Water use pattern and transpiration of Mongolian pine plantations in relation to stand age on northern Loess Plateau of China

Understanding the water use pattern and transpiration rate of revegetation species is crucial for efficient water management and sustainable vegetation restoration. Mongolian pine (P. sylvestris) plantation has been implemented for decades in desert regions of northwest China, achieving significant improvements in combating soil erosion and desertification. However, the characteristics of water use pattern and transpiration of P. sylvestris at different ages remain poorly understood. We investigated seasonal variations in water uptake pattern and transpiration of different-aged P. sylvestris (4-, 10-, and 18-yr) based on stable isotope and sap flow measurements over two growing seasons. The canopy transpiration of P. sylvestris and its discrepancy between the dry and rainy season increased with increasing stand age. The 4-yr P. sylvestris extracted water mainly from shallow soil layer (0–120 cm) across the growing season (63.21–69.40%). In contrast, the older P. sylvestris (10- and 18-yr) displayed a greater degree of ecological plasticity as they shifted water uptake to deeper soils and groundwater in times of drought. In the rainy season, 10-, and 18-yr P. sylvestris extracted water predominantly from shallow soil layer (68.62%) and shallow plus middle soil layers (0–200 cm) (66.04%), respectively; in the dry season, however, they both shifted main water sources to the middle plus deep soil layers (120–300 cm) (61.29% and 63.45%). The older P. sylvestris also increased absorption of groundwater in the dry season, leading to more severe soil desiccation and faster groundwater decline. The changes in water use patterns of P. sylvestris were related to differences in root distribution and biomass, seasonal variations of plant transpiration, and changes in soil water condition. As a consequence, the variations and controls of age-dependent plant water uptake and transpiration should be considered to implement the corresponding management measures in future reforestation activities to achieve sustainable development goals.