Monsoon forced evolution of savanna and the spread of agro-pastoralism in peninsular India

Nils Riedel,Dorian Q Fuller,Jayashree Ratnam,Norbert Marwan,Sushma Prasad,Mahesh Sankaran,Saswati Sarkar,Nathani Basavaiah,Martina Stebich,Dirk Sachse,Philip Menzel,Constantin Poretschkin

doi:10.1038/s41598-021-88550-8

Abstract

An unresolved issue in the vegetation ecology of the Indian subcontinent is whether its savannas, characterized by relatively open formations of deciduous trees in C4-grass dominated understories, are natural or anthropogenic. Historically, these ecosystems have widely been regarded as anthropogenic-derived, degraded descendants of deciduous forests. Despite recent work showing that modern savannas in the subcontinent fall within established bioclimatic envelopes of extant savannas elsewhere, the debate persists, at least in part because the regions where savannas occur also have a long history of human presence and habitat modification. Here we show for the first time, using multiple proxies for vegetation, climate and disturbances from high-resolution, well-dated lake sediments from Lonar Crater in peninsular India, that neither anthropogenic impact nor fire regime shifts, but monsoon weakening during the past ~ 6.0 kyr cal. BP, drove the expansion of savanna at the expense of forests in peninsular India. Our results provide unambiguous evidence for a climate-induced origin and spread of the modern savannas of peninsular India at around the mid-Holocene. We further propose that this savannization preceded and drove the introduction of agriculture and development of sedentism in this region, rather than vice-versa as has often been assumed.

Highlights

The savannas of peninsular India: natural or anthropogenic? Large tracts of peninsular India are characterized by savanna vegetation, with a continuous ground-layer, predominantly of C4-grasses and a woody layer of broadleaf or fine-leafed deciduous C3-trees[1,2]
Savannas: (1) anthropogenic activities such as livestock-grazing, forest clearing and burning since the Neolithic have led to a gradual degradation of what were deciduous forests to the savannas we see today[2,7,8]; or, (2) alterations in the monsoonal moisture supply drove the establishment of savanna vegetation and in turn compelled some hunter-gatherer communities to introduce agriculture to cope with diminishing food resources[9] while creating attractive habitat for immigrant p astoralists[10]
Archaeological research has established that the establishment of the first farming villages in the Northern Deccan took place around 4.5 kyr BP, while further to the northwest in Rajasthan and the Saurashtra Peninsula this could have begun from 5.5 kyr BP23,24

Summary

Introduction

The savannas of peninsular India: natural or anthropogenic? Large tracts of peninsular India are characterized by savanna vegetation, with a continuous ground-layer, predominantly of C4-grasses and a woody layer of broadleaf or fine-leafed deciduous C3-trees[1,2]. A mid-Holocene weakening of tropical circulation systems has been shown to be responsible for significant changes in the distribution of terrestrial biomes across the tropics, with a decline in forest extent relative to savannas, and the spread of deserts at the lower hygric limits of s avanna[16,17,18] These climate and vegetation changes are considered to be the driving factors for the cultural evolution of early societies in Africa[19]. Further south along the Tungabhadra and the region where it meets the Krishna river, the first food production systems were focused on livestock pastoralism and seasonal ashmound sites with sedentary villages appearing 4.2–4.0 kyr BP24–26 On this basis we expect anthropogenic savannas to emerge 4.5–4.0 kyr BP. The predominance of microliths occurs on sites in the Deccan from ~ 30,000 BP up to the N eolithic[31], and provides no indication for a shift in hunter-gatherer culture that might correlate with anthropogenic creation of savannas

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Conclusion