Impact of variable weather on vegetation phenology and eddy fluxes in tallgrass prairie

Authors: Wagle, Pradeep; Northup, Brian; Moffet, Corey; Gunter, Stacey

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2024
Publication Date: 4/19/2024
Citation: Wagle, P., Northup, B.K., Moffet, C., Gunter, S.A. 2024. Impact of variable weather on vegetation phenology and eddy fluxes in tallgrass prairie. Rangeland Ecology and Management. 94:215-225. https://doi.org/10.1016/j.rama.2024.03.011.
DOI: https://doi.org/10.1016/j.rama.2024.03.011

Interpretive Summary: This study compared the vegetation phenology obtained through satellite remote sensing and the dynamics of carbon dioxide (CO2) fluxes, evapotranspiration (ET), and ecosystem water use efficiency (EWUE) obtained through eddy covariance (EC) in a native tallgrass prairie pasture in central Oklahoma, USA. The study period (2019-2022) experienced highly variable growing conditions. The dynamics of vegetative growth and eddy fluxes of tallgrass prairie were mainly driven by the variations in rainfall. Due to the varying amounts of rainfall, we observed substantial differences in the duration and strength of carbon sink potential during growing seasons. The LSWI followed the patterns of GPP and showed its potential to successfully track drought-impacted tallgrass prairie vegetation. The EWUE showed large variations, especially during growing seasons. The reduction in EWUE during dry years was primarily driven by larger reductions in GPP than ET. Strong quantitative relationships between eddy fluxes and vegetation indices illustrate vegetation growth is a major driver of eddy fluxes. The results also indicate that CO2 fluxes and ET can be estimated using satellite imagery for large spatial scales of the tallgrass prairie.

Technical Abstract: Tallgrass prairie is one of the major land use types in the Southern Great Plains of the United States of America (USA). We compared the vegetation phenology obtained through satellite remote sensing and the dynamics of carbon dioxide (CO2) fluxes, evapotranspiration (ET), and ecosystem water use efficiency (EWUE) obtained through eddy covariance (EC) in a native tallgrass prairie pasture in central Oklahoma, USA. The study was conducted over the period of 2019 to 2022, considering varying growing conditions. Daily peak net ecosystem CO2 exchange (NEE), gross primary production (GPP), and ET were -8.7 g C m-2, 15.1 g C m-2, and 5.9 mm, respectively. Dynamics of eddy fluxes aligned with the phenology of vegetation, indicating the accuracy and reliability of satellite-based vegetation indices in tallgrass prairie. During the non-growing season (November to March), CO2 fluxes and ET rates remained constant across the years. However, eddy fluxes exhibited diverse patterns during growing seasons. During the regrowth phase after the hay harvest, the differences in eddy fluxes were particularly substantial due to large variations in late-season rainfall. Consequently, the strength and duration of carbon sink potential during growing seasons varied substantially by year (i.e., carbon sink for six months in 2019 vs. two months in 2022). The highly variable magnitude of EWUE over the years illustrates EWUE is not a constant property of this prairie ecosystem. A greater reduction in GPP than ET during dry years led to reduced EWUE. Strong relationships between eddy fluxes and vegetation indices suggest that CO2 fluxes and ET can be estimated from satellite imagery alone for tallgrass prairie across large spatial scales. Overall, this study provides valuable insights into the carbon and water cycles of tallgrass prairie and the impacts of environmental drivers and disturbances on their function.