The Pleistocene “Tundra-Steppe” and the productivity paradox: the landscape approach

The Pleistocene “Tundra-Steppe” and the productivity paradox: the landscape approach

Abstract

“Tundra-steppe” means either a certain type of plant community with codominance of both steppe and tundra species (including prostrate shrubs), or a type of landscape, codominated by both steppe and tundra (Yurtsev, Relic Steppe Complexes of Northeastern Asia. Nauka Press, Novosibirsk (in Russian) 1981; In: Hopkins, Matthews Jr., Schweger, Young, (Eds.), Paleoecology of Beringia. Academic Press, New York, 1982, pp. 157–177). A discrepancy between Pleistocene glacial climates that were much colder and drier than present in Beringia and the highly diverse herbivorous fossil fauna (the “productivity paradox”) is explained in terms of much greater diversity of herbaceous vegetation (grasses, sedges and forbs) in the mosaic of Beringian ‘tundra-steppe’ landscapes. Analysis of the relic distribution of some predominantly herbaceous plant communities throughout Beringia (Yurtsev, 1981, 1982, Komarovskiye chteniya (Vladivostok) 33 (1986) 3–53 (in Russian); Protection of Gene- and Coenotic Pool of the Herbaceous Biogeocoenoses, Sverdlovsk, 1988, pp. 128–129 (in Russian); Bridges of Science Between North America and the Russian Far East, 45th Arctic Science Conference, Abstracts, Vol. 1. Dalnauka Press, Vladivostok, 1994, p. 268; Paleontological Journal, 6 (1996)) provides the phytogeographic and landscape — ecological grounds for the reconstruction of plant cover of these landscapes. Dry watersheds and slopes had cryophytic (cold-adapted) steppes, cryoxerophytic (cold and dry-adapted) herbaceous and prostrate shrub-herbaceous communities, dry herb-prostrate shrub tundras, and tundra-steppe communities proper. All sorts of depressions on interfluves and in valleys along with concave pediments were occupied by dry steppe-meadows and brackish-water moist meadows. In some specific habitats sparse groupings of continental halophytes (plants growing in saline soils) of “arctic takkyrs”, zoochoric (plants with seeds dispersed by animals) groupings of annual-biennial “ruderals” and, finally, sparse xeric-psammophytic (plants adapted to dry, sandy soils) vegetation of “sand seas” occurred. Meadows in valleys and on slope pediments were the most productive as pastures for ungulates due to the redistribution of moisture and nutrients within landscapes. The lowest parts of the exposed shelf in the Bering Strait vicinities were the “bottle-neck” for the dispersal of steppe plants and animals.

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Nature and Wildlife

Natural Zone of Mongolia: Mongolia is one of the few countries, which possesses a great range of natural ecosystems within their borders. Mongolia is located at the junction of Siberian taiga, Central Asian prairie, steppe, and deserts. This junction offers distinct species of fauna and flora throughout the territory. Largely unknown to the rest of the world until recent years, Mongolia’s unique combination of diverse landscapes, unspoiled habitat, rare wild plant and animal species, clear water and fresh air has started to attract international nature conservation organizations and has become the subject of growing international attention. The six basic natural zones of Mongolia differ in climate, landscape, soil, flora and fauna.

1. High Mountains- Including the Altai Mountains and Tavan Bogda

Mongolia is a mountainous country. Though the high mountain zone, which includes the higher elevations of these ranges, makes up only about 5 percent territory of Mongolia, average elevation of the country is quite high, at 5,184 feet (1,580 m.) above sea level. In the Far Western Altai, Khuiten Peak in the top parts of Tavan Bogda Mountains reaches 14,350 feet (4,374 m.), the highest point in the country. Altai, Khangai and Khentii mountain ranges and the Khuvsgul Mountains are higher than the height of forest zone. Many mountain areas of Mongolia show signs of previous Ice Ages, with U-shaped valleys and boulders left behind by retreating glaciers. The climate in the high mountain zone is extremely cold, and there is a short growing season. Located above tree line, the zone is characterized by tundra, alpine-sedge meadows, highland swamps, and lichen-covered boulder fields.

Highest Mountains of Mongolia:

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Steppe

A steppe is a dry grassland with high temperatures in the summer and low temperatures in the winter.

