Spatial heterogeneity of land cover response to climatic change in the Nilgiri highlands (Southern India) since the last glacial maximum

Abstract

Fourteen hillslope soil profiles were sampled under natural vegetation (ie, grassland or forest) and plantations in the Nilgiri highlands, Southern India. Delta ¹³C ratios were measured at different depths and ¹⁴C ages determined for six profiles. In these highland soils where the turnover rate of organic matter is extremely low, the δ¹³C ratios of entire soil profiles have recorded signatures of past land cover. By correlating the data with results previously obtained from peat bogs and with knowledge concerning the history of human settlement, we distinguish three contrasting trajectories of palaeoenvironmental history and landscape change since the last glacial maximum. In the Central Nilgiris, between 18 and 10 ka BP, forest expansion occurred through the conjunction of a wetter climate (the maximum of southwest monsoon-related humidity occurring at c. 11 ka BP) and higher temperatures; since 10 ka BP, the reversal towards grassland vegetation is attributed to drier conditions. In the Western Nilgiris, where strong southwest monsoon winds permanently restrict forest patches to sheltered valley sites, steady but limited expansion of forest from 18 ka BP to the present is recorded and attributed to rising temperatures. The Southern and Eastern Nilgiris, where the northeast monsoon contributes 20% of the annual rainfall, are the less sensitive to fluctuations in the southwest monsoon. In these areas, rapid and extensive expansion of forest occurred mainly as a consequence of higher temperatures from 18 ka BP to the present. Massive deforestation by Badaga cultivators and Europeans planters followed after the sixteenth century AD.As a result, and in contrast with the Western Nilgiris where the land cover mosaic has remained remarkably stable in the last 18 ka BP, the current landscape differs sharply from the land cover pattern detected by the soil record.