Soil water components control plant water uptake along a subalpine elevation gradient on the Eastern Qinghai-Tibet Plateau

Soil water is crucial for maintaining forest health by providing plants with a dependable water source. However, the understanding of the impact of soil moisture on plant water uptake, particularly in conditions of excess soil water, is limited. In this study, we investigated root water uptake strategies of dominant species along an elevational gradient in the subalpine region of the eastern Qinghai-Tibet Plateau. Our results demonstrated divergent relationships between soil moisture and water uptake fractions in the shallow soil layer, where moisture generally exceeds field capacity (theta fc) during the growing season, and in the deep soil layer, showing negative and positive associations, respectively. This disparity is linked to soil water components: when plants mainly used shallow soil water, the relative proportion of plant-available water (the part of water between wilting coefficient and theta fc) in the shallow soil was smaller compared to instances where plants mainly used deep soil water. Conversely, the pattern for gravity water was the opposite. This phenomenon is attributed to precipitation replenishing shallow soil with gravity water and deep soil with plant-available water, leading to poor aeration in the shallow soil and an abundance of plant-available water in the deep soil, and plants tended to shift to deep soil water as a result. Furthermore, the decrease in the relative proportion of plant-available water in the shallow soil along the elevational gradient results in plants in the lowest-altitude broadleaf forest obtaining more shallow soil water and less deep soil water, which can also enhance our understanding of how soil moisture components influence plant water-use strategies. Additionally, no hydrological niche segregation was observed among species in this humid environment. These results can improve our comprehension of soil-plant interactions in subalpine areas.