CAN SOIL ORGANIC-CARBON ISOTOPES BE USED TO DESCRIBE GRASS-TREE DYNAMICS AT A SAVANNA-GRASSLAND ECOTONE AND WITHIN THE SAVANNA

We evaluated the use of soil organic carbon (SOC) isotopes to describe grass-tree dynamics at locations at the savanna-C-4 grassland ecotone and within a temperate semiarid Quercus savanna in southeastern Arizo na, USA. SOC will not describe grass-tree dynamics at locations within the savanna because isotope composition near the soil surface does no t correspond with the overlying vegetation and recent C-3 carbon has b een deposited at deep soil depths with no C-4 analog. In contrast, SOC can describe grass-tree dynamics at the savanna-grassland ecotone bec ause isotope composition near the soil surface corresponds with overly ing vegetation and significant deep soil deposition of C-3 carbon was not apparent. At the ecotone, trees became established in the last 700 -1700 years. There is no evidence to suggest an unstable grass-tree mi xture at the ecotone since that time.