Flooding Tolerance and Recovery Capacity of Carya illinoinensis

Carya illinoinensis is a relatively water-tolerant species widely planted in areas with high flood risk. Evaluating its adaptation strategies and tolerance thresholds is crucial for ecological restoration in the context of climate change. In this study, five treatments were applied to 1-year-old C. illinoinensis seedlings in a potting simulation experiment: T1 (field capacity: 75%), T2 (5 cm below the root collar), T3 (up to the root collar), T4 (10 cm above the root collar), and T5 (30 cm above the root collar). The flooding episode lasted for 4 months and was followed by a recovery period of 6 months. The results show that, at the end of flooding, total biomass (TB), stem-mass ratio (SMR), malondialdehyde (MDA), soluble protein (SP), superoxide dismutase (SOD), and catalase (CAT) were significantly increased in all the flooded groups (T2–T5) compared to the control (T1), while the root-to-shoot ratio (RSR), root-to-mass ratio (RMR), leaf-to-mass ratio (LMR), and leaf-mass fraction (LMF) were significantly decreased. Although survival in the high stress group (T5) temporarily decreased to 83.3% (T1–T4 remained 100%), survival in all treatment groups fully recovered (100%) after recovery. Significant decreases (p < 0.001) were observed when comparing post-recovery to end-flooding levels within each flooded group (T2–T5), with reductions ranging: LMR (21.0–30.8%), REL (14.0–26.7%), MDA (16.1–25.3%), SP (42.2–67.3%), SOD (27.6–49.8%), and CAT (47.0–61.3%) across treatments. At this time, T5 showed lower TB and higher MDA, soluble sugars (SS), and SP compared to T1. PCA analysis indicated that the damage ranked as T5 > T4 > T3 > T2 > T1 at the end of flooding, and as T5 > T1 > T4 > T3 > T2 at the end of recovery. Therefore, it could be concluded that 1-year-old C. illinoinensis exhibits high flooding tolerance, with self-recovery thresholds below the T5 treatment, making it suitable for ecological restoration in flood-prone areas.