Mid-late Holocene vegetation response to climatic drivers and biotic disturbances in the Banni grasslands of western India

Abstract

Citation:Pillai AAS, Anoop A, Sankaran M, Sanyal P, Jha DK and Ratnam J (2017) AbstractTropical grasslands and savannas are globally extensive, and are of significant environmental, economic, and ecological importance. These ecosystems are anticipated to be particularly sensitive to future changes in climate, and understanding how these systems have responded to climatic changes in the past can provide us with insights into their potential responses to future global change. In this study, the temporal dynamics of C3-C4 vegetation changes in response to changes in moisture availability, local fire events and changing levels of herbivory in a summer-rainfall region of Western India are reconstructed for the past ~4600 cal yr BP. Paleodata such as stable carbon isotope of bulk organic matter ( 13 Corg), oxygen isotope from carbonate shells ( 18 Oshell), macro-charcoal and herbivore dung fungal spores are reported from the retrieved cores of two wetland sites located in the Banni grasslands of Western India. Results show that vegetation in the Banni was composed mostly of C3 vegetation from ~4600 to ~2500 cal yr BP, after which there was a decline in C3vegetation. From the late-Holocene to the present, there was a mix of both C3 and C4 vegetation, with C4 grasses being more abundant in the ecosystem. These shifts were coincident with rainfall changes from more mesic conditions during ~4600 to ~2500 cal yr BP to more arid conditions towards the present as indicated by 18 Oshell isotope data. The period of increase in C4 vegetation also coincides with a period of increased biotic disturbances in the ecosystem, particularly fire. Given the current scenarios of global warming, recurrent drought events and increased anthropogenic use of similar ecosystems, such studies can provide us insights into potential future trajectories of these ecosystems.3

Multi-proxy evidence for an arid shift in the climate and vegetation of the Banni grasslands of western India during the mid- to late-Holocene

“…However, pollen that are representative of arid climate did not show clear declines during this period. Higher grass phytolith abundance in this phase relative to the previous phase can be attributed to increased arid conditions, while periods of lower grass phytolith abundance from ~2000 to ~1600 cal yr BP, and the slight decline after ~1000 cal yr BP following a peak at ~1200 cal yr BP (Figure 4) may be due to both climatic and disturbance factors (Pillai et al, 2017). The period from ~1000 cal yr BP towards the present is marked by an increase in all other herbaceous taxa (Figure 4).…”

“…Linear interpolation shows that the Chachi core spans ca.4600 cal yr BP and Luna core spans ca. 997 cal yr BP (Pillai et al, 2017). The sedimentation rate in the Chachi profile was 0.037 cm/yr from ~3300 to 2000 cal yr BP, followed by a gradual decline towards the present (0.018 cm/yr) (Figure 2(a)).…”

“…The period from ~1000 cal yr BP towards the present is marked by an increase in all other herbaceous taxa (Figure 4). This may have been caused by shifts in disturbance regimes such as fire and herbivory (Pillai et al, 2017) coupled with climatic factors. Evidence from contemporary studies suggests that changes in fire regimes and intense grazing by cattle can allow shrubs and herbs to replace C4 grasses and encroach onto savannas (Roques et al, 2001;Eldridge et al, 2011).…”

“…The slight increase in precipitation since ~600 cal yr BP as shown by the geochemical data is associated with an increase in the abundance of herb/shrub taxa in the pollen records. This may have been due to changes in biotic disturbance regimes (Pillai et al, 2017) and other human mediated processes acting on the ecosystem. Records from the arid, semi-arid belt in western India suggest that a mix of C3 and C4 vegetation during the late Holocene shifted to more C3 vegetation towards the present (Laskar et al, 2013).…”

Response of grassland ecosystem to monsoonal precipitation variability during the Mid-Late Holocene: Inferences based on molecular isotopic records from Banni grassland, western India

“…A previous study from the Banni grassland used the carbon isotopic composition of bulk organic matter (δ 13 C org ) and oxygen isotopic composition of gastropod shells (δ 18 O shell ) to decipher paleoenvironmental changes [12]. The δ 13 C org values from the Banni grassland suggested Late Holocene expansion of a C 4 grass dominated ecosystem in response to changes in moisture availability and local fire-events [12, 13]. However, δ 13 C org value based estimates of %C 4 plants have uncertainties due to thermal degradation of selective organic compounds [14], which can alter the pristine isotopic character [15].…”