Many grasslands and savannas around the world have experienced woody plant encroachment in recent history. In the Rio Grande Plains of southern Texas, subtropical woodlands dominated by C3 trees and shrubs have become significant components of landscapes once dominated almost exclusively by C4 grasslands. In this study, spatial variation of soil δ13C to was used to investigate patterns of transformation. Previous research has shown that grassland-to-shrubland transitions are initiated when discrete, multi-species shrub clusters organized around a honey mesquite (Prosopis glandulosa) tree nucleus established in grassland. It is inferred from space-for-time substitution and modeling studies that as new shrub clusters are initiated and existing clusters enlarge, coalescence will occur, leading to the formation of groves; and that groves will eventually merge to form woodlands. The hypothesis that present-day mesquite groves represent areas where individual discrete shrub clusters have proliferated and coalesced was evaluated by comparing patterns of soil δ13C within isolated shrub clusters (n = 6) to those in nearby groves (n = 3). Mean soil δ13C within discrete clusters was lowest in the center (−23.3‰), increased exponentially toward the dripline (−20.1‰), and stabilized at a relatively high value approximately 15 cm beyond the dripline (−18.9‰). The spatial structure of soil δ13C in groves was consistent with that which would be expected to occur if present-day grove communities were a collection of what once were individual discrete clusters that had fused. As such, it provides direct evidence in support of conceptual and mathematical models derived from indirect assessments. However, spatial patterns of soil δ13C suggest that groves are not simply a collection of clusters with respect to primary production and SOC turnover. This study illustrates how soil δ13C values can be used to reconstruct successional processes accompanying vegetation compositional change, and its consequences for ecosystem function.