delta13C values of soil organic carbon and their use in documenting vegetation change in a subtropical savanna ecosystem

摘要

In southern Texas, USA, C4 grasslands and savannas have been largely replaced by C3 subtropical thorn woodlands dominated by Prosopis glandulosa. delta13C values of soil organic matter, above- and below-ground plant biomass, and litter were used with radiocarbon dating and dendrochronology to test whether: (1) C3 Prosopis groves in uplands and C3 Prosopis woodlands in low-lying drainages have been long-term components of the landscape; and (2) Prosopis woodlands of low-lying drainages have expandedup-slope since Anglo-European settlement. Current organic matter inputs were not in isotopic equilibrium with soil organic carbon in any of the patch types sampled. In upland grasslands, delta13C values of vegetation (-20o/oo) were lower than those of soil organic matter (- 17o/oo), suggesting increased C3 forb abundance in response to long-term, heavy grazing (herbaceous retrogression). In wooded landscape elements, delta13C values of current organic matter inputs were characteristic of C3 plants (-28to -25o/oo), while those of the associated soil organic matter were typically -20 to -15o/oo. These delta13C values indicate that woodlands, groves, and shrub clusters dominated almost exclusively by C3 plants now occupy sites once dominated by C4 grasses. A particularly strong memory of the C4 grasslands that once occupied these sites was recorded in the delta13C values of organic carbon associated with fine and coarse clay fractions (-18 to -14o/oo). Herbaceous retrogression and a shift from C4 grassland to C3 woodland probably occurred within the last 50-100 years. Demographic characteristics of the dominant tree species corroborated the delta13C and 14C evidence, and indicated widespread establishment of P. glandulosa and associated shrubs over the past 100 years. The results show that delta13C values of soil organic matter afford a direct and powerful technique for reconstructing vegetation change in these areas.