Dormant season vegetation phenology and eddy fluxes in native tallgrass prairies of the U.S. Southern Plains

Authors: Wagle, Pradeep; KAKANI, VIJAYA - Oklahoma State University; GOWDA, PRASANNA - US Department Of Agriculture (USDA); XIAO, XIANGMING - University Of Oklahoma; Northup, Brian; NEEL, JAMES - US Department Of Agriculture (USDA); STARKS, PATRICK - US Department Of Agriculture (USDA); STEINER, JEAN - Kansas State University; GUNTER, STACEY - US Department Of Agriculture (USDA)

Submitted to: Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/27/2022
Publication Date: 5/31/2022
Citation: Wagle, P., Kakani, V.G., Gowda, P.H., Xiao, X., Northup, B.K., Neel, J.P., Starks, P.J., Steiner, J.L., Gunter, S. 2022. Dormant season vegetation phenology and eddy fluxes in native tallgrass prairies of the U.S. Southern Plains. Remote Sensing. 14(11). Article 2620. https://doi.org/10.3390/rs14112620.
DOI: https://doi.org/10.3390/rs14112620

Interpretive Summary: Carbon dioxide (CO2) fluxes and evapotranspiration (ET) during the non-growing season can be nontrivial and they can contribute significantly to annual carbon and water budgets. Most eddy covariance (EC) studies focus only for the growing season to measure and report dynamics of CO2 fluxes and ET. Comparison of the dynamics of CO2 fluxes and ET during the non-growing season in differently managed native tallgrass prairie systems from different landscape positions under the same climatic regime are scarce. In this study, we compared dynamics of satellite-derived vegetation phenology (enhanced vegetation index, EVI and normalized difference vegetation index, NDVI) and EC-measured CO2 fluxes and ET in six native tallgrass prairie systems during non-growing seasons (November through March). For the December-February period, vegetation phenology (EVI and NDVI), CO2 fluxes, and ET were similar across all pastures, but they showed some differences during March (the time of initiation of growth of dominant warm-season grasses) due to the influence of weather conditions and management practices such as burning. Results illustrated positive impacts of the combination of prescribed spring burn and non-drought conditions, and negative impacts of prescribed spring burn during drought conditions due to reduced soil moisture. The MODIS-derived vegetation indices were not fully capable of tracking some noticeable differences in rates and budgets of eddy fluxes among pastures, most likely due to their inability to capture the presence of some cool-season C3 species under residues and litters of tallgrass prairie. Thus, documentation of change in species composition (e.g., abundance of C3 species) during winter (non-growing season) is necessary to explain discrepancies in fluxes, and satisfactorily validate satellite remote sensing vegetation indices for tallgrass prairie in winter.

Technical Abstract: Carbon dioxide (CO2) fluxes and evapotranspiration (ET) during the non-growing season can contribute significantly to annual carbon and water budgets of agroecosystems. Comparative studies of land surface phenology and dynamics of CO2 fluxes and ET during the non-growing season of native tallgrass prairies from different landscape positions under the same climatic regime are scarce. Thus, this study compared dynamics of satellite-derived land surface phenology (enhanced vegetation index, EVI and normalized difference vegetation index, NDVI) and eddy covariance measured CO2 fluxes and ET in differently managed six native tallgrass prairie pastures during non-growing seasons (November through March). During December-February, land surface phenology (EVI and NDVI) and dynamics of eddy fluxes were comparable across all pastures in most years. Larger discrepancies in fluxes were observed during March (the time of initiation of growth of dominant warm-season grasses) due to the influence of weather conditions and management practices. Results illustrated interactive effects between prescribed spring burns and rainfall on land surface phenology (i.e., positive and negative impacts of prescribed spring burns under non-drought and drought conditions, respectively). The EVI showed better potential to track phenology of tallgrass prairie during the non-growing season than did NDVI. Similar EVI and NDVI values for the periods when flux magnitudes were different among pastures and years, most likely due to their inability to fully track the presence of some cool-season C3 species under residues, necessitated a multi-level validation approach of using ground-truth observations of species composition, EC measurements, Phenocams (digital cameras), and finer resolution satellite data to further validate the land surface phenology derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) during non-growing seasons. This study provides novel insights into dynamics of land surface phenology, CO2 fluxes, and ET of tallgrass prairie during the non-growing season in the U.S. Southern Great Plains.