Zonoecotone II/III - Climatic Savannas

Size: px

Start display at page:

Download "Zonoecotone II/III - Climatic Savannas"

Morris Banks
5 years ago
Views:

1 Zonoecotone II/III - Climatic Savannas This zonoecotone includes the climatic savannas, such as those of Southwest Africa, which have already been discussed. Similar conditions prevail in the Sahel zone, south of the Sahara, where the transition to the summer rain region of the Sudan (Zonobiome II) occurs. The Sahel zone has become completely degraded by overpopulation and overgrazing as a result of the typical and recurring years of drought in this zone. Due to the limited number of natural water sources in this area, it was only capable of supporting a very small population and a correspondingly small amount of cattle until foreign aid programs attempted to develop the land by drilling a large number of wells. This made water available for larger herds of cattle, and resulted also in a rise in the human population as long as the annual precipitation remained above the long-term average. The occurrence of several years of drought led to a catastrophe. Although enough water was available for both humans and the animals, the parched pastures yielded no more grass. The cattle died of starvation and the humans were forced to leave the area or were supported by foreign aid projects. The pastures, however, were irreparably damaged and were converted into a man-made desert. In Southwest Africa, with its similar climate, several successive years of drought also prove disastrous, although the small number of farmers is able to survive by reducing the size of their herds in time. The economy recovers quickly after a few good years of rain and the natives in the reservation receive the necessary aid. A further Zonoecotone II/III is situated in the border region of India and Pakistan in the Thar or Sind desert. This homogenous arid region between the Aravalli Mountains in the east and the Baluchistanian Heights in the west is also referred to as the Great Indian Desert (Fig. 53). The aridity increases from east to west. The reference in the literature to a Saharo-Sindic desert zone is incorrect. The Sahara, a rainless region (or one with scant winter rain) is floristically mainly part of the holarctic, and extends eastward into the Egyptian-Arabian desert as far as Mesopotamia. The Sind desert, however, is the final arid extension of the Indian Monsoon region and floristically, it must be considered part of the paleotropics. Climatically, the Indian Thar Desert is a Zonoecotone IIIIII, which may be compared to the "Sahel", the transitional region between the Sudan and the southern Sahara. Both receive light summer rains, although the Indian region lies north of the Tropic of Cancer and the mean annual temperature is 2-3 C lower than in the Sahel, meaning that frosts may occur between the months of December and February (Fig.

108 Zonoecotone II/III - Climatic Savannas o Fig. 53. Climate diagram map of the Sind-Thar Desert. Northwest of the dividing line A - B is the extremely arid region 53).

2 108 Zonoecotone II/III - Climatic Savannas o Fig. 53. Climate diagram map of the Sind-Thar Desert. Northwest of the dividing line A - B is the extremely arid region 53). Only the Indus valley has an actual rainfall of less than 100 mm and would, therefore, qualify as a desert. Because of the Indus and its tributaries, however, it is a water-rich irrigation region. On the other hand, the Great Indian Desert is a man-made desert. The area was settled 4000 years ago, but since the campaigns of Alexander the Great, the population has increased to an extent that overgrazing, wood clearing and, to some degree, farming have completely degraded the

Zonoecotone II/III - Climatic Savannas 109 Fig. 54. Barchane that has started to move recently in the region between Jaisalmer and Jodhpur with single specimens of Prosopis, Acacia, and Calotropis.

In its natural state, the region was a Prosopis savanna with an annual precipitation of of 400-150 mm on a deep reddish brown sandy savanna soil, as can be seen today in the area around Jodhpur,

3 Zonoecotone II/III - Climatic Savannas 109 Fig. 54. Barchane that has started to move recently in the region between Jaisalmer and Jodhpur with single specimens of Prosopis, Acacia, and Calotropis. (Photo courtesy of M. P. Petrov) countryside (Mann 1977). In its natural state, the region was a Prosopis savanna with an annual precipitation of of mm on a deep reddish brown sandy savanna soil, as can be seen today in the area around Jodhpur, which has been under protection for the past decades (Rodin et al. 1977). Here, the following thorny shrubs are found: Prosopis cineraria (= specigera), Ziziphus nummularia, Capparis decidua (= C. aphylla) etc. With an annual precipitation of mm, Prosopis grows to a height of 8 m, forming stands of individuals/ha. With a precipitation of mm, it grows to 5-6 m in stands of individuals/ha and with 200 mm it only grows to 3-4 m in stands of25-30 plants/ha. Decreasing amounts of rain also result in the replacement of high grass species (Lasiurus, Desmostachys) by lower growing species (Aristida) (Gaussen et al. 1972). The conditions are, therefore, similar to those in Southwest Africa (pp ). In regions with over 250 mm annual precipitation, the savannas are used for grazing and are degraded as a result of overstocking with cattle, whereby the annual grass species Aristida adscensionis becomes dominant in the pastures. The soils are extremely sandy in the Bikaner District. As a result of overgrazing in the vicinity of the villages, moving barchanes (dunes void of vegetation) give the impression of an extreme desert (Fig. 54). Actually, however, the water content in the sand of such naked dunes is much higher than of overgrown dunes as is illustrated by the following figures from an area with an annual precipitation of 260 mm:

