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The role of short-term disturbances in paleovegetation dynamics in a montane forest-grassland vegetation mosaic, Western Ghats, India
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  • Prabhakaran Ramya Bala,
  • Sarath Kavil,
  • Ichiro Tayasu,
  • Chikage Yoshimizu,
  • Kaustubh Thirumalai,
  • Krishnan Sajeev,
  • Raman Sukumar
Prabhakaran Ramya Bala
National Institute of Advanced Studies, University of Pennsylvania
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Sarath Kavil
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Ichiro Tayasu
Research Institure for Humanity and Nature
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Chikage Yoshimizu
Research Institute of Humanity and Nature
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Kaustubh Thirumalai
University of Arizona
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Krishnan Sajeev
Indian Institute of Science
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Raman Sukumar
Centre for ecological sciences, Indian institute of science
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The Sandynallah valley (Western Ghats, India) features one of the oldest peat accumulations in the world at >50 kyr and has been central to the reconstruction of late Quaternary paleoclimate using paleovegetation changes in the forest-grassland vegetation mosaic that coexist here. It is well-known that short-term disturbances (fire, frost, intense drought) can also cause vegetation switches when multiple stable states exist, but this framework has seldom been considered in paleoecology investigations. Using stable carbon isotope signatures (relative C3-C4 vegetation abundance) on the cellulose fraction from two well-dated peat cores ~170 m apart - Core 1 closer to the hillslope (32000 years old) and Core 2 from the centre of the valley floor (45,000 years old) - we looked at paleovegetation changes and the implications for paleoclimate reconstruction within the alternative stable states framework. Charcoal data from another sediment profile from the same valley was used to correlate with paleofires. We propose that the valley floor is bistable, switching between peat-forming vegetation ‘sedgeland’ and montane stunted evergreen forest ‘shola’, maintained by level of waterlogging. Core 1 shows shola-sedgeland dynamics with vegetation switching at c.22ka from shola (possibly due to fires) to a prolonged unstable state until 13 ka sustained by low waterlogging. Following a hiatus c.13-7 ka, sedgeland dominates, with a shift into shola at 3.75 ka driven by increasing aridity. Core 2 shows a relatively stable signature, enriched in C3-vegetation in the last glacial (45-20 ka) compared to the Holocene. Given temperature is the primary driver of abundance in C3-C4 mixed-grasslands, C4 dominance beginning c.18.5 ka followed by C4 enrichment is indicative of deglacial warming that continues into the Holocene except for a departure at ~10 ka. The record at Core 2 is indicative of changing climate while Core 1 shows disturbance-based vegetation dynamics. The simultaneously distinctive vegetation states in Cores 1 and 2 within the same valley is the first record of alternative stable states in the past in the montane tropics. Our results point to the need to account for short-term disturbances and site attributes before ascribing vegetation changes to changing climate in alternative stable states landscapes.