Increasing Trend in Ecosystem-Scale Photosynthetic Efficiency in the Yellow River Basin Since 2000 Caused by Afforestation and Climate Change

We used the leaf area index (LAI) and solar-induced chlorophyll fluorescence (SIF) to represent vegetation greenness and photosynthetic capacity, respectively, and the ratio of SIF to LAI (SIF/LAI) as an indicator of ecosystem-scale photosynthetic efficiency. We analyzed the spatial-temporal dynamics of SIF/LAI and its driving factors in the Yellow River Basin (YRB) of China from 2000 to 2021. The annual increase rate was 1.89% for SIF and 1.21% for LAI. The greater increase in SIF relative to LAI led to a significant increase in SIF/LAI ratio from 2000 to 2021, with an annual increase of 0.63%. This suggests an enhanced photosynthetic capacity per unit LAI over time. Forest cover explained 48% of the SIF/LAI rising trend, followed by temperature (26%) and the standardized precipitation-evapotranspiration index (SPEI, 16%). Regionally, the SIF/LAI had annual increasing rates of 1.25% and 1.10% in the upper and middle reaches of the YRB, respectively. Meanwhile, the SIF/LAI of the source region and lower reaches did not show a significant trend. This study deepens the understanding of the relationship between vegetation greening and photosynthetic capacity, which has implications for ecosystem management.