise in the erosive power of the rain. A large percentage of rainfall in the tropics occurs above this value (the so-called "erosion threshold").
The climatic pattern in the arid zones is frequently characterized by a relatively "cool" dry season, followed by a relatively "hot" dry season, and ultimately by a "moderate" rainy season. In general, there are significant diurnal temperature fluctuations within these seasons. Quite often, during the "cool" dry season, daytime temperatures peak between 35 and 45 centigrade and fall to 10 to 15 centigrade at night. Daytime temperatures can approach 45 centigrade during the "hot" dry season and drop to 15 centigrade during the night. During the rainy season, temperatures can range from 35 centigrade in the daytime to 20 centigrade at night. In many situations, these diurnal temperature fluctuations restrict the growth of plant species.
Growth of plants can take place only between certain maximum and minimum temperatures. Extremely high or low temperatures can be damaging to plants. Plants might survive high temperatures, as long as they can compensate for these high temperatures by transpiration, but growth will be affected negatively. High temperatures in the surface layer of the soil result in rapid loss of soil moisture due to the high levels of evaporation and transpiration. Although problems of low temperatures, in general, are less common in arid zones, when they do occur for relatively long periods of time, plant growth can be restricted; at temperatures below 0 centigrade, the plants can die.
Although rainfall and temperature are the primary factors upon which aridity is based, other factors have an influence. The moisture in the air has importance for the water balance in the soil. When the moisture content in the soil is higher than in the air, there is a tendency for water to evaporate into the air. When the opposite is the case, water will condense into the soil. Humidity is generally low in arid zones.
In many areas, the occurrence of dew and mist is necessary for the survival of plants. Dew is the result of condensation of water vapor from the air onto surfaces during the night, while mist is a suspension of microscopic water droplets in the air. Water that is collected on the leaves of plants in the form of dew or mist can, at times, be imbibed through the open stomata, or alternatively, fall onto the ground and contribute to soil moisture. The presence of dew and mist leads to higher humidity in the air and, therefore, reduced evapotranspiration and conservation of soil moisture.
Because of the scarcity of vegetation that can reduce air movements, arid regions typically are windy. Winds remove the moist air around the plants and soil and, as a result, increase evapotranspiration.
Soil erosion by wind will occur wherever soil, vegetative, and climatic conditions are conducive to this kind of erosion. These conditions (loose, dry, or fine soil, smooth ground surface, sparse vegetative cover, and wind sufficiently strong to initiate soil movement) are frequently encountered in arid zones. Depletion of vegetative cover on the land is the basic cause of soil erosion by wind. The most serious damage from wind-blown soil particles is the sorting of soil material; wind erosion gradually removes silt, clay, and organic matter from the surface soil. The remaining materials may be sandy and infertile. Often, sand piles up in dunes and presents a serious threat to surrounding lands.
Precipitation represents the main transfer of moisture from the water vapor of the air to the ground. The completion of this hydrologic cycle is through evaporation. Loss of water from the soil due to evaporation is important when considering "effective" rainfall. Evaporation increases with strong winds, high temperatures, and low humidity.
As mentioned above, plants must transpire to compensate for high temperatures. Transpiration accounts for great losses of moisture from the soil. The intensity of transpiration depends on wind, temperature, humidity, and the plant itself. Some plants are more adapted to dry conditions and transpire less than others. Therefore, the composition of the vegetation has a great influence on the rate of transpiration. The combination of evaporation and transpiration, called evapotranspiration, is the principal component of the water cycle that can be influenced by land management to increase water yield.
8. Arid zone soils and importance of soil properties
Soils are formed over time as climate and vegetation act on parent rock material. Important aspects of soil formation in an arid climate
- Significant diurnal changes in temperature, causing mechanical or physical disintegration of rocks.
- Wind-blown sands that score and abrade exposed rock surfaces.
The physical disintegration of rocks leaves relatively large fragments; it is only chemical weathering which can break up these fragments. The process of chemical weathering in arid zones is slow because of the characteristic water deficit. Also, extended periods of water deficiencies are important in the elimination or leaching of soluble salts, for which the accumulation is enhanced by the high evaporation. Short periods of water runoff do not permit deep penetration of salts (only short-distance transport), often resulting in accumulation of salts in closed depressions.
Vegetation plays a fundamental role in the process of soil formation by breaking up the rock particles and enriching the soil with organic matter from aerial and subterranean parts. However, this role of the vegetation is diminished in arid zones because of the sparse canopy cover and the limited development of aerial parts. Nevertheless, the root systems often exhibit exceptional development and have the greatest influence on the soil.
A forester is usually more concerned with the soil properties that are important to support growth of trees and shrubs than with the evolution of the soil profile or with the systems of regional soil classification. Of primary importance for arid zone soils are the water-holding capacity and the ability to supply nutrients.