Related terms:

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Steppes and Prairies

J.M. Briggs, . S.L. Collins, in Encyclopedia of Ecology , 2008

Steppes and prairies (grasslands) are ecosystems that are dominated by grasses and to help understand grasslands, it is important to know something about grass morphology and growth forms. The remarkable ability of grasses to thrive in so many ecological settings and their resilience to disturbance is largely attributable to their growth form. Grasses are characterized by streamlined reduction and simplicity with tillers being the key adaptive structural element of the plant ( Figure 1 ). Tillers originate from growing parts (meristems) typically just near, at, or below the surface of the soil. The meristems that produce tillers are generally well protected by their location near or beneath the soil surface. It is the location of the meristem that explains much of the resilience of grasses and thus grasslands to disturbance.

Figure 1 . Common oat, Avena sativa, × ½. From Hubbard (1984).

Ecology of amphibians in north-west Africa

Daniel Escoriza, Jihène Ben Hassine, in Amphibians of North Africa , 2019

Steppes

The steppe is a transitional ecosystem between the Mediterranean and the Saharo-Arabian phytogeographical regions, mainly formed by perennial grasses and small shrubs ( Fennane et al., 1999 ) ( Fig. 3.6 ). Steppes become deserts when rainfall is below 100 mm per year and savannahs when it exceeds 300 mm per year ( Tucker et al., 1991 ; De Waroux and Lambin, 2012 ). The steppes occupy a vast area in north-west Africa, extending from the Atlantic coast of Morocco to northern Libya. The temperatures are moderate to high (15°C–19°C), with low precipitation (100–300 mm per year) in no month surpassing the arid threshold (30 mm). The diversity of amphibians is low (three to five species): B. brongersmai, B. boulengeri, Discoglossus pictus, P. saharicus, and S. mauritanica, relegated to wadis, irrigation fields, and oases ( Salvador, 1996 ; Ibrahim, 2014 ).

Figure 3.6 . Steppes: annual grasses and thorny scrub with scattered Phoenix dactylifera trees in the surroundings of Sbih (southern Tunisia). Habitat of Bufotes boulengeri.

Photo: Daniel Escoriza.

Study Areas

V.V. Snakin, . E. Kovács-Láng, in Soil Liquid Phase Composition , 2001

3.3 VEGETATION

Wooded steppe , the zonal vegetation of the Great Hungarian Plain (Alföld) is a continuation of the Southern Russian steppe zone forming its Western border.

The vegetation of the Bugac site has a mosaic pattern. The mosaic is formed of forest patches of Populus canescens, sparse woodland patches of Juniperus communus — Populus alba, mainly on tops of hills enclosing patches of denser grassland formed by Calamagrostis epigeios and the open perennial sand grassland of Festuca vaginata typical for the place. Bare patches with open sand surface nudum or with dense moss and lichen cover also occur.

The mosaic pattern of the vegetation is controlled by herbivores (deer, rabbit, etc).

Our investigation was predominantly concerned with the widespread and characteristic community of Festucetum vaginatae danubiale ( Table 28 ). The canopy cover equals 40%. The number of species is rather low, but these are adapted to the water limited habitat.

Table 28 . The vegetation of the research sites

Site Vegetation type Ecosystem type Phytomass * (g/m 2 ) Projective cover (%) Number of species / m 2
aboveground roots
living total (0-20 cm)
Rugac Primary edaphophytic semidesert vegetation perennial sandy open community Festucetum vaginatae danubiale 59 294 1050 60 32
Császártöltés A fragment of relic primary steppe vegetation loess steppe community Salvio-Festucetum rupicolae stipetosum pannonicum 273 420 1500 100 44
Khomutovskaya steppe A xerophytic version of primary steppe vegetation Mixed fescue-stipa steppe community Salvio-Festucetum rupicolae stipetosum ponticum 350 500 1800 100 82
Kameničky Secondary meadow vegetation mesophytic grassland community Polygalo-Nardetum strictae 242 990 2200 100 43

The dominating species is the subendemic Festuca vaginata W. et K., the condominating species are Koeleria glauca (Schk.) DC, Carex liparicarpos Gaud. The most typical and widespread are Stipa borysthenica Klokov, Alkanna tinctoria (L.), Tausch., Euphorbia seguieriana Necker, Fumana procumbens (Dun.) Gr. et Godc., Ephedra distachya L., Potentilla arenaria Berkh., Colchicum arenarium W. et K., Linum hirsutum L.. The abundance of over 32 Mediterranean and continental species may found there is abundance. The moss-lichen synusia are well developed.

