Divergent climatic effects on forest greenness modulated by temperature and precipitation gradients in the Yellow River Basin, China

Abstract

How climatic effects on forest greenness vary between different temperature and precipitation gradients in climate–sensitive and ecologically fragile areas, such as the Yellow River Basin (YRB) in China, is not well understood. Here we used meteorological data and remotely sensed leaf area index (LAI) data to investigate climatic effects on forest LAI changes along temperature–precipitation gradients in the YRB from 2001 to 2020. We found that changing precipitation thresholds divided forests into higher and lower LAI areas. The precipitation thresholds increased from 400 mm mean annual precipitation (MAP) in areas with a mean annual temperature (MAT) below 4 °C to 500 mm MAP in areas with a MAT above 4 °C. Forest LAI increased significantly from 2001 to 2020. The MAT and MAP thresholds changed when dividing forests into different areas with different LAI values and climatic effects. Forests greening was mainly driven by temperature in areas with LAI values between 3 and 6 m2 m−2 but was mainly driven by precipitation in areas with LAI values less than 3 m2 m−2. The threshold of MAP isopleth increased from 500 mm to 700 mm and then decreased to 500 mm, while the MAT increased from less than 0 °C to 5 °C and then increased to 12 °C. These divergent climatic effects were caused by the combined effects of water resource limitations and increasing evapotranspiration. Identifying MAT and MAP thresholds is helpful for decision–maker and managers to optimize forest restoration more accurately in future ecological protection projects facing the challenge of limited water resources in the YRB.