title: CO2exchange and evapotranspiration across dryland ecosystems of southwestern North America.
abstract: Global-scale studies suggest that dryland ecosystems dominate an increasing trend in the magnitude and interannual variability of the land CO2sink. However, such analyses are poorly constrained by measured CO2exchange in drylands. Here we address this observation gap with eddy covariance data from 25 sites in the water-limited Southwest region of North America with observed ranges in annual precipitation of 100-1000 mm, annual temperatures of 2-25°C, and records of 3-10 years (150 site-years in total). Annual fluxes were integrated using site-specific ecohydrologic years to group precipitation with resulting ecosystem exchanges. We found a wide range of carbon sink/source function, with mean annual net ecosystem production (NEP) varying from -350 to +330 gCm-2across sites with diverse vegetation types, contrasting with the more constant sink typically measured in mesic ecosystems. In this region, only forest-dominated sites were consistent carbon sinks. Interannual variability of NEP, gross ecosystem production (GEP), and ecosystem respiration (Reco) was larger than for mesic regions, and half the sites switched between functioning as C sinks/C sources in wet/dry years. The sites demonstrated coherent responses of GEP and NEP to anomalies in annual evapotranspiration (ET), used here as a proxy for annually available water after hydrologic losses. Notably, GEP and Recowere negatively related to temperature, both interannually within site and spatially across sites, in contrast to positive temperature effects commonly reported for mesic ecosystems. Models based on MODIS satellite observations matched the cross-site spatial pattern in mean annual GEP but consistently underestimated mean annual ET by ~50%. Importantly, the MODIS-based models captured only 20-30% of interannual variation magnitude. These results suggest the contribution of this dryland region to variability of regional to global CO2exchange may be up to 3-5 times larger than current estimates.
authors.affiliation: Southwest Watershed Research Center, Agricultural Research Service, Tucson, AZ, USA.
authors.forename: Joel A
authors.identifier: 0000-0002-1831-461X
authors.initials: JA
authors.lastname: Biederman
authors.affiliation: Southwest Watershed Research Center, Agricultural Research Service, Tucson, AZ, USA.
authors.forename: Russell L
authors.initials: RL
authors.lastname: Scott
authors.affiliation: Earth Research Institute, University of California Santa Barbara, Santa Barbara, CA, USA.
authors.forename: Tom W
authors.initials: TW
authors.lastname: Bell
authors.affiliation: Department of Biology, University of Utah, Salt Lake City, UT, USA.
authors.forename: David R
authors.initials: DR
authors.lastname: Bowling
authors.affiliation: School of Forestry, Merriam-Powell Center for Environmental Research, Northern Arizona University, Flagstaff, AZ, USA.
authors.forename: Sabina
authors.identifier: 0000-0003-2956-5251
authors.initials: S
authors.lastname: Dore
authors.affiliation: Departamento de Ciencias del Agua y Medio Ambiente, Instituto Tecnológico de Sonora, Ciudad Obregón, Sonora, Mexico.
authors.forename: Jaime
authors.initials: J
authors.lastname: Garatuza-Payan
authors.affiliation: School of Forestry, Merriam-Powell Center for Environmental Research, Northern Arizona University, Flagstaff, AZ, USA.
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Kolb
authors.affiliation: Atmospheric Turbulence and Diffusion Division, Air Resources Laboratory, National Oceanographic and Atmospheric Administration, Oak Ridge, TN, USA.
authors.forename: Praveena
authors.initials: P
authors.lastname: Krishnan
authors.affiliation: Department of Biology, University of New Mexico, Albuquerque, NM, USA.
authors.forename: Dan J
authors.initials: DJ
authors.lastname: Krofcheck
authors.affiliation: Department of Biology, University of New Mexico, Albuquerque, NM, USA.
authors.forename: Marcy E
authors.initials: ME
authors.lastname: Litvak
authors.affiliation: Department of Biology, University of New Mexico, Albuquerque, NM, USA.
authors.forename: Gregory E
authors.initials: GE
authors.lastname: Maurer
authors.affiliation: Atmospheric Turbulence and Diffusion Division, Air Resources Laboratory, National Oceanographic and Atmospheric Administration, Oak Ridge, TN, USA.
authors.forename: Tilden P
authors.initials: TP
authors.lastname: Meyers
authors.affiliation: Global Change Research Group, Department of Biology, San Diego State University, San Diego, CA, USA.|Department of Geography, College of Life and Environmental Sciences, Exeter, UK.
authors.forename: Walter C
authors.initials: WC
authors.lastname: Oechel
authors.affiliation: School of Natural Resources and the Environment, University of Arizona, Tucson, AZ, USA.
authors.forename: Shirley A
authors.initials: SA
authors.lastname: Papuga
authors.affiliation: Southwest Watershed Research Center, Agricultural Research Service, Tucson, AZ, USA.
authors.forename: Guillermo E
authors.initials: GE
authors.lastname: Ponce-Campos
authors.affiliation: Departamento de Agricultura y Ganaderia, Universidad de Sonora, Hermosillo, Sonora, Mexico.
authors.forename: Julio C
authors.initials: JC
authors.lastname: Rodriguez
authors.affiliation: School of Natural Resources and the Environment, University of Arizona, Tucson, AZ, USA.
authors.forename: William K
authors.initials: WK
authors.lastname: Smith
authors.affiliation: Department of Plant and Soil Sciences, University of Delaware, Newark, DE, USA.
authors.forename: Rodrigo
authors.initials: R
authors.lastname: Vargas
authors.affiliation: Departamento de Fisica, Universidad de Sonora, Hermosillo, Sonora, Mexico.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Watts
authors.affiliation: Departamento de Ciencias del Agua y Medio Ambiente, Instituto Tecnológico de Sonora, Ciudad Obregón, Sonora, Mexico.
authors.forename: Enrico A
authors.initials: EA
authors.lastname: Yepez
authors.affiliation: Department of Earth System Science, University of California Irvine, Irvine, CA, USA.
authors.forename: Michael L
authors.initials: ML
authors.lastname: Goulden
mesh_terms: D002245:Carbon Dioxide; D017753:Ecosystem; D065928:Forests; D009656:North America; D013696:Temperature
chemical_list: D002245:Carbon Dioxide
keywords: climate; moderate resolution imaging spectroradiometer; net ecosystem exchange; photosynthesis; remote sensing; respiration; semiarid; water