Mid-Holocene Vegetation Shifts and Climate Change in the Temperate Belt of Garhwal Himalaya
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The Palaeobotanist 60(2011): 291-298 0031-0174/2011 $2.00 Mid-Holocene vegetation shifts and climate change in the temperate belt of Garhwal Himalaya ANJALI TRIVEDI1*, B.S. KOTLIA2 AND L.M. JOSHI2 1Birbal Sahni Institute of Palaeobotany, 53 University Road, Lucknow 226 007, India. 2Geology Department, Kumaon University, Nainital, Uttarakhand, India. *Corresponding author: [email protected] (Received 6 April, 2011; revised version accepted 24 May, 2011) ABSTRACT Trivedi A, Kotlia BS & Joshi LM 2011. Mid-Holocene vegetation shifts and climate change in the temperate belt of Garhwal Himalaya. The Palaeobotanist 60(2): 291-298. Pollen analysis of 3.06 m deep sediment core from the temperate lake-Nachiketa Tal, has brought out the vegetation shifts and climatic oscillations in the temperate region of Garhwal Himalaya since Mid-Holocene. The pollen sequence depicts that between 5,304 and 3,912 yr BP the mixed conifer forests dominated by Pinus cf. wallichiana together with Cedrus and Abies occupied most of the landscape under a regime of a cold and dry climate. The record of pollen of aquatic element, Potomogeton and freshwater alga-Botryococcus implies the existence of the lake. The broad-leaved forests mainly constituted of oak (Quercus cf. semecarpifolia) occurred sparsely in the moist and shady situations. Between 3,912 and 2,975 yr BP the mixed conifer forests got transformed into mixed oak-broad-leaved forests as evidenced from the much expansion of Quercus and improvement of its close allies Alnus, Betula, Carpinus, Juglans, Salix, etc. This change in the vegetation pattern suggests that a warm and humid climate prevailed in the region. Around 2,975 to 1,872 yr BP the abrupt decline in Quercus and its broad-leaved associates and a corresponding spurt in Pinus cf. wallichiana reflect the re-establishment of pine dominated conifer forests with reversal of cool and dry climate. Subsequent expansion of Quercus and broad-leaved taxa around 1,872 to 767 yr BP and declining trend of conifers took place with the prevalence of warm and moderately humid climate. The sporadic encounter of Cerealia pollen during this phase indicates that the area was under cereal-based agricultural practice. Since 767 yr BP onwards a warm and less humid climate prevailed than before and consequently relatively less-diversified mixed broad-leaved oak forests continued to thrive in the region, with relatively reduced frequencies of Quercus and other thermophillous broad-leaved elements. 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The lake is elliptical in outline, Chauhan, 2006; Chauhan et al., 2000; Bhattacharyya, 1988) measuring about 150 m in length and 40 m in breath with and Jammu & Kashmir (Vishnu-Mittre & Sharma, 1966; Sharma irregular margin. The lake remains covered with thick sheet of & Singh, 1968; Singh, 1964; Gupta et al., 1984; Sharma et al., ice during the cold months of January and February. 1985; Trivedi & Chauhan, 2008, 2009) from northwest Himalaya, based on the pollen evidence from the lacustrine deposits. CLIMATE However, the Garhwal region with a large number of potential lakes/bogs for the Quaternary palaeoclimatic studies have The climate of this region is temperate and typical hitherto not got adequate attention on this aspect, barring the montane type with well defined rainy, winter and summer sketchy proxy records retrieved from the subtropical lakes, seasons. The maximum annual mean temperature ranges from viz. Dewar Tal (Chauhan & Sharma, 2000) and temperate belt 15ºC to 25ºC, but during winter it descends to 5ºC and Deoria Tal (Sharma et al., 1996; Sharma & Gupta, 1997) and Sat occasionally it descends below 0ºC, during the cold months Tal (Chauhan et al., 1997), depicting the changing vegetation of January and February. Average annual rainfall is about 2,000 scenarios, contemporaneous climatic shifts and impact of mm. During the months of severe cold the precipitation is in human activity during the Late Holocene. In the present paper, the form of snow. All the surrounding mountains remain snow an attempt has been made to extend such studies in the clad during the winter. Fig. 1—Map showing the site of investigation in Uttarkashi District, Garhwal Himalaya. TRIVEDI et al.—MID-HOLOCENE VEGETATION SHIFTS AND CLIMATE CHANGE OF GARHWAL HIMALAYA 293 VEGETATION Sample No. Depth AMS Age The hill slopes contiguous to the lake are largely covered with oak forest dominated by Quercus semecarpifolia. The NKT-I 10 cm Modern other prominent broad-leaved of oak such as associated NKT-II 109 cm 1,855 yr BP Rhododendron arboreum, Aesculus indica, Alnus nepalensis, Acer caesium, Fraxinus excelsior, Ulmus wallichiana, Myrsine Fig. 2—AMS ages for sediment core. africana, Salix elegans, Pyrus malus, etc. also occur frequently in the forest. Other arboreals such as Myrica esculanta, Lyonia from the sediments. Samples for microscopic examination were ovalifolia, Symplocos sp. and Engelhardtia bellutianum have prepared in 50% glycerine solution. sporadic distribution in the forest. The shrubby vegetation comprises Berberis asiatica, B. chirta, Rosa moschata, Rubus POLLEN ANALYSIS ellipticus, Lonicera obavata, Crataegus cranulata, Zanthoxylum alatum, Viburnum cotonifolium, V. fruticosa, All the samples analysed from this sediment core were etc. found very productive in pollen and spores (Pl. 1). The pollen The herbaceous vegetation on the forest floor is quite sums vary from 300 to 500, which include only the terrestrial luxuriant and constituted of Anaphalis adnata, Primula sp., pollen. The pollen of aquatic plants and fern spores were Potentilla fulgens, Pedicularis sp., Berginia ligulata, Swertia excluded from the pollen sum due to their origin from local chirta, Geranium nepalense, Agremonia sp., Aster sp., provenance. However, their frequency percentages have been Pedicularis sp., Sedum trifidum, Saxifraga diversifolia, calculated from the pollen sums for their representation in the Thalictrum chelidonii, Galium suga, Jasminum humile, Rumex pollen diagram.