Links of apple tree water uptake strategies with precipitation and soil water dynamics in the deep loess deposits

Understanding how trees absorb water is important for the sustainable management of water resources and vegetation restoration. However, the water uptake strategy of apple trees and its link with precipitation and soil water dynamics (i.e., contents, recharge proportion and residence time) remains poorly understood. In this study, we determined the source water of apple trees, the recharge proportion and mean residence time (MRT) of soil water through collecting precipitation, soil water and xylem water samples with a high frequency. Results indicated that apple trees mainly absorbed water from shallow soil layers of 0-2 m (>64%) in the early growing season (May to July), but deep soil water below 2 m (similar to 60%) in the late growing season (August to October). As such, apple trees had seasonal variations in water uptake strategies by shifting water sources from shallow to deep soils. In addition, the proportions of water uptake from the same soil layer varied with tree ages. The proportions of water uptake from different soil layers were not significantly correlated to the corresponding soil water contents (p > 0.05), which may be attributed to the long residence time of soil water (14-99 days). Therefore, the water uptake strategies of apple trees were likely to be influenced by precipitation and MRT of soil water, as well as stand ages. The indicated relationship between water uptake strategies and precipitation and soil water dynamics can improve insights into the interaction between soil and vegetation.