Productivity and CO2 Exchange of Great Plains Ecoregions. I. Shortgrass Steppe: Flux Tower Estimates

Abstract

The shortgrass steppe (SGS) occupies the southwestern part of the Great Plains. Half of the land is cultivated, but significant areas remain under natural vegetation. Despite previous studies of the SGS carbon cycle, not all aspects have been completely addressed, including gross productivity, ecosystem respiration, and ecophysiological parameters. Our analysis of 1998 - 2007 flux tower measurements at five Bowen ratio-energy balance (BREB) and three eddy covariance (EC) sites characterized seasonal and interannual variability of gross photosynthesis and ecosystem respiration. Identification of the nonrectangular hyperbolic equation for the diurnal CO2 exchange, with vapor pressure deficit (VPD) limitation and exponential temperature response, quantified quantum yield α, photosynthetic capacity Amax, and respiration rate rd with variation ranges (19 < α < 51 mmol mol-1, 0.48 < Amax < 2.1 mg CO2 m-2 s-1, 0.15 < rd < 0.49 mg CO2 m-2 s-1). Gross photosynthesis varied from 1 100 to 2 700 g CO2 m-2 yr-1, respiration from 900 to 3,000 g CO2 m-2 yr-1, and net ecosystem production from - 900 to + 700 g CO2 m-2 yr-1, indicating that SGS may switch from a sink to a source depending on weather. Comparison of the 2004-2006 measurements at two BREB and two parallel EC flux towers located at comparable SGS sites showed moderately higher photosynthesis, lower respiration, and higher net production at the BREB than EC sites. However, the difference was not related only to methodologies, as the normalized difference vegetation index at the BREB sites was higher than at the EC sites. Overall magnitudes and seasonal patterns at the BREB and the EC sites during the 3-yr period were similar, with trajectories within the ± 1.5 standard deviation around the mean of the four sites and mostly reflecting the effects of meteorology. © 2017 The Authors. Published by Elsevier Inc. on behalf of The Society for Range Management.