The water-holding capacity of a soil depends on its physical characteristics, including texture, structure, and soil depth. Texture refers to the relative distribution of the particles (clay, sand, and silt). In general, the finer the texture, the greater is the water retention. Structure, the internal arrangement of the soil particles, is influenced by the amount of organic matter that binds the soil particles. Sandy soils have no structure; clayey soils have different forms of structure and the spaces between particles enable circulation of air and water. The larger these spaces, the greater is the permeability.
The soil depth governs the amount of soil moisture and the type of root disposition of trees. In general, colluvial and alluvial soils are deep; but residual soils are highly variable in depth, depending on the degree of slope, the length and intensity of weathering, and the biotic influences (cultivation, livestock grazing, etc.). Soils of the ridges and upper slopes are often shallow, while those of the midslopes and valleys are moderately deep to very deep. The depth of soils in arid regions is often limited by a "hardpan" layer. Such hardpans, which consist of ironstone or laterite gravel in the tropical zone and consolidated calcite in the Mediterranean region, can be more or less continuous and from 5 to 60 centimeters below the surface.
As there is little deposition and accumulation of organic litter in arid zones, the organic matter content of the soil is low. When these soils are cultivated, the limited organic matter content that exists is quickly lost.
The chemical properties of soil control the availability of nutrients. Arid soils are characterized by significant leaching of nutrients and intensive weathering of minerals, although these two activities are slowed with decreasing rainfall. Natural fertility (which largely depends upon the organic matter content of the topsoil) is often low.
Because of the aridity of the climate, edaphic characteristics which ease the water constraints will be favorable to planting of trees or shrubs. Some of these edaphic characteristics are:
- The presence of a water table at a depth attainable by the roots.
- A soil thickness adequate to allow a water reserve.
- A soil texture which retains the maximum amount of water.
It should not be overlooked that the topography of the terrain can also play an important role. For instance, the shallows and the lower parts of sand dunes can accumulate a considerable quantity of water which can be used by an adapted vegetation.
Finally, because arid zone soils are vulnerable to both wind and water erosion, soil fixation and conservation are important.
The vegetation cover in arid zones is scarce. Nevertheless, three plant forms can be distinguished:
- Ephemeral annuals.
- Succulent perennials.
- Nonsucculent perennials.
Ephemeral annuals, which appear after rains, complete their life cycle during a short season (± 8 weeks). Their growth is restricted to a short wet period. Ephemerals do not have the xeromorphic features of perennials. In general, ephemerals are small in size, have shallow roots, and their physiological adaptation consists of their active growth. Ephemerals live through the dry season, which may last a number of years, in the form of seeds. At times, ephemerals can form dense stands and provide some forage.
Succulent perennials are able to accumulate and store water (that may be consumed during periods of drought); this is because of the proliferation and enlargement of the parenchymal tissue of the stems and leaves and their physiological feature of low rates of transpiration. Cacti are typical succulent perennials.
Nonsucculent perennials comprise the majority of plants in the arid zone. These are hardy plants, including grasses, woody herbs, shrubs and trees that withstand the stress of the arid zone environment. Many nonsucculent perennials have "hard" seeds that do not readily germinate; these seeds often must be treated (by soaking in water or acid) before they will germinate. Three growth forms of nonsucculent perennials can be distinguished:
- Evergreen - biologically active throughout the year.
- Drought-deciduous - biologically dormant during the dry season.
- Cold-deciduous - biologically dormant during the cold season.
Ephemerials are drought-escaping species and, in general, are not considered true xerophytes; succulent and nonsucculent perennials are drought-enduring and drought-resisting species and are true xerophytes. Xerophytism refers to adaptive attributes of plants which can subsist with small amounts of moisture. Some of the features of xerophytic plants are:
- Development of an extensive root system - the main growth of the roots can be vertical, horizontal, or both, and seems to depend on the site conditions. Roots penetrating 10 to 15 meters in depth are not unusual; horizontally extending roots are common in shallow soils. Some xerophytic species produce "rain roots" below the soil surface, in response to light rainfall or during periods of dew formation.
- Shoots not as large as their roots - shoot-to-root ratios of 1:3.5 to 1:6 are frequent.
- Reduction of the transpiring surface - transpiring surfaces reduced by shedding of foliage and rolling of leaves.
- Seasonal reduction of the transpiring surface of the plant this feature results in a reduction of the water loss during the dry season.
- Special adaptations in "evergreen" species lessen transpiration their leaves are leathery and often heavily wax-coated; these plants are referred to as sclerophylls.
Other distinguishing anatomical characteristics associated with xerophytism are:
- Cuticularization - the formation of a surface plaster-like layer of cutin.
- Cutinazation - the impregnation of the cell wall with cutin, which forms a watertight layer with abundant hairs.