The grass cover of the Császártöltés site is represented by a Salvio-Festucetum rupicolae stipetosum pannonicum community. The loess patches have preserved fragments of this primary steppe vegetation and these relic plots are the rephugiums for the steppe flora. As a result of agricultural activities, the area of these steppes drastically decreased and now there are only a few patches with an area of 100-500 m 2 each.

A characteristic community of the Great Hungarian Plain is the mixed grass-fescue-stipa steppe, with Salvio nutanti-nemorosae-Festucetum rupicolae. It is rich in species of Pontic (Black Sea) and continental origin. This colorful community is characterized by high density sward and the canopy cover is 100% with a high biodiversity. The dominant species are fescue (Festuca rupicola Heuff), feather grass (Stipa capillata L.), bluestem (Andropogon ischaemum L.). In addition Astragalus onobrychis L., Medicfgo falcata L., Seseli varium Trev., Centaurea sadleriana Janka and some others are widespread. The number of species is 44/m 2 and there is almost no moss-lichen cover.

The grass cover of the Khomutovskaya steppe is as a xeric version of mixed sheep’s fescue stipa steppes ( Kleopov & Lavrenko, 1933 ). The dominant species are sheep’s grass and feather grass, Festuca rupicola Heuff., Stipa lessingiana Trin. et Rupr., Stipa capillata L.

We have studied the mixed community Salvio-Festucetum rupicolae stipetosum ponticum at strict conservation and periodical mowing regime. It partly found spread on the plain and steep slopes The projective canopy cover is 95-100%, including. Stipa lessingiana (30-45%), Festuca rupicola (15-25%), Stipa capillata (10%), Salvia nutans L. (5-10%) and Achillea setacea W. et K. (3%). The characteristic species of the community are Phlomis tuberosa L., Crambe tataria Sebeok, Medicago romanica Prod., Otites chersonemis Cleop. and Linum austriacum L.

Some 82 flowering plant species have been registered. The biodiversity is high, with 24-27 species per m 2 . No moss-lichen cover has been found.

In the grass cover of the Khomutovskaya steppe, the creeping Agropyron community can be found under the same conditions as the Festuca grassland. The predominance of a certain species in the grass cover is controlled by the land use regime. In the territories of strict protection, which rules out mowing and grazing, the species of Agropyron (Agropyron repens P. B., Agropyron trychophora (Link.) Nevski), bluegrass (Poa angustifolia L.), bromus (Bromus inermis Leyss) are widespread. They have a high density of predominantly monodominant sward of poor species composition and a rich layer of litter (up to 15 cm) which is preserved during the whole vegetation period. Periodic mowing or grassing favours the development of species rich bunchgrass communities and in the Khomutovskaya steppe they are Salvio-Festucetum rupicolae stipetosum ponticum.

The mowed and unmowed plots of the Polygalo-Nardetum community in the Kameničky site is a typical secondary community emerged from the wet submontaneous alder and spruce stands and is described as Piceo-Alnetum ( Rybničkova & Rybniček, 1979 , 1988 ).

The species composition of the studied community (Polygalo-Nardetum strictae) is characterized by the predominance of Nardus stricta L., covering almost the whole area (80 — 100 %). Festuca capillata L. (cover 60-80%) and Sanguisorba officinalis L. (cover 60-80%) are spread as codominants. From the total amount of 43 higher plants species, the most frequent are Potentilla erecta (l.) Raeu., Luzula capestris (l.) Dc., Carex pilulifera L.. Anthoxanthum odoratum L., Ranunculus Acer L., Bryza media L. Mosses are: Climacium dendroides (Hedw.), Web. et Mohr, and Aulacomnium palustre (Hedw.) Schwaegr ( Balatova-Tulackova et al., 1977 ; Zelena, 1979 ). Shrubs also frequently occur. The gradual accumulation of dead wood and increase in the amount of Nardus stricta, Sanguisorba officinalis, Deschampsia caespitosa and Avenula flexuosa takes place in the unmowed plots.

Western Steppic Rivers

A.N. Sukhodolov, . M.A. Usatii, in Rivers of Europe , 2009

13.3.3 Land Use Patterns

The Western Steppe was relatively untouched by humans until the 17th century. Earlier, the territory was inhabited by nomadic tribes that used the land as pasture for domestic cattle and for hunting. The first agricultural settlements were developed by the Romans in the western part of the steppe along river valleys of the Dniester. The northern-forested area along the Dnieper was used for timber and agriculture. Land use consisted of a semi-nomadic agriculture of ancient Slavs that burned the forests as fertilizer for crops of millet oats, barley, and wheat, often moving from place to place (Kluchevsky 2000). This land use process eliminated the pristine deciduous and coniferous forests over a large area that stretched as far south as Kiev (Gumilev 1989).

