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 world's 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 C ₃ 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.