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 b een questioned. To investigate the possibility of vegetation change at this boundary, we used the natural difference in the stable carbon is otope ratio (C-13/C-12, expressed as deltaC-13) of C3 (forest) and C4 (savanna) plants, which is preserved in the isotopic composition of so il organic carbon. Past changes in the position of the ecotone can thu s be documented by C-13/C-12 analysis of soil organic matter (SOM). Me asurements were made on soil samples taken along a transect across the forest savanna boundary. In both ecosystems, deltaC-13 values of SOM in upper soil layers appeared to be in equilibrium with the current pl ant community: SOM was near - 14 parts per thousand under savanna cove r, and near - 28 parts per thousand under forest. By contrast, deltaC- 13 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-d ominated community. These results indicate that a change in ecosystem dominance occurred recently, and that forest has invaded an area occup ied previously by a C, plant community (savanna and cultivated grass l and.