Paleovegetation dynamics in a montane vegetation mosaic in the Western
Ghats, India: evidence for alternative stable states in the past?
Ecologists have proposed that montane grassland-shola (stunted evergreen
forest) mosaics in the Western Ghats may represent alternative stable
vegetation states. But paleoecology investigations seldom consider this
framework, especially the role of short-term disturbances (fire, intense
drought) other than long-term climatic changes, that can cause
vegetation switches in landscapes with alternative vegetation states.
The Sandynallah valley that hosts one of the oldest peat accumulations
in the world at >50 kyr has been central to the
reconstruction of paleovegetation in the montane Nilgiris, Western
Ghats. Although the peat-forming vegetation here (dominated by sedges)
is a unique vegetation state, its contribution to the paleovegetation
signal has not been explicitly considered. We propose a conceptual
framework of a tri-stability landscape with sedgeland on the valley
floor, grassland on the hill slopes and shola vegetation in the boundary
between sedgeland and grassland. While frost prevents shola saplings
from establishing in grassland, waterlogging provides a barrier for
their establishment in sedgeland, thus maintaining these distinct
vegetation states under the same climate. We investigated the stable
carbon isotope signatures of the cellulose fraction from two well-dated
peat cores (Cores 1 and 2) collected from ~170m apart in
the Sandynallah valley within the alternative stable states framework.
We find that Core 1, which is closer to the boundary of valley and hill
slope, shows dynamic switches between sedgeland and shola whereas Core
2, located in the centre of the valley floor, represents a stable
sedgeland state. The vegetation switches and maintenance mechanisms at
Core 1 is connected to a disturbance (fire) and to changing climate
while Core 2 seems to be responding primarily to climatic changes. The
simultaneously distinctive vegetation states in Cores 1 and 2 at such
close proximity within the same valley is the first record of
alternative stables states in the past in the Western Ghats.