CO2 exchange and evapotranspiration across dryland ecosystems of southwestern North America

Authors: Biederman, Joel; Scott, Russell - Russ; BELL, T.W. - University Of California - Cooperative Extension Service; BOWLING, D.R. - University Of Utah; DORE, S. - Northern Arizona University; GARATUZA-PAYAN, J. - Sonora Institute Of Technology; KOLB, T.E. - Northern Arizona University; KIRISHNAN, P. - National Oceanic & Atmospheric Administration (NOAA); KROFCHECK, D.J. - University Of New Mexico; LITVAK, M.E. - University Of New Mexico; MAURER, G.E. - University Of New Mexico; MEYERS, T.P. - National Oceanic & Atmospheric Administration (NOAA); OECHEL, W.C. - San Diego State University; PAPUGA, S.A. - University Of Arizona; Ponce Campos, Guillermo; RODRIGUEZ, J.C. - Universidad De Sonora; SMITH, W.K. - University Of Arizona; VARGAS, E.A. - University Of Delaware; WATTS, C.J. - Sonora Institute Of Technology; YEPEZ, E.A. - Sonora Institute Of Technology; GOULDEN, M.L. - University Of California

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/7/2017
Publication Date: 10/19/2017
Publication URL: http://handle.nal.usda.gov/10113/5802449
Citation: Biederman, J.A., Scott, R.L., Bell, T., Bowling, D., Dore, S., Garatuza-Payan, J., Kolb, T., Kirishnan, P., Krofcheck, D., Litvak, M., Maurer, G., Meyers, T., Oechel, W., Papuga, S., Ponce Campos, G.E., Rodriguez, J., Smith, W., Vargas, E., Watts, C., Yepez, E., Goulden, M. 2017. CO2 exchange and evapotranspiration across dryland ecosystems of southwestern North America. Global Change Biology. 23:4204-4221. https://doi.org/10.1111/gcb.13686.
DOI: https://doi.org/10.1111/gcb.13686

Interpretive Summary: Global-scale modeling studies suggest semiarid lands dominate the interannual variability of atmospheric CO2 and the increasing trend of terrestrial CO2 uptake. Paradoxically, the remote sensing models and biospheric models used in such studies are poorly validated by semiarid ecosystems measurements, which have lagged as compared with forests. We address this gap with 150 site-years of measurements from 25 ecosystems in southwestern North America including deserts, grasslands, shrublands, and forests. We identified seven subregions with unique seasonal patterns of weather and plant growth, enhancing our ability to understand climate change (e.g. seasonal precipitation shifts). Large precipitation variability drove larger spatial and temporal variability of CO2 uptake than reported in wetter regions. Models predicted only 20 – 30% of the measured interannual variability of plant growth and net CO2 uptake, suggesting that semiarid region impacts on the global carbon cycle could be as much as 3 – 5 times larger than currently estimated.

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 analyses are poorly constrained by measured CO2 exchange 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-2 across 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 Reco were 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 CO2 exchange may be up to 3 – 5 times larger than current estimates.