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Paleoecological clues help understand ecological history of Banni grasslands

Priyanka Runwal

Left: sediment core collection from a dry lake bed in Banni Right: top - fungal spore, middle - snail shells , bottom - charcoal particles observed in the sediment layers
Left: sediment core collection from a dry lake bed in Banni Right: top - fungal spore, middle - snail shells , bottom - charcoal particles observed in the sediment layers   (Photo: Anusree A S)

Standing in the middle of a dry lake bed, scientists drive a PVC pipe into the mud below, sometimes up to few meters deep, to collect sediment cores. Interspersed between grains of soil, are bits and bobs of preserved biological material. These have assimilated over hundreds and thousands of years and now serve as proxies for the past vegetation, land-use and climate.  An ecological treasure trove of the region’s past, one might add. 

Using this time-travel technique, researchers from the National Centre for Biological Sciences (NCBS), Bangalore and Indian Institute of Science, Education and Research, Mohali and Kolkata have reconstructed the ecological history of Banni – a mixed tree-grass (savanna) ecosystem in western India in Gujarat for the last 4600 years.  “We hoped to understand what led to shaping today’s vegetation and where it might go in the future, based on what we can derive from the recent past,” says Jayashree Ratnam, from NCBS and one of the authors of the study. 

Using a combination of indirect evidences - carbon isotope fingerprints of organic soil and microscopic silica crystals from dead plant tissues, the team estimated the abundance of trees and shrubs, relative to grasses. By measuring the ‘geochemical’ signature on shells of Cerithium snails, they assessed past rainfall regimes.

The team found that trees and shrubs dominated in this mixed tree-grass system of Banni between the last 4600 and 2500 years. Periods of high rainfall between these years potentially favoured this dominance.

Conditions became arid between the last 2500 and 1000 years and this time around, grasses emerged as the dominant life-forms. Researchers found frequent appearance of charcoal grains, which are basically burnt remnants of woody vegetation in the sediment cores, indicating recurrent fires during this dry phase. 

It is in the same time period that evidences for pastoralism and settled agriculture start showing up in western India’s history. This period of grass-dominance and frequent fires was also accompanied by a greater abundance of grazing animals, most possibly livestock.

In the last 1000 years, the rainfall levels slowly increased and trees gradually began to make a come-back. But it is also likely that the escalated livestock numbers reduced grass abundance and additionally allowed for trees to proliferate. 

“Our work suggests a cumulative role of climate and anthropogenic factors in maintaining the ecology of the Banni grasslands,” says Anusree A.S.from NCBS, the lead researcher of the study. 

Kathleen Morrison, an expert in paleoecology and historical anthropology at the University of Pennsylvania, told IndiaBioscience, “This is a timely and important study. What’s most impressive is the range of analytical techniques used. It’s really cutting-edge work”. She added, “Tropical grasslands and savannas, although very large (both on a global scale and in India) and quite sensitive to climate change, are surprisingly understudied. We need a lot more studies like this in India”.

Like most other tropical grassland and savanna ecosystems, the Banni is predicted to experience recurrent droughts and increased human-use in the future. Given this knowledge of past ecological dynamics, a study like this has the potential to predict what the landscape will look like in the future and can inform management initiatives. 

The study was published in the journal of Palaeogeography Palaeoclimatology Palaeoecology.