Following the appearance and growth of cities, development of the states, and establishment of military control against nomadic invasions, exploration of the steppe occurred rapidly. The union of Ukraine to the Russian Empire, and specific colonization projects by Ekaterina II at the end of 18th century, attracted colonists from Germany that settled and cultivated lands along the Southern Bug and on the southeast area of the steppe (north of Crimea). Zaporizhzhya Cossacks were transferred to the Northern Caucasus in 1792 to colonize the Kuban catchment and resulted in agricultural military settlements that began exploration of pristine steppic lands. By the end of the 19th century, 65–70% of the Western Steppe was used for agriculture (mainly rye, corn, potato and flax). Government organizations began to express concern about preserving the natural steppe at this time, and a national reserve, Askania Nova (33 307 ha), was established in 1898 in the Kherson region (Krutyporokh & Treus 1967).

Since the times of Ancient Rus, the southern steppe region has been mined for the rich deposits of table salt by Ukranian salt traders. Intensive industrialization in the mid-20th century brought new significant land use changes to the Western Steppe. The Donets and western Don catchments were mined for coal, while the southern Dnieper catchment housed a massive manufacturing industry ( Golobutsky 1970 ). Energy demands of the growing industry resulted in the construction of large hydropower stations on the Dnieper below a vast cascade of reservoirs. The reservoirs flooded ∼6900 km 2 of fertile land, withdrawing it from agriculture ( Vishnevsky 2000 ; Vishnevsky & Kosovets 2003). Subsequently, industrial development and accelerated urbanization have caused even higher water demands, promoting irrigational and water transfers from other watersheds via the Volga–Don and North-Crimea canal systems.

Presently, the agricultural system of land use is based heavily on the collective farming scheme inherited in the post-soviet period. Although some private farming has developed during the last decade, a significant reduction in agriculture has occurred. Increases in costs for fertilizers and chemicals (pesticides, herbicides) traditionally used during the soviet period have caused a significant reduction in their use today. However, other factors such as industrial growth, degradation of canals and irrigation systems, and inefficient sewage treatment have become more pronounced and are primarily responsible for ground and surface water pollution.

Climate-Smart Soil Management in Semiarid Regions

Noelia Garcia-Franco, . Martin Wiesmeier, in Soil Management and Climate Change , 2018

Semiarid Grasslands

Semiarid grasslands, also known as steppes , are among the largest ecosystems globally, distributed predominantly across continental areas of temperate zones and to a lesser extent in subtropical zones. The largest temperate steppe of the world is the Eurasian steppe, stretching from northern China in the east to eastern Europe in the west. Further important temperate semiarid grasslands are the prairies of North America, the veld in southern Africa, and the semiarid pampas in South America. Subtropical steppes are located in Mediterranean areas of Europe, North America, and Australia.

Semiarid grasslands are among the most important terrestrial C sinks, storing approximately 15% of total SOC stocks ( Lal, 2004 ). Steppe soils are characterized by high soil organic matter (SOM) content, a result of the high productivity of grasslands and the comparatively low decomposition during dry summer months. Ordered along decreasing precipitation and SOM content, typical soils of steppe ecosystems are Phaeozems, Chernozems, and Kastanozems ( IUSS Working Group WRB, 2015 ).

Whereas large-scale commercial livestock grazing was introduced to semiarid grasslands in relatively recent times in America and Australia, the steppe ecosystems in Africa and Eurasia reveal ancient grazing systems, having been used for extensive nomadic pastoralism for several millennia. In response to high spatial and temporal variability of climate and vegetation in semiarid conditions, nomadic peoples developed a flexible, mobile grazing strategy, characterized by a seasonal change of pastures, a diversity of grazing animals, and a detailed knowledge of animal physiology and plant ecology, enabling sustainable use of limited grassland resources.

However, steppe ecosystems have been subject to increasing degradation since the second half of the 20th century. In addition to expanding crop production, the introduction of static grazing management and increased stocking density has resulted in overgrazing and severe soil degradation. In northern China, for example, the traditional extensive nomadic pastoralism of the Mongols changed to an intensive sedentary livestock production system within the short period between the emerging communism in the 1950s and the privatization of livestock and pastures in the 1980s ( Thwaites et al., 1998; Akiyama and Kawamura, 2007 ). This has resulted in degradation of up to 70% of the total steppe area in some regions ( Tong et al., 2004 ).