4 110 Zonoecotone II/III - Climatic Savannas Water content (in mm) in the sand of naked (I) and overgrown (II) dunes, according to Mann (1976) Depth (in cm) March June September January This is explained by the fact that a growth of Prosopis requires approximately 220 mm of water for transpiration. The often-planted grass species, Pennisetum typhoides, requires mm. The inhabitants take advantage of the water content of the sand in the naked dunes by planting watermelons every 2 m. They use branches to prevent the sand from blowing away. No information is available on the natural vegetation of the driest parts of the Sind Desert in the Indus Valley. This irrigation region is densely populated and there are no areas with natural vegetation to be found. Due to irrational irrigation practices, the water table is rising, resulting in a secondarily increased salt concentration in the moist soil. This has caused the loss of 40,000 ha of agricultural land annually, and means that the food production will not be able to keep pace with the growing population. A rehabilitation of the brackish soil would be an extremely costly endeavor in this level landscape. Naturally salty soils are widely distributed in the southern part of the Thar Desert on the Gulf of Kutch. Mangroves grow in the tidal zone, followed inland by salt marshes with Salicorina, Suaeda, Atriplex and the salt grass species Urochondra. In the region of Ran of Kutch, with its high water table, nearly sterile salty clay soils are found with halophytes (Haloxylon salicornicum, Aeluropus, Sporobulus), and Chenchrus spp., Cyperus rotundus, or scattered woody plants on good locations (Blasco 1977). A difficult region to cataegorize ecologically, is the arid "Polygono da Seca" in the Caatinga of northeastern Brasil. It is characterized by a precipitation which may vary extremely from year to year. In the driest location in Cabaceira, for example, the years with plentiful rainfall ( mm) were followed by the drought years of with precipitation below 80 mm (1952 only 24 mm, 1958 only 22 mm), excepting the years 1954 (170 mm) and 1955 (187 mm). In such an unreliable climate large succulent columnar cacti and large thorny bromeliads, which grow close to the ground survive best, as well as bottle trees (Ceiba) or deciduous shrubs, which are leaftless for long periods of time. This region is difficult to use agriculturally and is only sparsely settled since the drought periods are unpredictable and force the inhabitants to leave the area. Similar conditions are found in the trade wind deserts on the Venezuelan-Columbian border on the north coast of South America and on the Galapagos Islands. These dry regions also experience years with very high precipitation.

In the foreground lie the cut-off thorny-tipped succulent leaves of Sanseviera. Thorny brush on the southern foothills of the Pare Mountains in East Africa. (Photo H.

5 Zonoecotone II/III - Climatic Savannas 111 Fig. 55. The tuberlike trunk of Aderia globosa (Passifloracease) from the top of which the bow-shaped branches armed with green thorns and which normally envelop the trunk, were cut. In the foreground lie the cut-off thorny-tipped succulent leaves of Sanseviera. Thorny brush on the southern foothills of the Pare Mountains in East Africa. (Photo H. Winkler) Finally, and still belonging to the Paleotropic realm, there are the extensive arid regions in the tropical parts of eastern Africa, as well as a small area in the rain shadow between the Pare and West Usambara mountains where very odd succulents are found [Adenia globosa (Fig. 55), the boulderlike Pyrenacantha, Euphorbia tirucalii, Caraliuma, Cissus quadrangularis, Sansevieria, etc.]. This is probably the driest region near the equator, with an annual mean temperature of 28 C and a rainfall of only mm. In northern Kenya, western Ethiopia, Somaliland, and on Socotra, there are

112 Zonoecotone II/III - Climatic Savannas Fig. 56. Adenium socotranum (Apocynaceae) with a diameter around the trunk of 2 m on western Socotra. (Photo F.

6 112 Zonoecotone II/III - Climatic Savannas Fig. 56. Adenium socotranum (Apocynaceae) with a diameter around the trunk of 2 m on western Socotra. (Photo F. Kossmat) even more extensive arid regions where Adenium socotranum (Apocynaceae), a plant with bizarre succulent stems achieving diameters of up to 2 m, is found (Fig. 56), and Dracaena cinnabari with a trunk diameter of 1.6 m. The driest southeastern corner of Madagascar is distinguished by Baobab trees and plants of the columnar cactuslike family Didieraceae, which only occurs her.

GLOBAL CLIMATES FOCUS

GLOBAL CLIMATES FOCUS which you will learn more about in Chapter 6. Refer to the climate map and chart on pages 28-29 as you read the rest of this chapter. FOCUS GLOBAL CLIMATES What are the major influences on climate? Where