A Three-Dimensional Assessment of Soil δ13C in a Subtropical Savanna: Implications for Vegetation Change and Soil Carbon Dynamics

Abstract

Tree/shrub encroachment into drylands is a geographically widespread vegetation change that often modifies soil organic carbon (SOC) storage and dynamics, and represents an important yet uncertain aspect of the global carbon (C) cycle. We quantified spatial patterns of soil δ13C to 1.2 m depth in a subtropical savanna to evaluate the magnitude and timing of woody encroachment, and its impacts on SOC dynamics. Woody encroachment dramatically altered soil δ13C spatial patterns throughout the profile; values were lowest in the interiors of woody patches, increased towards the peripheries of those patches, and reached highest values in the surrounding grasslands. Soil δ13C and 14C revealed this landscape was once dominated by C4 grasses. However, a rapid vegetation change occurred during the past 100–200 years, characterized by (1) the formation and expansion of woody patches across this landscape, and (2) increased C3 forb abundance within remnant grasslands. Tree/shrub encroachment has substantially increased SOC and the proportion of new SOC derived from C3 plants in the SOC pool. These findings support the emerging perspective that vegetation in many dryland ecosystems is undergoing dramatic and rapid increases in SOC storage, with implications for the C cycle at regional and global scales.

Woody encroachment happens via intensification, not extensification, of species ranges in an African savanna

“…2016, Zhou et al. 2019). Furthermore, few studies have considered woody encroachment as a community‐level process, and we mostly lack an understanding of how woody encroachment results from changes in the community and species composition of the savanna woody layer.…”

“…Improving our understanding of the patterns and drivers of woody encroachment across savannas is paramount to achieving optimal outcomes in managing and conserving this ecologically and economically important biome, which covers around 20% of Earth's land surface (Scholes andArcher 1997, Sankaran et al 2005). Work on encroachment to date has relied overwhelmingly on ecosystem-level approaches, quantifying woody cover or biomass as a whole, without differentiating among components of the woody community (Wigley et al 2010, Stevens et al 2016, Zhou et al 2019. Furthermore, few studies have considered woody encroachment as a community-level process, and we mostly lack an understanding of how woody encroachment results from changes in the community and species composition of the savanna woody layer.…”

“…Another important question regarding woody encroachment in savannas is whether woody species are thickening within their existing ranges (i.e., intensification) or are actively expanding into new ranges (i.e., extensification). Woody encroachment has been extensively reported based on time-series of aerial and satellite images at continental, regional, and local scales (e.g., Stevens et al 2017, Venter et al 2018, Zhou et al 2019, but these studies generally fall short of identifying individual woody species and their community assemblages, such that it has so far been difficult to distinguish to what extent intensification vs. extensification of species distributions are contributing to ecosystem changes. Though traditional ground-based surveys are able to examine species-level changes in woody plant abundance through time (Roques et al 2001, Case and, these studies are usually limited in the number of sampling points at scales that can be spatially applied to detect range shifts at the species level.…”

“…2018, Zhou et al. 2019), but these studies generally fall short of identifying individual woody species and their community assemblages, such that it has so far been difficult to distinguish to what extent intensification vs. extensification of species distributions are contributing to ecosystem changes. Though traditional ground‐based surveys are able to examine species‐level changes in woody plant abundance through time (Roques et al.…”