Understanding the relationship between biomass production and water use of Populus tomentosa trees throughout an entire short-rotation

Understanding the relationship between tree production and water use, as well as the main environmental and plant-related drivers of water use, is crucial for the development of production prediction models and reliable water management strategies under different climatic conditions. However, the relation between tree water use and biomass production has never been assessed throughout the entire rotation for poplar trees. Meanwhile, it remains poorly understood how the transpiration driving factors of poplars will change with stand age. Therefore, we investigated the relationship between transpiration (E) and aboveground biomass (ABM), as well as the main drivers of E for 2- to 5-year-old (2016–2019) Populus tomentosa trees under three water treatments. The annual increase in ABM depended on annually accumulated E and their relationship could be fitted with a logistic curve in each growing season (R² > 0.89). Throughout the whole rotation period, compared with the non-irrigated trees, full irrigation trees produced 59% more biomass with only 12% more E, while deficit irrigation trees attained 46% more biomass with 32% more E. The daily E had a strong exponential relationship with vapor pressure deficit (D) during years 3–5 of the rotation, and the asymptote of this relationship increased with tree age (1.6 kPa (2017), 2 kPa (2018), 2.5 kPa (2019)). The E was also strongly linearly correlated to solar radiation (Rₛ) for each year although with slightly weaker relationships than for D. Similar to other poplar species, P. tomentosa showed effective stomatal control on E. However, soil water content had almost no effect on E for all treatments, no matter which soil layer was considered. D and Rₛ were the major drivers of P. tomentosa transpiration during the growing season, even during drought periods, in the study region. Our findings will not only help to deepen the understanding of poplar water use characteristics and the underlying mechanisms, but also help to develop models for predicting the biomass production.