Abstract Forage production in the Southern Great Plains of the United States relies heavily on Old World bluestem (OWB, Bothriochloa spp.). This study analyzed 4 years (2021–2024) of continuous eddy covariance measurements in a managed OWB pasture in central Oklahoma, spanning both drought and wet periods. The main objectives were to quantify the dynamics of carbon dioxide (CO 2 ) fluxes and evapotranspiration (ET) and determine their primary controlling factors. The OWB pasture exhibited substantial interannual variability in productivity and carbon balance. Annual net ecosystem carbon dioxide exchange (NEE) ranged from a carbon source of 58 g C m −2 to a strong carbon sink of −228 g C m −2 , with peak aboveground dry biomass varying between ~3 and 9 t ha −1 . Similarly, growing season total NEE ranged from −55 to −380 g C m −2 . Despite this variability, the ratio of growing season to annual ET and gross primary production (GPP) remained remarkably consistent (0.86–0.94). Daily peak ET, NEE, GPP, and ecosystem respiration (ER) were about 6 mm day −1 , −8 g C m −2 day −1 , 15 g C m −2 day −1 , and 10 g C m −2 day −1 , respectively. Nongrowing season (November–March) monthly ET remained similar and consistently low (<20 mm) despite substantial fluctuations in monthly rainfall. Increased ET in the year with higher growing season rainfall suggests that the OWB ecosystem utilizes surplus water input when available. The carbon uptake magnitude was highly sensitive to climatic conditions, as pasture acted as a carbon sink for 3–5 months each year, depending on the amount and distribution of rainfall. The strong correlation between the enhanced vegetation index (EVI) and fluxes underlines the critical role of vegetation greenness in regulating the OWB ecosystem and validates the use of satellite‐derived EVI for monitoring ecosystem productivity and estimating regional carbon and water budgets.
Wagle et al. (2026) studied this question.