Paleovegetation dynamics in an alternative stable states landscape in the montane Western Ghats, India

Abstract

Peat deposits (>50 ka) in the montane Nilgiris (Western Ghats, India), have been central to the reconstruction of late Quaternary paleoclimate using paleovegetation changes in the forest-grassland vegetation mosaic that coexist here. However, it is well-known that short-term disturbances can also cause vegetation switches when multiple stable vegetation states exist. We studied paleovegetation changes within the alternative stable states framework using stable carbon isotopes (relative abundance of C3-C4 vegetation) on the cellulose fraction from two high-resolution radiocarbon-dated peat cores ~170 m apart in the Sandynallah valley: Core 1 closer to the hillslope (32,000 years old) and Core 2 from the centre of the valley (45,000 years old). Core 1 is located in an ecotone showing shola-sedgeland dynamics with vegetation switching at c.22 ka from shola (possibly due to fire) to a prolonged unstable state until 13 ka sustained by low waterlogging. Following a hiatus c.13 ka, sedgeland dominates, with a shift into shola at 3.75 ka driven by increasing aridity. Core 2 shows a stable sedgeland mixed C3-C4 composition responding to temperature, enriched in C3-vegetation in the last glacial with C4-dominance beginning c.18.5 ka, indicative of deglacial warming. The distinctive vegetation states at corresponding times in Cores 1 and 2 within the same valley, responding independently to disturbances and climate, respectively, is the first paleo-record from an alternative stable states landscape in the montane tropics. Thus, short-term disturbances and site attributes need to be accounted for before ascribing vegetation change to changing climate in such vegetation mosaics.

Holocene precipitation hydrogen isotopic values on Nilgiri Plateau (southern India) suggest a combined effect of precipitation amount and transport paths

“…The vegetation in the Nilgiri Plateau is characterized by a mosaic of grassland, sedgeland and shola (i.e. stunted evergreen forest) (Blasco, 1971(Blasco, , 1987Blasco and Tassy, 1975;Meher-Homji, 1967;Ramya Bala et al, 2022;Ranganathan, 1938). The grassland occurs on the hilltops; shola, in the sheltered valleys; and sedgeland, on the waterlogged valley floors (Ranganathan, 1938).…”

“…The grassland occurs on the hilltops; shola, in the sheltered valleys; and sedgeland, on the waterlogged valley floors (Ranganathan, 1938). While shola vegetation is mainly C3, grassland and sedgeland comprise both C3 and C4 vegetation (Ramya Bala et al, 2022). At the sampling site, the shola vegetation is in proximity in the south, east and west; and the grassland, in the north and northwest (Supplemental Figure S1, available online).…”

“…The Nilgiri Plateau, southern India, hosts paleoclimate archives such as peat deposits that are ideal for the preservation of pollen (Sutra, 1997;Sutra et al, 1997;Vasanthy, 1988) and other organic matter (Ramya Bala et al, 2022;Sukumar et al, 1993), soil profiles (Caner et al, 2007), and lake sediments (Raja et al, 2018). In contrast to the other archives in the Indian region, peat deposits offer a continuous record of >40 kyr (Rajagopalan et al, 1997;Ramya Bala et al, 2022) and are important to reveal the orbital-scale climate variability and the factors influencing it. The pollen records from the Nilgiri Plateau have established not only the long-term stability of the Shola-Grassland mosaic but also the heterogeneity of landcover change, going back >30,000 years (Sutra, 1997).…”

“…The pollen records from the Nilgiri Plateau have established not only the long-term stability of the Shola-Grassland mosaic but also the heterogeneity of landcover change, going back >30,000 years (Sutra, 1997). The carbon isotopic composition (δ 13 C) of peat (Rajagopalan et al, 1997;Ramya Bala et al, 2022;Sukumar et al, 1993) and soil profiles (Caner et al, 2007) from the Nilgiri Plateau, reflecting the relative proportion of C3 and C4 plants in the catchment area, has been used to reconstruct the past climate variability. However, as the trees to grass ratio in the Nilgiri Plateau is controlled by the winter frost (Joshi et al, 2020), δ 13 C and pollen-based proxies might not reflect past precipitation variability alone.…”

“…However, as the trees to grass ratio in the Nilgiri Plateau is controlled by the winter frost (Joshi et al, 2020), δ 13 C and pollen-based proxies might not reflect past precipitation variability alone. Further, short-term disturbances such as fire and local attributes led to divergence in the δ 13 C records of peat cores (Ramya Bala et al, 2022). Thus the potential of peat deposits from the Nilgiri Plateau in reconstructing past rainfall variability is yet to be fully realized.…”