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Research Project: Understanding Water-Driven Ecohydrologic and Erosion Processes in the Semiarid Southwest to Improve Watershed Management

Location: Southwest Watershed Research Center

Title: Ecosystem hydrologic and metabolic flashiness are shaped by plant community traits and precipitation

Author
item Potts, D.l. - State University Of New York (SUNY)
item Barron-gafford, G.a. - University Of Arizona
item Scott, Russell - Russ

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/24/2019
Publication Date: 8/27/2019
Citation: Potts, D., Barron-Gafford, G., Scott, R.L. 2019. Ecosystem hydrologic and metabolic flashiness are shaped by plant community traits and precipitation. Agricultural and Forest Meteorology. 279:1-8. https://doi.org/10.1016/j.agrformet.2019.107674.
DOI: https://doi.org/10.1016/j.agrformet.2019.107674

Interpretive Summary: Understanding the hydrologic and carbon cycling consequences of precipitation variability in dryland ecosystems requires improved appreciation and accounting of how above- and belowground physical processes differ in their response to rainfall. Our objective was to contrast the sensitivity of dryland ecosystem evapotranspiration (ET), gross ecosystem productivity (GEP), and ecosystem respiration (Re) in response to seasonal and annual precipitation changes in a grassland, savanna, and shrubland ecosystems in southeastern Arizona. We modified a hydrologic index which quantifies the flashiness of a stream’s hydrograph, to calculate analogous indices of ecosystem hydrologic and metabolic flashiness. In contrast to our prediction, annual GEP flashiness was consistently greater than annual Re. Furthermore, we predicted that increasing rooting depth would correlate with a decline in annual ET and GEP. In fact, annual GEP flashiness was similar between the grassland, savanna, and shrubland. Whereas the response of annual ET and GEP flashiness to annual precipitation was plant community dependent and generally declined with increasing rainfall, annual Re flashiness did not vary in response to precipitation. The effect of late summer storms on GEP flashiness was plant community dependent such that shrubland GEP and Re strongly declined in response to rainfall whereas grassland and savanna GEP was relatively unresponsive. Conceptually similar to hydrologic flashiness, ecosystem flashiness may provide an additional lens through which to observe the influence of resource availability, shifts in community composition, and disturbance on ecosystem hydrologic and carbon cycling.

Technical Abstract: Understanding the hydrologic and carbon cycling consequences of precipitation variability in dryland ecosystems requires improved appreciation and accounting of how above- and belowground biophysical processes differ in their response to rainfall. Our objective was to contrast the sensitivity of dryland ecosystem evapotranspiration (ET), gross ecosystem productivity (GEP), and ecosystem respiration (Re) in response to inter- and intra-annual precipitation variability in a nearby grassland, savanna, and shrubland ecosystems in southeastern Arizona. To do this, we modified the Richards-Baker index, which quantifies the flashiness of a stream’s hydrograph, to calculate analogous indices of ecosystem hydrologic and metabolic flashiness. In this way, ecosystem flashiness describes the frequency and rapidity of short-term fluctuations in H2O and CO2 exchange in response to precipitation while preserving the sequence of day-to-day variation in fluxes using tower-based time-series of daily averaged ET, GEP and Re. We calculated annual hydrologic, GEP, and Re flashiness (ETf, GEPf and Ref respectively) using 6 years of daily-averaged fluxes estimated from eddy covariance. In contrast to our prediction, annual GEPf was consistently greater than annual Ref. Furthermore, we predicted that increasing rooting depth would correlate with a decline in annual ETf and GEPf. In fact, annual GEPf was similar between the grassland, savanna, and shrubland. Whereas the response of annual ETf and GEPf to annual precipitation was plant community dependent and generally declined with increasing rainfall, annual Ref did not vary in response to precipitation. The effect of late summer storms on GEPf was plant community dependent such that shrubland GEPf and Ref strongly declined in response to rainfall whereas grassland and savanna GEPf was relatively unresponsive. Conceptually similar to hydrologic flashiness, ecosystem flashiness may provide an additional lens through which to observe the influence of resource availability, shifts in community composition, and disturbance on ecosystem hydrologic and carbon cycling.