Changes in Water and Carbon Fluxes in the USA Southern Great Plains Grassland Due to Evergreen Forest Encroachment

The southern Great Plains (SGP) grassland of the United States has been largely encroached by evergreen forest in recent decades. The response of the grassland water and carbon cycles to the encroachment is not clear yet. Given so, this study quantified the changes in gross primary production (GPP), evapotranspiration (ET), and ecosystem water use efficiency (EWUE) between grassland pixels with evergreen forest encroachment and neighboring pure grassland pixels (500 m). We also assessed the spatial variation of the changes in relation to precipitation (mm), air temperature (°C), and evergreen forest coverage (%). These analyses were repeated, respectively for the years of 2004, 2008, 2016, and 2019 as robustness check. Results suggest that across the 4 years, 62–72% of encroached grassland pixels exhibit higher annual GPP, 67–74% exhibit higher annual ET, whereas 65–71% exhibit lower annual EWUE. The change ratio of ET is positively correlated with that of GPP but negatively correlated with that of EWUE. Additionally, the spatial variation in the change ratios of annual GPP and ET can be explained to a certain degree by the encroachment amount. These results further clarify the response of water and carbon cycles to evergreen forest encroachment in the SGP grassland.