39th Sir Albert Charles Seward Memorial Lecture: Climate changes over space and time: their repercussions on the flora and vegetation

Abstract

The changes in the climatic types from region to region over the vast expanses of the Indian sub-continent have been examined in terms of quantum of rainfall and distribution of rainy pattern: length of the dry period and season(s) of occurrence of rains. The interyearly variability introduces a further dimension in the diversity of the climate types. The vegetation pattern generated by the climatic diversity is briefly reviewed. Of particular interest is the transition from the tropical type of climate in India to the Mediterranean type further westwards.

As to the climate changes through the geological ages, the impressive record of megafossils collected by the scientists of the Birbal Sahni Institute has yielded valuable evidence of the vicissitudes. For the Quaternary period, palynology has played a key role in reconstructing the climate-vegetational history. A criticism may not be out of place. Many of the reconstructions refer to climate change only in terms of rainfall amount ignoring the distributional pattern (number of dry months, season of occurrence of rains). Sometimes the botanical markers selected are not fidel indicators of climate. In the much acclaimed and frequently cited palaeopalynological study of Gurdip Sing et al., (1974), the taxa selected like Artemisia, Maytenus, Mimosa, Oldenlandia, Cyperus, Syzygium cumini are not very fidel indicators of a humid phase. If the climate of the Thar had changed to humid type in true sense of the term, the pollen of the forest species of the Aravallis should have turned up in the profile of the Sambhar Lake.

In other cases, palynologists in their enthusiasm attribute even man-induced change in the vegetation to climate. The meticulous study of Caratini et al (1991) brings out a distinct change from forest vegetation to savannah type in the Western Ghats 3,500 years B.P., which incidentally pinpoints the beginning of the anthropic activity with burning of vegetation promoting the growth of grass-savanna. In yet other cases, the change in vegetation appears likely due to shifts in the course of river rather than fluctuations in rainfall (Jolly & Bonnefille, 1992).

A hypothesis has been suggested as to how the species responded or adjusted phenologically to the drift over a vast latitudinal span from 30° S to 30° N. it has also been pointed out that major deforestation can bring about a climate change in disrupting the normal rainy pattern decreasing the number of rainy days and drastically altering the micro-climates. The physical mechanisms involved have been enumerated.

Vegetation mapping has provided clues for the areas to be protected on priority basis for conservation of biodiversity and for permitting the migration of species through these protected corridors in the wake of climatic change: from the cooler’s zones to the warmer and from wetter to the drier. The need for study of populations in plant taxonomy has been stresses for assessing finer changes in climate.