Shrubland carbon sink depends upon winter water availability in the warm deserts of North America

Authors: Biederman, Joel; Scott, Russell - Russ; ARNONE, J. - Desert Research Institute; JASONI, R. - Desert Research Institute; LITVAK, M. - University Of New Mexico; MOREO, M. - Us Geological Survey (USGS); PAPUGA, S. - University Of Arizona; Ponce Campos, Guillermo; SCHREINER-MCGRAW, A. - Arizona State University; VIVONI, E. - Arizona State University

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2017
Publication Date: 3/1/2018
Publication URL: http://handle.nal.usda.gov/10113/5858117
Citation: Biederman, J.A., Scott, R.L., Arnone, J., Jasoni, R., Litvak, M., Moreo, M., Papuga, S., Ponce Campos, G.E., Schreiner-Mcgraw, A., Vivoni, E. 2018. Shrubland carbon sink depends upon winter water availability in the warm deserts of North America. Agricultural and Forest Meteorology. 249:407-419. https://doi.org/10.1016/j.agrformet.2017.11.005.
DOI: https://doi.org/10.1016/j.agrformet.2017.11.005

Interpretive Summary: Shrublands are the most common land cover type on Earth and cover an estimated 85% of the deserts in the Southwest region of North America. Shrublands provide critical agroecosystem services including grazing, habitat, water supply, and atmospheric CO2 uptake. However, shrubland flux measurements lag far behind those in wetter ecosystems such as forests. Here we assembled 33 years of data from six shrubland eddy covariance sites of the Chihuahuan, Sonoran and Mojave Deserts. The deserts vary in average precipitation and seasonal distribution of rainfall between summer and winter. Our objective was to determine how precipitation amount and seasonality regulated gross and net shrubland productivity (carbon uptake). We found that while gross productivity was greatest in summer, net productivity was higher in winter, when respiration returned less carbon to the atmosphere. Therefore, reduced winter precipitation across the Southwest in the 21st century has reduced the region’s net productivity

Technical Abstract: Global-scale studies suggest that dryland ecosystems dominate an increasing trend in the magnitude and interannual variability of the land CO2 sink. However, such model-based analyses are poorly constrained by measured CO2 exchange in open shrublands, which is the most common global land cover type, covering ~14% of Earth’s surface. Here we evaluate how water availability regulates desert shrubland CO2 exchange. We use eddy covariance data from six sites across the three warm deserts of North America with observed ranges in annual precipitation of ~100 – 400 mm, annual temperatures of 13 – 18 'C, and records of 2 – 8 years (33 site-years in total). The Chihuahuan, Sonoran and Mojave Deserts present gradients in both mean annual precipitation and its seasonal distribution. We found that due to hydrologic losses during the wettest summers in the Sonoran and Chihuahuan Deserts, evapotranspiration (ET) was a better metric than precipitation of water available to drive ecosystem CO2 exchange. As expected, ET and gross ecosystem productivity (GEP) were positively related across sites, but net ecosystem productivity (NEP) appeared unrelated to the amount or seasonality of water availability. Ecosystem water use efficiency (WUE = GEP/ET) did not differ between winter and summer. Carbon use efficiency (CUE = NEP/GEP), however, was greater in winter because the average return of carbon dioxide to the atmosphere by ecosystem respiration (Reco) was smaller in winter (23% of GEP) than in summer (77%), making winter precipitation and associated CO2 fluxes crucial to annual NEP in these warm deserts. Combining the water-carbon relations found here with historical precipitation since 1980, we estimate that lower average winter precipitation during the 21st century reduced the net carbon sink of the three deserts by an average of 6.8 Tg C yr-1.