A direct result of overgrazing is a substantial loss of SOC and N. This decline is attributable to the lower input of SOM due to overgrazing-induced decreases in above- and below-ground net primary productivity (NPP), and a concurrent decrease in surface roughness leading to enhanced erosion of C-rich topsoils ( Hoffmann et al., 2008a ). In addition to reduced inputs, heavy grazing leads to soil compaction and a deterioration of soil structure ( Hamza and Anderson, 2005 ), destroying soil aggregates and resulting in the decomposition of formerly protected SOM ( Steffens et al., 2009; Wiesmeier et al., 2012b ).

Grazing-induced losses of SOC and N are documented for steppe ecosystems in northern China ( Steffens et al., 2008 ), North America ( Neff et al., 2005 ), South America ( Abril and Bucher, 1999 ), and Africa ( Hiernaux et al., 1999 ). Despite this, global metaanalyses on grazing effects on grassland SOC stocks reveal no uniform trend ( Milchunas and Lauenroth, 1993; Pineiro et al., 2010; McSherry and Ritchie, 2013 ), as effects may be confounded by interactions of precipitation with soil texture, as well as grazing-associated, vegetation-specific shifts in belowground C allocation ( Reeder and Schuman, 2002 ). In addition to SOC and N losses, intensive grazing destroys heterogeneous vegetation patterns characteristic of semiarid lands ( Nash et al., 2004; Wiesmeier et al., 2009 ), resulting in increased erosion and reduced water infiltration, with a feedback loop to further SOM losses.

Climate-Smart Management Practices of Semiarid Grasslands

Optimized Grazing Intensity

In view of severe declines of SOC and N due to heavy grazing, a reduction of grazing intensity and stocking rates is a promising option for sustainable steppe soil management ( Yu et al., 2004 ), and the positive effects of reduced grazing pressure on above- and below-ground NPP as well as SOC and N stocks are well documented. Biomass strongly increases shortly after decreasing stocking rates ( Schönbach et al., 2010 ), resulting in marked increases of SOC and N stocks, particularly in labile SOC fractions ( Zhou et al., 2010; Cao et al., 2013 ). Studies have shown that no more than 40%–50% of aboveground NPP may be removed for sustainable grazing management ( Biondini et al., 1998; Wang et al., 2008 ).

Net GHG emissions can be effectively neutralized via reducing stocking rates in grasslands ( Schönbach et al., 2012 ). However, socioeconomic constraints and meat demand constrain the implementation of low-intensity management systems and a reduction of grazing intensity in one region may be offset by intensified grazing in other steppe regions. As such, maintaining a high proportion of ungrazed land next to intensively grazed lands may result in optimal economic returns and minimal GHG emissions ( Schönbach et al., 2012 ). Rotational grazing systems with periodic changes of grazed/ungrazed areas is a promising sustainable management option for semiarid grasslands ( Pineiro et al., 2010; Xiong et al., 2016 ) and may be a suitable alternative to the static separation of high-intensity grazed lands and ungrazed areas.

Grazing Exclusion

Grazing exclusion (i.e., the cessation of grazing by fencing) is a common practice for the regeneration of degraded semiarid ecosystems across the globe ( Conant and Paustian, 2002; Xiong et al., 2016 ). Grazing exclusion effectively regenerates steppe vegetation, resulting in increased above- and below-ground NPP and litter mass ( Xiong et al., 2016 ) and reversing the decline in surface roughness ( Hoffmann et al., 2008b ). Concurrently, soil structure improves due to the absence of mechanical stress combined with higher organic matter inputs, leading to increased physical protection of SOM within soil aggregates ( Wiesmeier et al., 2012b ). As a result, SOC and N stocks significantly increase after grazing exclusion, particularly in the topsoil ( Yong-Zhong et al., 2005; Wiesmeier et al., 2012a ).

Although a metaanalysis revealed that grazing exclusion led to a mean SOC increase of 14% in China ( Xiong et al., 2016 ), the results indicated the effects are short-lived, and a new SOC equilibrium is reached after approximately 10 years. As such, it is questionable whether observed SOC increases contribute to long-term C sequestration, especially because SOC gains after grazing exclusion mainly occur as labile particulate organic matter, whereas more stable silt and clay fractions are close to C saturation, even in degraded soils ( Steffens et al., 2009; Wiesmeier et al., 2015 ) ( Fig. 2A and B ).

