Effects of understory removal and thinning on water uptake patterns in Pinus massoniana Lamb. plantations: Evidence from stable isotope analysis

Understanding the dynamic relationships between water uptake patterns and forest management in plantations is important for predicting the impacts of climate change and tailoring forest restoration to mitigate potential impacts. However, the water uptake patterns of overstory trees and associated drivers in plantations with different management practices are poorly understood. In this study, analyses of stable isotopes (δ2H and δ18O), in addition to a MixSIAR model, were applied to assess the water uptake patterns of Pinus massoniana Lamb. under four forest management measures (no thinning: NTN; understory removal: USR; light-intensity thinning: LIT; and heavy-intensity thinning: HIT) under three rainfall events (8.9 mm, 13.3 mm and 67.7 mm) in the Three Gorges Reservoir Area in China. A random forest model and variation partitioning analysis were further used to identify the effects of tree and soil properties on the water uptake patterns of overstory trees. The results indicated that P. massoniana in the NTN and USR stands mainly derived water from deep soil, whereas P. massoniana in the HIT stand absorbed more water from surface soil. P. massoniana in the LIT stand consistently used deep soil water after 8.9 mm of rainfall, and preferred to use shallow soil water after 67.7 mm of rainfall. This result suggested that P. massoniana in the LIT stand has a more competitive advantage over that in the HIT stand through consistent uptake of deep soil water following light rainfall; however, this strategy is more conducive to avoid excessive consumption of deep soil water than the strategies used in the NTN and USR stands as it allows the extraction of shallow soil water following heavy rainfall. The different water uptake patterns of P. massoniana among the four treatments were mainly attributed to differences in tree properties, including leaf biomass, fine-root biomass density, and leaf water potential. Overall, light-intensity thinning may be a suitable forest management practice to optimize the water use strategies of P. massoniana to cope with the changes in precipitation pattern.