COS 61-2 - Long-term influence of woody plant removal on soil organic carbon pools in a semi-desert grassland

Wednesday, August 8, 2007: 8:20 AM
J2, San Jose McEnery Convention Center
Heather L. Throop, Biology Department, New Mexico State University, Las Cruces, NM, Steven R. Archer, School of Natural Resources and the Environment, University of Arizona, Tucson, AZ and Mitchel P. McClaran, School of Natural Resources, University of Arizona, Tucson, AZ
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.
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