Fig. 2 . Intensively grazed steppe (A) and experimental sites with long-term grazing exclusion vs. continuous sheep grazing, Inner Mongolia, Northern China (B).

Long-term grazing exclusion is an effective way to restore patchy steppe vegetation and thus restore heterogeneous ecosystem functionality. However, grazing exclusion cannot be regarded as a panacea, as several studies in North America found no benefit of grazing exclusion on SOC and N ( Reeder and Schuman, 2002; McSherry and Ritchie, 2013 ). Alternatively, the regeneration of heavily degraded grasslands can be actively promoted by creating heterogeneously distributed obstructions, mimicking the natural distribution of soilscape features ( Tongway and Ludwig, 1996; Xiong et al., 2016 ).

Haymaking

An alternative management strategy to grazing exclusion is rotational haymaking and grazing, with strict delimitation of grazing and haymaking areas ( Schönbach et al., 2010; Baoyin et al., 2014 ). Annual alternation of grazing and haymaking has been shown to be more resilient to moderate/high stocking than traditional grazing systems ( Schönbach et al., 2010; Ren et al., 2015 ), as well as being more effective in offsetting GHGs ( Schönbach et al., 2012 ) and enhancing vegetative precipitation responsiveness ( Wan et al., 2010 ). However, mixed grazing/haymaking systems require additional labor as well as fencing materials ( Schönbach et al., 2012 ). Despite these extra costs, mixed grazing/haymaking systems with annual mowing in high production years and grazing in low production years is a promising option for sustainable livestock management in semiarid grasslands ( Zhou et al., 2006; Wan et al., 2010; Baoyin et al., 2014 ).

Regions of initial colonization

Several localities are known in the Barabinskaya Steppe (Vengerovo V, Novy Tartas). Of particular interest is Volchya Griva, a short-term camp at the place of an animal salt lick. The faunal remains were dated in the range between 17,800 ± 110 BP, GIN-11463/21,550 ± 170 cal. yr BP/ and 11,090 ± 120 BP, SOAN-4291/12,940 ± 110 cal. yr BP/. Among the bones (dominated by mammoth), there were found more than 30 stone pieces, mostly bladelets and some burins. That suggests occasional human visits to the salt lick ( Zenin, 2002 ).

TROPICAL SOILS | Arid and Semiarid

Climatic Controls

The terms arid, deserts, semiarid, and steppes are used variously to describe dryland conditions. Arid (an adjective) describes a climatic condition of low rainfall – commonly taken to be less than 250 mm (10 in.) of mean annual precipitation ( Figure 1a ). Desert (a noun) is a region of the Earth’s land surface within an arid climate. Likewise, semiarid is a climatic condition characterized by a mean annual precipitation between 250 and 500 mm ( Figure 1a ). A region of the Earth’s surface within a semiarid climate is a steppe.

Figure 1 . (a) Climate categories based on mean annual precipitation. (b) Desert and steppe boundaries as a function of both mean annual precipitation and temperature according to the Köppen system. Arrows show how boundaries shift according to whether precipitation falls mainly in the summer or winter.

Closely linked to annual precipitation, and especially to soil moisture, is vegetation. The driest deserts are often barren of plant cover, but most deserts have scattered shrubs, cacti, forbs, and grasses. Though some steppe vegetation can occur in areas like the Badia of Jordan that receive as little as 100 mm of rainfall, generally steppe vegetation is characterized by higher amounts of rainfall in which short-grass prairie is bordered by desert vegetation on the arid side and tall-grass prairie, savanna, or woodlands on the subhumid side. Thus, by these definitions arid soils are synonymous with desert soils and semiarid soils are synonymous with steppe soils.

But to define deserts and steppes by precipitation alone is to ignore other important climatic variables, mainly temperature. One expression of the combined influences of both precipitation and temperature is the de Martonne aridity index based on the formula:

where the aridity index (Ia) is equal to the mean annual precipitation in millimeters (Pmm) divided by the mean annual temperature in degrees Celsius (T°C) plus 10. According to this index, values below 5 characterize true deserts, values of approximately 10 demarcate dry steppes, values of about 20 represent prairies, and values above 30 typify forest. The boundary for the Chihuahuan desert of Mexico and the USA, for example, is based on an aridity index of 10 or lower.

