Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/25/2005
Publication Date: 1/24/2006
Citation: Scott, R.L., Huxman, T.E., Williams, D., Goodrich, D.C. 2006. Ecohydrological impacts of woody plant encroachment: seasonal patterns of water and carbon dioxide exchange within a semiarid riparian environment. Global Change Biology, 12:311-324. Interpretive Summary: Encroachment by mesquite is arguably the most pervasive land-cover change in the southwestern US. It is important to understand how this on-going change in vegetation will affect water and nutrient cycling in order to predict the outcomes of this change on society. We investigated the consequences of mesquite encroachment on water and carbon exchange in a Southwestern riparian area along the San Pedro River. Results suggest that the deep roots of mesquite will lead to an increase in ecosystem water use as the invading mesquites mature in former grasslands. This ability of mesquite to acquire stable groundwater sources rather than precipitation enhanced net carbon uptake in the dry periods and net carbon loss in rainy periods. These results suggest that mesquite encroachment in riparian areas will increase groundwater use and lead to additional carbon sequestration.
Technical Abstract: Across many dryland regions, historically grass-dominated ecosystems have been encroached upon by woody plant species. In this paper, we compare ecosystem water and carbon dioxide fluxes over a grassland, a grassland-shrubland mosaic, and a fully developed woodland to evaluate potential consequences of woody plant encroachment on these ecosystem exchanges. All three sites were located in the riparian corridor of a perennial river in the southwest U.S. As such, plants in these ecosystems, unlike their upland counterparts, may have access to the additional source of moisture at the capillary fringe of the near-surface water table. We compared the fluxes that were measured in 2003 using eddy covariance and found that ecosystem evapotranspiration and net carbon uptake increased with the amount of woody plants. Growing season evapotranspiration totals were 407 mm, 450 mm, and 639 mm in the grassland, shrubland, and woodland, respectively. All sites had evapotranspiration in excess of precipitation (227 mm, 265 mm, and 473 mm in the same order) and thus acquired some amount of groundwater, especially during the extremely dry pre-monsoon period when plants had leafed out, but soil surface moisture was unavailable. The greater access to groundwater for the deeper-rooted woody plants apparently causes a decoupling of ecosystem evapotranspiration from gross ecosystem production (GEP) with respect to precipitation. The woody plants were better able to use the stable groundwater source, which increased net carbon gain during the dry periods, but also potentially decreased net carbon gain during rainy periods due to high microbial respiration from decomposition of accumulated leaf litter. Estimated April through December (primary growing season) totals of carbon dioxide flux was 63 g C m-2, 212 g C m-2, and 233 g C m-2 in the grassland, shrubland, and woodland, respectively, indicating a strengthening sink of carbon at the woodier sites that did not scale with ecosystem water use. Despite a higher density of woody plants and a greater productivity than the shrubland, the mesquite woodland had a much higher respiration response to rainfall that largely offset its higher accumulation of carbon. These initial data suggest that the ability of the woody plants to better exploit water resources in riparian areas results in enhanced carbon sequestration at the expense of increased groundwater use under current climate conditions. These results highlight the important roles of water sources and ecosystem structure on the control of water and carbon balances in dryland areas.