Stable isotopes in ecosystem science: structure, function and dynamics of a subtropical savanna

Abstract

Stable isotopes are often utilized as intrinsic tracers to study the effects of human land uses on the structural and functional characteristics of ecosystems. Here, we illustrate how stable isotopes of H, C, and O have been utilized to document changes in ecosystem structure and function using a case study from a subtropical savanna ecosystem. Specifically, we demonstrate that: (1) δ¹³C values of soil organic carbon record a vegetation change in this ecosystem from C₄ grassland to C₃ woodland during the past 40–120 years, and (2) δ²H and δ¹⁸O of plant and soil water reveal changes in ecosystem hydrology that accompanied this grassland‐to‐woodland transition. In the Rio Grande Plains of North America, δ¹³C values of plants and soils indicate that areas now dominated by C₃ subtropical thorn woodland were once C₄ grasslands. δ¹³C values of current organic matter inputs from wooded landscape elements in this region are characteristic of C₃ plants (−28 to −25‰), while those of the associated soil organic carbon are higher and range from −20 to −15‰. Approximately 50–90% of soil carbon beneath the present C₃ woodlands is derived from C₄ grasses. A strong memory of the C₄ grasslands that once dominated this region is retained by δ¹³C values of organic carbon associated with fine and coarse clay fractions. When δ¹³C values are evaluated in conjunction with ¹⁴C measurements of that same soil carbon, it appears that grassland‐to‐woodland conversion occurred largely within the past 40–120 years, coincident with the intensification of livestock grazing and reductions in fire frequency. These conclusions substantiate those based on demographic characteristics of the dominant tree species, historical aerial photography, and accounts of early settlers and explorers. Concurrent changes in soil δ¹³C values and organic carbon content over the past 90 years also indicate that wooded landscape elements are behaving as sinks for atmospheric CO₂ by sequestering carbon derived from both the previous C₄ grassland and the present C₃ woody vegetation. Present day woodlands have hydrologic characteristics fundamentally different from those of the original grasslands. Compared to plants in remnant grasslands, tree and shrub species in the woodlands are rooted more deeply and have significantly greater root biomass and density than grasslands. δ¹⁸O and δ²H values of plant and soil water confirm that grassland species acquire soil water primarily from the upper 0.5 m of the soil profile. In contrast, trees and shrubs utilize soil water from throughout the upper 4 m of the profile. Thus, soil water that formerly may have infiltrated beyond the reach of the grassland roots and contributed to local groundwater recharge or other hydrologic fluxes may now be captured and transpired by the recently formed woodland plant communities. The natural abundances of stable isotopes revealed fundamental information regarding the impacts of human land use activities on the structure and function of this subtropical savanna. Stable isotopes provided direct, spatially explicit evidence for dramatic changes in ecosystem physiognomy and demonstrated some functional consequences for the hydrologic cycle. Furthermore, grassland‐to‐woodland conversion has been geographically extensive in the worlds' drylands, suggesting that these ecosystem‐level changes in vegetation structure, carbon cycling, and hydrology may have implications for regional/global biogeochemistry and climate. Copyright © 1999 John Wiley &­Sons, Ltd.