Native shrubland and managed buffelgrass savanna in drylands: implications on ecosystem carbon and water fluxes

Authors: HINOJO-HINOJO, C. - Universidad De Sonora; CASTELLANOS, A. - Universidad De Sonora; HUXMAN, T. - University Of California; RODRIGUEZ, J. - Universidad De Sonora; VARGAS, R. - University Of Delaware; ROMO-LEON, J. - Universidad De Sonora; Biederman, Joel

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/21/2019
Publication Date: 1/30/2019
Citation: Hinojo-Hinojo, C., Castellanos, A., Huxman, T., Rodriguez, J., Vargas, R., Romo-Leon, J., Biederman, J.A. 2019. Native shrubland and managed buffelgrass savanna in drylands: implications on ecosystem carbon and water fluxes. Agricultural and Forest Meteorology. 268:269-278. https://doi.org/10.1016/j.agrformet.2019.01.030.
DOI: https://doi.org/10.1016/j.agrformet.2019.01.030

Interpretive Summary: Dryland ecosystems cover ca. 40% of Earth’s land mass and play an important role in global cycles of water and carbon dioxide (CO2). Management of semiarid agroecosystems has included replacement of shrubs with exotic grass species to improve grazing. These non-native plants often out-compete native plants, resulting in their spread well beyond the areas of intended management, with unknown impacts on CO2 and water cycling. In this study, we measured exchanges of water and CO2 for three years between agroecosystems and the atmosphere over a native shrubland and a nearby exotic buffelgrass grassland in Sonora, Mexico. When rainfall wet the soil, the shrubland used a greater portion of the soil moisture than the grassland, due to the deeper roots of shrubs. However, the grassland used the water available with greater efficiency, making the two sites comparable sinks for atmospheric CO2.

Technical Abstract: Land cover and land-use change between woody- and grass-dominated ecosystems in drylands comprise one of the largest uncertainties in the land CO2 sink. This is especially true for the widespread transition from shrublands to grasslands/savannas caused by the establishment of exotic C4 grass species for grazing or through biological invasion of these species, where information about its impacts on ecosystem CO2 fluxes is limited. For studying this, we used three years of eddy covariance measurements of net ecosystem production (NEP), gross primary production (GPP) and ecosystem respiration (Reco) over a shrubland and an adjacent exotic buffelgrass (Cenchrus ciliaris L.) savanna within the Sonoran Desert. At daily, seasonal and annual time scales, we assessed whether between-site differences in CO2 fluxes were related to differences in ecosystem water use, measured as the fraction of evapotranspiration (ET) to precipitation, water use efficience (WUE), the ratio between GPP and ET or carbon use efficiency (CUE), the fraction of GPP retained as NEP. Although the savanna had higher WUE than the shrubland, its summer NEP was limited by water, likely due to limitations of rooting pattern and leaf area index. Conversely, the savanna had higher NEP than the shrubland during fall to spring seasons due to increased WUE, possibly due to buffelgrass activity or to remaining native woody species using water from deeper soil layers. However, these seasonal differences compensated each other over time, making both sites comparable carbon sinks over the three-year study period.