The Köppen system also demarcates deserts and steppes as a function of both precipitation and temperature ( Figure 1b ). For example, some cold regions in the high latitudes of North America and Eurasia that get semiarid-amounts of precipitation are coniferous forest instead of steppes, and many cold areas that receive arid-amounts of precipitation are steppes instead of deserts.

Seasonality of precipitation is another climatic factor that affects desert and steppe boundaries. For a given mean annual temperature, the boundary of a steppe will extend into wetter climates if its precipitation falls mainly in the summer ( Figure 1b ). In other words, if precipitation falls in summer, the area of a steppe will be larger because summer evapotranspiration depletes soil moisture more thoroughly than winter evapotranspiration. Similarly, the size of a desert will be larger if its precipitation falls in the summer rather than in the winter.

Soil water potential is another way of defining boundaries of arid and semiarid soils. Soil moisture measured as soil water potential, which includes the influence of particle size and salts, is more important to vegetation than annual precipitation alone. The Soil Taxonomy system, for example, uses soil water potential to define moisture regimes as a criterion for classifying soils. The aridic moisture regime, for example, includes soils too dry to support nonirrigated crops, and is defined as soil that is moist (i.e., water held at tensions greater than −1500 kPa) for no more than 90 consecutive days when the soil temperature at a depth of 50 cm is above 8°C. Soils with the ustic and xeric moisture regimes are transitional between the aridic moisture regime and soils of humid climates that have the udic moisture regime. Semiarid soils occur within ustic and xeric moisture regimes, their drier subdivisions, and wetter subdivisions of the aridic moisture regime, namely, the aridic ustic, aridic xeric, xeric aridic, and ustic aridic regimes.

Classification Systems: FAO☆

Set 7

Set 7 groups soils that are mainly found in steppe regions (pampa in South America, prairie in Northern America), characterized by a vegetation of ephemeral grasses and dry forests, where accumulation of organic matter and less-soluble salts dominates over leaching processes. This set holds four different Great Soil Groups, all characterized by a mollic horizon:

The Chernozems (from Russian, chern, black, and zemlja, earth), characterized by their great depth and dark surface horizon, rich in organic matter, with a pronounced accumulation of calcium carbonate with depth;

The Kastanozems (from Latin, castaneo, chestnut, and Russian, zemlja, earth) that generally occur in the driest part of the steppe zone, with lighter-colored topsoils and stronger accumulations of calcium carbonate or gypsum;

The Phaeozems (from Greek, phaios, dusky and Russian, zemlja, earth) that occur in the wettest part of the steppe and that show no signs of enrichment of calcium carbonate, but remain rich in bases;

The Greyzems (from Anglo-Saxon, grey, and Russian, zemlja, earth) that occur in the colder parts of the steppe and that show gray coatings of silica powder in the topsoil.

Introduction

1.5 Droughts, Aridity, and Desertification

Dryness is a common feature in the desert and steppe areas of the world, where there are water deficits throughout many years and centuries. The environmental life in such areas is adapted to water scarcity; therefore, only water deficit-bearing plants and animals can survive. Great deserts of the world are areas of dryness and aridity. The duration of dryness, steppe, and desert conditions are time continuous and have regional extensiveness. The desert areas occur in the northern and southern flanks of the tropical regions around the equator. Aridity is a continuous climate property, whereas drought is an extreme event that occurs during some time period. Rainfall variability is very high in arid and semiarid regions.

Droughts are among the rare events related to water availability, and people start to feel drought when there is not enough water. Drought periods may correspond to scarce rainfall events, excessive runoff exploitation, and groundwater abstraction more than safely rechargeable amounts from the rainfall directly and subsequent runoff indirectly. When drought comes, everybody is concerned; if it lasts, everybody tries to do his/her best to combat it but when it goes away everybody forgets except those who have been hurt ( Yevjevich, et al., 1983 ). In general, humans realize droughts with the start of water shortages and stresses. With the appearance of drought everybody starts to worry and do everything in their power to handle it during its elongated duration, but after its disappearance all the stresses and pains are forgotten. In other words, traditional style of life restarts without any more worry until the next drought occurrence.

