Spatial variation of soil δ13C values reflects pattern of woody plant encroachment in a subtropical savanna

In recent history, many grasslands and savannas around the world have experienced woody plant encroachment. In the Rio Grande Plains of southern Texas, subtropical woodlands dominated by C3 trees and shrubs have become significant components of landscapes that were once almost exclusively dominated by C4 grasslands. In this study, we used δ13C of soil organic matter (SOM) in conjunction with geostatistical methods to investigate patterns of vegetation change at the landscape scale. In an upland portion of the landscape, we set up 6 plots in small woody clusters organized around a central mesquite tree, and 3 plots in larger woody groves which appear to be comprised of clusters that have fused together. Three transects were established from the center of each woody patch type out into the adjacent grassland area. In a lower-lying closed-canopy drainage woodland, two 70 m transects were established. Soils (0-15cm) were sampled along upland and lowland transects, and δ13C of SOM was measured. There were significant spatial patterns in δ13CSOM associated with upland woody patches. In clusters, δ13CSOM was lowest near the bole of the central mesquite tree (mean δ13CSOM = -23.25‰), increased exponentially to the edge of the canopy (mean δ13CSOM = -20.08‰), and stabilized at a relatively high value approximately 15 cm outside of the dripline (mean δ13CSOM = -18.89‰). In groves, δ13CSOM generally increased along the transect from the grove center to the adjacent grassland, but the lowest δ13CSOM values were not always in the grove center. Points closer to a mesquite tree tended to have lower δ13CSOM values (r2=0.44; p<0.0001), and lowest values in groves were associated with the largest (oldest) mesquite trees. Hence, spatial structure of δ13CSOM in groves appears to reflect the age and spatial patterns of the clusters that fused to form the larger grove. In closed-canopy drainage woodlands, variographic analysis indicated δ13CSOM had less spatial structure [(sill-nugget)/sill= 41%] than upland groves [(sill- nugget)/sill=85% to 96%], and the distribution of individual mesquite trees along transects in the drainage woodlands had little impact on spatial patterns of δ13CSOM. Instead, δ13CSOM in drainage woodlands appeared to be correlated with clay content, with higher δ13CSOM where % clay was high. High clay content would protect 13C-enriched organic matter derived from the original grassland from decay, and lengthen its mean residence time. Our study shows that the union of soil stable isotope analyses with geostatistical techniques can generate sophisticated insights regarding patterns of vegetation change at the landscape scale.