Forest savanna ecotone dynamics in India as revealed by carbon isotope ratios of soil organic matter.

In India, the stability of the forest savanna boundary (ecotone) has been questioned. To investigate the possibility of vegetation change at this boundary, we used the natural difference in the stable carbon isotope ratio (13C/12C, expressed as δ13C) of C3 (forest) and C4 (savanna) plants, which is preserved in the isotopic composition of soil organic carbon. Past changes in the position of the ecotone can thus be documented by 13C/12C analysis of soil organic matter (SOM). Measurements were made on soil samples taken along a transect across the forest savanna boundary. In both ecosystems, δ13C values of SOM in upper soil layers appeared to be in equilibrium with the current plant community: SOM was near-14‰ under savanna cover, and near-28‰ under forest. By contrast, δ13C values of SOM from deep layers of soil profiles under forest cover near the ecotone, were less negative than would be expected for a C3-dominated community. These results indicate that a change in ecosystem dominance occurred recently, and that forest has invaded an area occupied previously by a C4 plant community (savanna and cultivated grassland).