Effects of stand age and structure on root distribution and root water uptake in fast-growing poplar plantations

Severe soil desiccation in mature forests has been discovered in many water-stressed regions around the world, threatening sustainable forest development. Only by understanding fine root distributions and root water uptake patterns of different forest stands can we timely deal with the severe water stress in tree growth. During the 2019 growing season, we repeated isotopic and soil water content sampling for four Populus tomentosa stands in the North China Plain (two young stands with lower- or higher-competition structure, and two mature stands with lower- or higher-competition structure), and fine root sampling was performed at the end of the growing season. The hydrogen-oxygen stable isotope method and the Bayesian mixture model were used to determine root water uptake patterns. The findings revealed that stand age had no effect on fine root distributions in the 0–2 m profile. However, P. tomentosa became more reliant on the deeper soil water with stand development. The stand structure did not affect fine root distributions of young stands but significantly affected that of mature stands. Regardless of developmental stage, the higher-competition structure would increase trees' relative water uptake from the middle layers. However, this increase was at the expense of a decrease in the relative water uptake from shallow layers during the young stage and from deep layers during the mature stage. Furthermore, we discovered that the groundwater level in this area may have dropped to an extent that groundwater cannot provide sufficient water supply for P. tomentosa. This study clearly shows that fine root distribution patterns cannot be used to replace root water uptake patterns. In addition, the findings of this article will serve as a theoretical foundation for sustainable forest management in fast-growing plantations in other water-stressed areas around the world.