Palaeoecology of the Rajasthan Desert Duri G the Last 10,000 Years

PALAEOECOLOGY OF THE RAJASTHAN DESERT DURI G THE LAST 10,000 YEARS VISHNU-MITTRE Birbal Sahni Institute of Palaeobotany, Lucknow-226007, India ABSTRACT In the light of fresh and exhaustive information now available on the ecology of vegetation of the Rajasthan desert, the reinterpretation of the published pollen diagrams reveals that there was increasing trend towards dryness until 5,000 years ago and thereafter between 5,000-3,000 years ago the environment was characterized by maximum warmth and dryness and by high velocity winds and dust storms. The precipitation which was within the range of 150-400 mm showed a gradient from the extreme west of the desert to its east. The renewed sand dune activity during the hypsithermal period had encroached upon the eastern tracts of the desert. The salt lakes which had originated about 10,000 years ago had already high salt content in their waters and during a continuous phase of recession in the entire Holocene rose once between 6,000-5,000 years B.P. The vegetation had comprised of Calligonum series and Prosopis-Capparis• Ziziplms series in the vicinities of Lunkaransar, Didwana and Sambhar and of Anogeisslts-Acacia series in the vicinity of Pushkar lake. The earliest farming episode did not precede 5,000 years B.P. though inexpli• cable evidence of burning and disturbance of vegetation preceded it perhaps caused by natural means and grazing of animals. The subsequent desertifIcation was caused by combined effect of climatic and biotic factors. INTRODUCTION from is based upon inadequate knowledge of the factors governing the distribution of the plant life. It is intended in this paper a regionreconstructionthrough ofpollenpalaeoecologyanalysis ofis to infer information of palaeoecology of THElargely dependent upon the ecological Rajasthan desert from the recently pub• requirements of modern representatives of lished palaeobotanical work by Singh et at. the fossil plant species identified from their (1973,1974) in the light of information now remains. It becomes highly imperative available on the ecology and vegetation of therefore that the investigator posts himself the Rajasthan desert by Gaussen et at. fully with the ecological information of the (1972), the information which was not modern plant life of the region under available to Singh et at. (1973, 1974). investigation. Luckily the ecology and The writing of this paper was also mapping of modern vegetation of the prompted by the glaring anomaly between Rajasthan desert exhaustively accomplished the hitherto inferred Holocene climatic by the French Institute, Pondicherry sequence in Rajasthan (Singh et at., 1974) (Gaussen et at., 1972) if properly made use with global climatic fluctuations. For ins• of, valuable information can be had of the tance between 5,000 B.P. to about 2,760 former plant life and the climatic, biotic B.P. (the Sub-Boreal Period), a recog• and edaphic factors governing its distribu• nized hypsithermal interval during which tion. Further it can provide information maximum warmth and dryness prevailed on the origin and history of the desert, all over the world (with rare exceptions of past meteorological conditions, origin and course) and minor cool oscillations did history of the lakes and their salinity and occur in some parts of the world (Wendland & aspects of human and animal influence Bryson, 1974), wet climate turning dry upon the desert vegetation towards land• about 1800 B.c. has been inferred in use and agriculture. The technical excel• Rajasthan (Singh, 1971; Singh et at., 1974). lence of the recovery and presentation of Nearer home the chemical analy~is of the palaeobotanical and palynological data Indian ocean cores in the Gulf of Aden has often becomes offset if the inference there- revealed that not only the pre-Boreal was 550 THE PALAEOBOTA -1ST the driest and a time of great wind erosion, PALAEOECOLOGY OF THE RAJASTHAN the first millennium of the sub-Boreal was DESERT drier than the Atlantic and thereafter: about 1800 B.c. a slight increase in precipitation TEMPERATURE, PRECIPITATION AND did occur. Thereafter, between 750 B.C. SAND DUNE ACTIVITY to 500 B.C., a new and gradual decrease in rainfall had occurred (Olausson, 1972). Gaussen ct al.'s (1972) ecological survey In westem Iran dry climate prevailed of the Rajasthan flora brings to light until 6,000 B.P. and there was either fall climatically and edaphically controlled in temperature or rise in precipitation but series of vegetation occurring in the desert. the pollen stratigraphy of lake Mirabad in Finding the inadequacy of the rainfall the same region suggests that the summers isohyets as determiners of the distribution during 5,000-4,500 B.P. were drier causing of plant societies in Rajasthan, Gaussen intermittent desiccation of the lake (Van et al. (1972) have derived bioclimates Zeist, 1967). Macrofossil study at this site from temperature, duration of dry months, suggests that the lake level during 5,000 number of biologically