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 Arizona, USA. SOC will not describe grass-tree dynamics at locations within the savanna because isotope composition near the soil surface does not correspond with the overlying vegetation and recent C-3 carbon has been deposited at deep soil depths with no C-4 analog. In contrast, SOC can describe grass-tree dynamics at the savanna-grassland ecotone because isotope composition near the soil surface corresponds with overlying 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 mixture at the ecotone since that time.