One of the dramatic long-term drought impacts, combined with human activities, is the degeneration of productive ecosystems into desert in the process called desertification. It is not exclusively a consequence of drought, but it may be accelerated by droughts through such phenomena as wind action in dry years, soil erosion during drought and postdrought periods, and particularly through human activities that are responsible for poor management of land, soil, crops, and herds. The desertification areas reflect more solar irradiation than the original land by causing changes in the thermal regime of the atmosphere that may tend to extend or intensify droughts.

Even though many societies have been subject to droughts for a long time, the effects of droughts cannot be assessed completely with the proper solutions. For instance, management of water, food, irrigation, and settlement issues continue to lack proper and sufficient planning where drought risk levels, necessary precautions, and early warnings are not established. Present categorizations are based on different activities such as meteorology, hydrology, agriculture, sociology, physiology, economy, and so forth. Socioeconomic impacts are not related to water supply only, but also to the water demand; hence, it is concerned with local population, and accordingly, with water resources management. Droughts trigger socioeconomic events and become sensible due to their temporal durations and spatial coverages.

The effects of drought are more harmful in humid regions than arid regions because settlers in humid regions are not ready for water scarcity impacts. Accordingly, agricultural investments face maximum harm as a result of drought periods. Among the primary economic hazards due to the drought impacts are crop yield, husbandry, hydroelectric energy generation, decrease in industrial products, and navigation problems in low river flows. On the other hand, secondary effects include soil erosion, dust storms, forest fires, increase in plant diseases, insect hurdle, decrease in social and individual health, pollution concentration increase, deterioration in water quality, and so on. Partial and continuous migrations appear due to droughts.

Generally, droughts are expected in arid and semiarid regions due to rainfall reduction, but the most devastating ones appear in humid regions with comparatively abundant precipitation. It is, therefore, necessary to distinguish between drought and aridity. For instance, 200 mm of precipitation may be enough for husbandry but wheat production needs at least 500 mm of precipitation; hence, 200 mm of precipitation implies dry conditions for the husbandry owners. The importance lies with the precipitation variability and fluctuations. In the assessment of droughts, generally regional extents are considered with climatic features (“see chapter: Spatiotemporal Drought Analysis and Modeling ”). Additionally, the relative balance of water supply and demand is also important, especially at settlement areas. If water supply is less than demand, then a drought starts to appear and it affects many social, agricultural, and industrial activities. A continuation of persistent successive periods (months, seasons, years) with water supply less than demand constitutes extensive drought durations (“see chapter: Temporal Drought Analysis and Modeling ”). Even though precipitation reduction plays a role as one of the input variables in the modeling of droughts, additionally water supply, demand, and conservation enter the modeling domain. For instance, precipitation does not provide direct water supply for plants; first, the soil moisture is recharged and then plant roots carry water inside the plant body. Likewise, precipitation does not provide direct water supply or irrigation water; instead, water is provided by rivers and groundwater resources. If a region is dependent on surface water resources, then the effect of drought is felt earlier than the areas where the water supply is heavily dependent on groundwater resources. A simple but effective rule that can be derived from this statement is that surface waters can be impounded behind surface reservoirs for short-term usages. However, they must be directed toward groundwater storage through natural and artificial recharge practices for use in the long run; especially during drought periods (“see chapter: Drought Hazard Mitigation and Risk ”). Groundwater resources are primary alleviation sources during any drought or any other natural hazard cases such as earthquakes. Local hydrological cycle productivity possibilities must be enhanced for such activities. Conjunctive use of surface and groundwater resources comes into play for more beneficial consequences.

It is possible to deduce rationally that, although droughts may prevail at every corner of the world, there is a reverse relationship between annual average precipitation and drought impacts. Droughts are more intensive in arid and semiarid regions for two reasons:

Low precipitation has extreme variability, which means that compared to the average precipitation, the standard deviation from the mean is very big, showing the unreliability in the precipitation occurrences because the amounts vary from season to season and even from year to year.

In dry regions the length of drought is longer. In humid regions, drought durations may be persistent for a few or several months or years as a result of deviations from the average precipitation.

In arid and semiarid regions, accumulative effect of dryness starts to show its effects after some years and may continue for long durations of 10–15 years. For instance, drought that was effective from almost 1975 onward in Europe continued for almost 16 years. Such a situation in South Africa reached the level of famine and even worse cases existed in some areas.

The interaction between the land surface and the atmosphere is also a mechanism for changing the progression of prolonged droughts, which may cause desertification. For example, any change in albedo causes a decrease in vegetation cover and reduction in water vapor transfer associated with a decrease in soil moisture and surface water volumes.

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