Age-Dependent Water-Use Strategies in Amygdalus Pedunculata Pall. Plantations: Implications for Sustainable Afforestation in Desert Ecosystems

Amygdalus pedunculata Pall. (A. pedunculata) plantations play a critical role in combating desertification and mitigating soil erosion in arid and semi-arid regions. However, the ontogenetic shifts in water-use strategies of A. pedunculata and the influence of soil water content (SWC) and root distribution on water uptake remain poorly characterised, limiting targeted management strategies. Here, we integrated stable isotope (δ²H and δ¹⁸O) weighted by SWC and root weight density with the MixSIAR model to quantify seasonal water sources for 6-, 10- and 18-yr-old A. pedunculata in artificial sand-fixing plantations during three consecutive growing seasons (2019–2021). Results revealed distinct ontogenetic and seasonal shifts: 6-yr-old A. pedunculata relied predominantly on shallow soil water (0–90 cm; 58.90% ± 1.60%) year-round. In contrast, 10- and 18-year-old trees extracted water primarily from deeper soil layers (90–350 cm and 60–200 cm, respectively) during dry seasons, contributing 55.85% ± 6.03% and 57.11% ± 1.07% of total uptake; during rainy seasons, however, the main water sources for older trees were the shallower layers (0–250 cm for 10-year-old: 70.85% ± 6.86%; 0–120 cm for 18-year-old: 64.50% ± 8.91%). The proportion of groundwater contribution to older trees increased significantly during the dry season compared to the rainy season (p < 0.05). The variation in water use patterns of A. pedunculata was associated with vertical root distributions, plant water requirements and seasonal variability in soil water availability. Notably, the weighted contribution demonstrated that unweighted approaches underestimated contributions from moist soil layers and groundwater by 6.23% ± 0.56% (dry season) and 3.45% ± 0.92% (rainy season), highlighting the necessity of incorporating SWC dynamics and root distribution for accurate water source apportionment. These findings underscore the age-related differences in water uptake patterns of A. pedunculata and emphasise the need for age-specific afforestation strategies to optimise water resource allocation and ensure long-term sustainability of vegetation restoration in water-limited desert ecosystems.