dry days (i.e. xero• B.P. did not fluctuate much rather it was thermic index) besides precipitation. Their lower than today (Wasylikowa, 1967). The painstaking work suggests that the series arid period in Sahara began at about 4,300 Calligonum polygonoides and the series B.P. (Zinderen Bakker, 1972). The sub• Prosopis-Capparis-Ziziphus occur under the Boreal optimum temperature was secular same climatic conditions: annual precipita• in nature in Europe, S. America and Tropi• tion 150-400 mm, dry period extending cal Africa though Central Africa was slightly over 9-11 months a year, xerothermic index warmer and wetter during 4,700 B.P. over 250 and mean temperature of the (Zinderen Bakker, 1972). Although syn• coldest mO:1th being 15-20°C or less than chroneity in global fluctuations of tempera• 15°. However, the Calligonum series occurs ture has to a large extent been proved than on sand dunes (active & stabilized) and in fluctuations in moisture, most amazingly the Prosopis-Capparis-ZiziPhus series especi• the sub-Boreal has been found to be secu• ally on old sandy alluvium. Likewise the larly dry with rare exceptions. other series are characterized by increase in The distribution and ecology of the annual precipitation and less number of Rajasthan flora reveal that distinct plant dry months and slightly higher winter species fall into ecological groups such as temperature. The Acacia-Capparis series halophytes, xerophytes, mesophytes, etc. for instance occurs under a rainfall of the temperature, moisture and soil require• 400-700 mm with 8-9 dry months, 15-20°C ments of the individual constituents of each mean temperature of the coldest month. group will not permit, for instance the M aytenus emerginata of which pollen grains members of the mesophytes to occur with constitute an important curve in pollen those of the xerophytes (planted or intro• diagrams belongs to the shrubby savannah duced plant species excepted). Further the in the Prosopis-Capparis-Ziziphus series. It plant societies in Rajasthan desert consti• occurs abundantly with Calligonum and tute a mosaic of plant associations having Prosopis in the Rajgarh area and Bikaner particular edaphic preference. Thus pollen in the Bikaner District (Joshi, 1956). grains may be derived from several plant Capparis and M aytenus constitute dominant societies. For proper palaeoecological in• vegetation in the sandy areas in the ference it would be profitable to group the Shekhawati region in Jaipur District (Joshi, pollen grains in the societies they may be 1957). It also occurs in other series under derived from. Likewise pollen grains trans• higher rainfall but the associated constitu• ported from plants in different climatic ents are entirely different. regimes within or outside the Rajasthan Singh (1971) and Singh et al. (1974) desert too be grouped together for any based the inference of climatic fluctuations palaeoecological indication they may sug• upon the pollen curves of Cyperaceae, gest. From the published data of which grasses, Artemisia, Mimosa rubicaulis, details are also preserved with us the Oldenlandia and Typha as indicators for inferences drawn here aFe based upon such wet climate and it has already been sug• recognizable ecological groupings. gested elsewhere (Vishnu-Mittre, 1974a, b) VISHf\"U-MITTRE - PALAEOECOLOGY OF RAJASTHAN DESERT 551 that these pollen indicators for wet climate revealed that the pollen grains of Mimosa have not been judiciously chosen. Granted rubieaulis and Mimosa hamata are indistin• for a moment that they are, a comparison guishable from one another (Saxena & of the three pollen diagrams would reveal Vishnu-Mittre, 1976) and so are those of that during 9,500 B.P. to 5,000 B.P. Oldenlandia species. Sambhar and Didwana areas were much The Lunkaransar pollen diagram shows wetter than Lunkaransar (the curves of during the last 10,000 years the develop• most of them show a progressive decrease ment of the desert vegetation from Ephedra• from east to west) suggesting that the Maytemts-Calligonum-Capparis to Maytemts• moisture belts remained much the same as Ephedra-Capparis with little ZiziPhus to today. And this would be contrary to the Calligonum- M aytenus -Ephedr a-Pros opis -Cap• conclusion of Singh et al. (1974, p. 494) paris' and eventually to Ca.lligonum-Capparis that the westward shift of the rainfall belts Society*. During the same period of time took place during phases II-IV. Amazingly Sambhar Lake pollen diagram shows the behaviour of Cyperaceae and Artemisia Mimosa-Acacia-Ephedra developing into as shown below has been found to be Maytenus-Capparis-Aeaeia to Capparis-Pro• inversely related at the two sites, Lunkaran• sopis-Aeaeia-Maytemts and eventually to sar in the driest belt and Sambhar in the Capparis-Prosopis-Aeaeia and Euphorbia. least dry belt. The declining Artemisia The sequence in Didwana is ± similar to pollen curve at Didwana, a site in between that of Sambhar with the difference that the two, remains enigmatic.

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