Long-term influence of woody plant removal on soil organic carbon pools in a semi-desert grassland

The abundance of woody species in grasslands and savannas has increased globally over the past century. Recent estimates suggest that this proliferation of woody plants may account for a significant fraction of the Northern Hemisphere carbon (C) sink. A large degree of uncertainty in the direction and magnitude of soil C pool response to woody encroachment exists; resolving this uncertainty is important given that the majority of C in dryland systems is typically belowground. In the western US, landscapes may be a mosaic of sites undergoing shrub encroachment and sites recovering from 'brush management' practices imposed to reduce shrub cover. We coupled field data with CENTURY, a process-based ecosystem model, to project changes in SOC in response to woody encroachment and subsequent brush management in a southern Arizona semi-desert grassland. Encroachment by velvet mesquite (Prosopis velutina), estimated to have initiated ca. 1900, had a strong, positive influence on SOC, with SOC pools doubling after 100 years of encroachment (946 g C m-2 for unencroached grassland and 1886 g C m-2 for the shrubland developing on former grassland). Removal of mesquite in simulation year 1960 (SOC = 1030 g C m-2) caused SOC pools to decline to 956 g C m-2 by simulation year 2005. The model suggests SOC losses would continue for ca. 105 years after shrub removal, before stabilizing around 920 g C m-2. The degree of soil disturbance strongly affected SOC loss dynamics, with more rapid initial loss rates following management techniques that disturb the soil and coarse root material.