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Research Project: Understanding Ecological, Hydrological, and Erosion Processes in the Semiarid Southwest to Improve Watershed Management

Location: Southwest Watershed Research Center

Title: Interannual variability and trends of gross primary production and transpiration in savannas and grasslands from 2000 to 2021

Author
item MENG, C. - University Of Oklahoma
item XIAO, X. - University Of Oklahoma
item PAN, L. - University Of Oklahoma
item PAN, B. - University Of Oklahoma
item Scott, Russell
item Wagle, Pradeep
item ZHANG, C. - University Of Oklahoma
item YAO, YUAN - University Of Oklahoma
item QIN, YUANWEI - University Of Oklahoma

Submitted to: Frontiers of Earth Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2024
Publication Date: 3/21/2025
Citation: Meng, C., Xiao, X., Pan, L., Pan, B., Scott, R.L., Wagle, P., Zhang, C., Yao, Y., Qin, Y. 2025. Interannual variability and trends of gross primary production and transpiration in savannas and grasslands from 2000 to 2021. Frontiers of Earth Science. 19:246-260. https://doi.org/10.1007/s11707-024-1136-8.
DOI: https://doi.org/10.1007/s11707-024-1136-8

Interpretive Summary: Knowing the amount of carbon sequestered by plant photosynthesis and water from plant water use (known as transpiration) is important for understanding the earth’s water and carbon cycle. However, determining the amount of annual photosynthesis and transpiration using computer simulations has proven to be difficult. In this study, we selected five savanna and grassland sites in Arizona and California, each of them having specialized measurements of photosynthesis and transpiration, and we compared them to simulation to investigate the seasonal dynamics, fluctuations from year to year due to changing weather, and trends across two decades. After some simulation model improvements, the simulations were shown to track well the seasonal, annual and decadal patterns seen in the data indicating the potential to use the simulations for more accurate regional to global grassland photosynthesis and transpiration estimates.

Technical Abstract: Carbon and water fluxes of savannas and grasslands have large interannual variability, making them difficult to predict accurately with models. In this study, we selected five savanna and grassland sites, each of them having multi-year (7-14 years) eddy covariance (EC) data for a total of 57 site-years. We ran a long-term simulation (2000-2022) of the vegetation photosynthesis model (VPM) and vegetation transpiration model (VTM) to investigate the seasonal dynamics, interannual variability, and decadal trends of modeled gross primary production (GPPVPM) and transpiration (TVTM) at these sites. Both GPPVPM and TVTM tracked the seasonal dynamics of EC-based GPP (GPPEC, R2=0.87) and evapotranspiration (ETEC, R2=0.82) well. At the annual scale, GPPVPM tracked the interannual variability of annual GPPEC well, with the regression slopes for GPPEC versus GPPVPM-EC ranged from 0.95 to 1.23. The GPPVPM and TVTM simulations using two different meteorological forcing datasets (site measurements and model reanalysis, European Center for Medium-Range Weather Forecasts Reanalysis v5 dataset (ERA5)) were similar, suggesting that the ERA5 data can be used for regional and global VPM/VTM simulations. Using the GPP model estimates (GPPVPM) in VTM showed the potential for regional and global T estimation. From 2000 to 2022, GPPVPM and TVTM showed no significant interannual trends at one savanna and three grassland sites. However, they increased significantly at one savanna site that experienced woody plant encroachment over these years. The results demonstrate the performance of the VPM and VTM in predicting the seasonal dynamics and interannual variability of GPP and T in savannas and grasslands.