- Special arrangements of the stomata in recesses and grooves which provide protection from the arid atmosphere.
10. Classification of vegetation
In classifying the vegetation of the arid zones, the major "delineations" are usually characterized in terms of rainfall amount and the pattern of occurrence.
- Desert - In this manual, the term "desert" is used in its narrowest sense to classify land where vegetation is virtually absent, except by watercourses. Ephemeral grasses and herbs can appear after infrequent rain showers. On the average, rainfall is less than 100 millimeters per year.
- Semidesert - Vegetation in semidesert regions is a mixture of grasses, herbs, and small, short trees and shrubs up to 2 meters in height, interspersed with bare areas. Semidesert grasslands occur in areas where geological erosion has been less intense and the soils can absorb the limited rainfall that falls on them; the resulting vegetation is a uniform cover of mixed grasses and herbs. Scattered trees and scattered shrubs occur on areas where there is an excess of water, along drainage lines, and on catchment sites. Succulent shrubs consist of open plant communities dominated by succulent plants; grasses may or may not be present. In general, the vegetation in semidesert regions is characterized by an abundance of plants with extreme reduction of leaves, the development of storage tissues to form succulent stems, and the presence of thorns and spines. The rainfall in these regions varies from 100 to 300 millimeters; most of this is unreliable and confined to several months, occurring as local storms or scattered rain showers.
- Low rainfall woodland savanna - The vegetation in this region includes a mixed type of grasses and herbs, with shrubs or trees (or both), in which the proportion of grass to shrubs or trees is determined by the frequency and intensity of fires. The trees and shrubs often have flat, umbrella-like crowns. Their crowns do not form a closed canopy, but leave large openings filled with low shrubs, grasses, and herbs, although bare spots occur as well; sometimes these bare areas are covered with ephemerals after rains. Grasses seldom attain 2 meters, and the shrubs and trees are not higher than 6 meters. The shrubs and trees normally do not provide sufficient shade to prevent the development of grasses. During the dry season, these plants become a potential fire hazard. However, the species of the woodland savanna have some degree of fire tolerance. When grasses and herbs are dominant and shrubs and trees cover less than 50 per cent of the ground, the woodland is classified as open and wooded grassland. This kind of woodland savanna is typical of the dry tropics, with a short rainy period followed by a long, hot dry period. Rainfall ranges from 300 to 600 millimeters.
- Evergreen scrub - This type of arid zone vegetation consists of a closed scrub of evergreen or semievergreen shrubs, small trees, climbers, and occasionally, some large trees. Shrubs have glossy, leathery leaves or thorny, succulent leaves and are 2 to 3 meters high. The larger trees are widely scattered. Annual rainfall exceeds 500 millimeters.
The arid environment can be defined as one in which the amount of precipitation an area receives, divided by the amount which is lost to evapotranspiration, yields a fraction which is less than 0.50. The arid environment can be stratified into three zones: hyper-arid, arid, and semi-arid. The zones are characterized by low annual precipitation (0 to 800 millimeters), which occurs infrequently and irregularly, and is vegetated by drought resistant species. Almost one-third of the total land area of the world is arid.
In the arid zones, there are three major climatic types: Mediterranean, tropical, and continental. Rainfall distribution of these types varies between winter and summer and from year to year. Also, rainfall intensity affects the arid zones differently, as losses due to evaporation and runoff can vary among the climatic types; the potential for soil erosion differs depending on edaphic characteristics. Temperature is characterized by significant diurnal fluctuation and affects plant growth. Plant growth also is affected by atmospheric humidity, which is generally low. Evapotranspiration is increased by wind, which also can cause soil erosion. Soils in the arid zones are formed when the significant diurnal temperature changes mechanically disintegrate rocks, wind-blown sand abrades rock surfaces, or the root systems of plants break up rock particles. In general, chemical weathering is not significant, as the low water content of the soil slows this process. The water-holding capacity of arid zone soils depends on the structure (internal arrangement of soil particles), texture (relative distribution of different-sized particles), and depth (often limited by a hardpan layer). Leaching of nutrients and weathering of minerals are slowed with decreasing rainfall.
When choosing planting sites, soils and edaphic characteristics which ease water constraints are favored. Vegetative growth in arid zones are: ephemeral annuals (growth that is restricted to the short wet period), succulent perennials (which can store water to use during drought), and nonsucculent perennials (the majority of plants which can withstand the stress of the arid environment). Xerophytic plants, also found in arid environments, have extensive root systems, reduced transpiring surfaces, a layer of cutin, and other anatomical characteristics that protect the plant from the arid atmosphere. Arid zone vegetation is classified as desert (less than 100 millimeters of rainfall annually), semidesert (100 to 300 millimeters of annual rainfall, low rainfall woodland savanna (300 to 600 millimeters), and evergreen scrub (in excess of 500 millimeters).