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Plant species distributions and transitions between vegetation types are determined by numerous factors, including disturbances such as fire. Documentation of past changes in the distribution and structure of fire-dependent ecosystems is necessary to assess the success of land management in maintaining historic vegetation types. In our study system—longleaf pine (Pinus palustris)-wiregrass (Aristida stricta) savannas and embedded wetlands (i.e., pocosins)—wiregrass is dependent on frequent burning. We used soil carbon (C) isotopes to test for past changes in the abundance of wiregrass and for shifts in the ecotone between savanna and pocosin in four sites at Fort Bragg, North Carolina. Wiregrass has a C₄ photosynthetic pathway and therefore produces organic matter enriched in ¹³C compared to other dominant species in the community that have a C₃ photosynthetic pathway. At each of the four sites, we measured profiles of soil δ¹³C to a depth of 1 m at four locations along the vegetation gradient from upland savannas to lowland pocosins. Surface soil δ¹³C, which reflects C inputs from current vegetation, did not differ along the gradient despite a decrease in wiregrass cover from savanna, where it is abundant, to pocosin, where it is absent. Enrichment of soil δ¹³C with depth was indistinguishable from effects of decomposition, indicating that past variation in the abundance and distribution of wiregrass was not detectable in our study sites. Our results suggest that wiregrass currently does not produce, and historically has not produced, enough biomass to influence soil δ¹³C values along the savanna-pocosin gradient.
