Landscape-scale vegetation change indicated by carbon isotopes in soil organic matter for a semidesert grassland in southeastern Arizona

Abstract

Vegetation change, particularly from the grass to shrub lifeform, is a critical issue on the world's rangelands. The plant community present on a site is the primary determinant of the land's value for watershed protection, wildlife habitat, livestock production, and recreation. Studying past vegetation composition can help separate natural from anthropogenic sources of change and guide natural resource conservation and management decisions. 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. The utility of the carbon isotope method was verified for this site based on the clear and wide separation in δ¹³C values between grasses having the C₄ photosynthetic pathway and shrubs having the C₃ pathway. The direction and timing of vegetation dynamics differed with landscape position along a gentle elevation gradient from the basin outlet to a nearby volcanic ridge top. Warm-season C₄ perennial grasses have dominated the basin outlet, center, and toe slope landscape positions since at least 5000-6000 yr BP, except for a dramatic increase in C₃ plants at the bottom of the outlet excavation around 5000 yr 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, warm-season perennial grass, composition decreased from approximately 60% as recently as 400 yr BP to only 1.5% now. SOM density separates were also analyzed. The youngest SOM is represented by the <2 g/cm³ density fraction that turns over in a few years to several decades and has a post-bomb radiocarbon age. For the ridge top landscape position, this fraction yielded 39% C₄ vegetation, suggesting that the conversion from grass to shrub vegetation occurred recently.