关键词: 水分利用策略, 氢氧稳定同位素, 黄土地区, 吸水可塑性

Abstract: Abstract Aims This study aims to investigate the water use strategies of typical plantation species in the loess area of western Shanxi under different stand compositions. The objective is to elucidate the regulatory effect of mixed planting on plant water uptake plasticity, providing a scientific basis for vegetation restoration and management in the Loess Plateau. Methods We selected four stand types: pure stands of Robinia pseudoacacia, Pinus tabuliformis, Platycladus orientalis, and a mixed stand of Robinia pseudoacacia-Pinus tabuliformis. Soil water content monitoring, stable isotope (δ¹⁸O and δD) analysis, and root distribution measurements were conducted. The MixSIAR model and Pianka’s niche overlap index were applied to quantify the seasonal variation in plant water sources and the plasticity of water uptake under different stand compositions. Important findings The results showed that: (1) Plant water use patterns exhibited clear seasonal variation. During the wet period, all species predominantly utilized shallow soil water (0-40 cm). During the dry period, water uptake depths shifted downward. Under both mixed Robinia pseudoacacia-Pinus tabuliformis forest and the corresponding pure forest conditions, Robinia pseudoacacia primarily shifted to utilizing middle and deep soil water (>40 cm), whereas Pinus tabuliformis mainly utilized water from shallow and intermediate soil layers (10-60 cm). A similar pattern was observed for Platycladus orientalis in pure stands. (2) The matching degree between plant water uptake and root biomass (M_RW), as well as that between water uptake and soil moisture (M_SW), showed dynamic changes throughout the growing season. The mean M_RW ranged from 0.86 to 0.96, while the mean M_SW ranged from 0.92 to 0.98. The higher M_SW relative to M_RW indicates that plant water uptake is more strongly regulated by soil moisture than by root biomass. (3) Based on the coefficient of variation of the proportion of water uptake from different soil layers for each species, its water uptake plasticity was characterized, the plasticity of water uptake under different afforestation patterns followed the order: mixed Robinia pseudoacacia > pure Robinia pseudoacacia > mixed Pinus tabuliformis > pure Platycladus orientalis > pure Pinus tabuliformis. The water uptake plasticity of Robinia pseudoacacia was highest in the mixed plantation, which helped reduce its dependence on the same soil water layer as Pinus tabuliformis. This study elucidates the dynamic interactions among plant water use strategies, soil moisture conditions, and root distribution under different species compositions, and provides important theoretical support for optimizing plantation structure and promoting sustainable forest management in the Loess Plateau.

Key words: water use strategy, hydrogen and oxygen stable isotopes, the loess area, water uptake plasticity