Reduced transpiration without changes in root water uptake patterns in degraded trees in semi-arid afforestation ecosystems

Abstract. While reforestation in water-limited areas has increased vegetation coverage, an increasing number of studies have reported that large-scale plantations are suffering from tree degradation, which is characterized by short trees, small size, and dieback. Moreover, the changes in hydrological processes of degraded trees within the soil-plant system remain poorly understood. Here, the stable isotope method (²H, ¹⁸O, and ¹³C) and thermal dissipation technique were used to compare the soil water dynamics, tree transpiration, root water uptake patterns, and intrinsic water-use efficiency (WUE_i) of Populus simonii under different degradation degrees (no degradation, ND; light degradation, LD; or severe degradation, SD) in the 2021 growing season. As tree degradation intensified, the root weight density decreased significantly (P<0.05) and the root proportion of the shallow layer (0–40 cm) increased. Influenced by precipitation recharge, the soil volumetric water content (SWC) of the shallow layer did not show significant differences (P>0.05) among the different degradation degrees. The SWC of the middle (40–80 cm) and deep (80–200 cm) layers were significantly (P<0.05) lower in the ND plot than the LD and SD plots. Despite the differences in SWC, the isotopic characteristics of the soil water and xylem water were similar among the ND, LD, and SD plots. Although tree transpiration in the LD and SD plots was significantly reduced (P<0.05) compared to that in the ND plot, the main depths and proportions of water uptake by the root system did not change. P. simonii trees in the ND, LD, and SD plots were able to shift the water source from shallow to deep layers in the process of soil wetting to drying. Moreover, compared to healthy trees, WUE_i of degraded trees was more sensitive to SWC. Our study shows that although degraded trees alleviate the exhaustion of deep soil water reservoirs to some extent, the sustainable development of afforestation ecosystems requires appropriate stand management measures to maintain balanced forest-water relationships.