Landscape patterns of vegetation change indicated by soil carbon isotope composition

Vegetation change, particularly from the grass to shrub life form, is a critical issue on the worldʹs semiarid rangelands. Stable carbon isotope (δ13C) values and associated radiocarbon ages from soil organic matter (SOM) were used to evaluate vegetation change across five landscape positions at a small enclosed basin in southeastern Arizona. Light and dense SOM fractions were separated to distinguish recent vegetation changes. The direction and timing of vegetation change differed with landscape position along a gentle elevation gradient from the basin outlet to a nearby volcanic ridge top. C4 perennial grasses have dominated the basin outlet, center, and toe slope landscape positions since at least 5000–6000 years BP, except for the dominance of C3 plants at the bottom of the outlet excavation at 5000 years BP. This isotopic change is associated with rounded cobbles that may have been a stream channel, suggesting the presence of C3 herbaceous or woody riparian vegetation. On mid-slope and ridge top landscape positions, where semidesert shrubs now dominate, the proportion of plants with C4 metabolism calculated from mass balance mixing formulas decreased from approximately 60% as recently as 400 years BP to only 1.5% observed today. The light SOM fraction from mid-slope and ridge top surface soil horizons was approximately 30% C4 and had a post-bomb date, suggesting that the conversion from grass to shrub occurred over the last several decades.