Changes in water-use strategies and soil water status of degraded poplar plantations in water-limited areas

Poplar plantations play an active role in windbreak and sand-fixation and timber production in water-limited areas, but the large-scale plantations are experiencing degradation, characterized by short trees, small size, and dieback. Moreover, the potential impacts of plantation degradation on ecohydrological processes in soil-plant systems remain unclear. We continuously measured soil water content (SWC), hydrogen and oxygen isotopic compositions in the soil water and plant xylem water, carbon isotopic compositions in the leaf, and sap flow velocity of poplar trees under various degraded plantations (no degraded, ND; lightly degraded, LD; severely degraded, SD) during the 2021 growing season (May–September). We also investigated tree root systems at a depth of 0–200 cm. Our results showed that as plantation degradation intensified, the root weight density at different depths decreased and the root proportion of the shallow layer (0–40 cm) increased. Although the SWC of the shallow layer did not change in the degraded plantations, the SWCs at middle layer (40–80 cm) and deep layer (80–200 cm) were higher in the LD and SD plantations than in the ND plantations, which might be related to reduced transpiration of degraded plantations. The Bayesian mixing model showed that all plantations can shift the water source from shallow to deep layers in the process of soil wetting to drying. Evidence from leaf carbon isotopes suggested that degraded plantations increased the sensitivity of intrinsic water-use efficiency to SWC. Our findings demonstrate that the normal growth of poplar plantations is prone to soil desiccation of deep layers due to high transpiration demand in water-limited areas, and degraded poplar plantations alleviate deep soil water depletion due to low transpiration. For rain-fed poplar plantations, proper thinning and measures of reducing soil evaporation may be necessary to avoid water excess consumption from deep soils.