Early physiological response of Pinus yunnanensis Franch. seedlings to simulated karst rocky desertification stress

Rocky desertification in the karst regions of southwestern China is characterized by shallow soils, nutrient depletion, and ionic imbalance, which severely restrict vegetation recovery and ecological restoration. Pinus yunnanensis Franch., a native conifer adapted to nutrient-poor karst habitats, has been widely used for ecological restoration. However, its early physiological responses to rocky desertification stress remain poorly understood, particularly the coordination among growth, carbon metabolism, nutrient dynamics, and ion homeostasis. In this study, one-year-old P. yunnanensis seedlings were grown for 60 days in soils collected from Yunnan Province, China, representing a non-karst rocky desertification control (CK) and three rocky desertification levels: light (LRD), moderate (MRD), and severe (SRD). Growth, non-structural carbohydrates (NSC), nutrient stoichiometry, and ion profiles were evaluated. The results indicated that P. yunnanensis responds to rocky desertification stress via coordinated physiological adjustments. Seedling growth was maintained under CK and LRD but was progressively suppressed under MRD and SRD. NSC analysis showed that LRD promoted soluble sugar accumulation and increased the sugar-to-starch ratio, which may enhance metabolic flexibility and stress tolerance. Organ-specific N: P ratios were 2.42 in roots, 0.73 in stems, and 1.04 in needles, with an overall mean of 1.40, indicating nitrogen limitation and pronounced organ-specific nutrient allocation. Under SRD, K⁺ content declined significantly, whereas Mg²⁺ and Ca²⁺ accumulated in stems and needles, suggesting dynamic ion regulation under severe stress. Na⁺ content was markedly reduced, indicating restricted ion uptake and transport under degraded substrate conditions. Overall, P. yunnanensis exhibited ecological adaptability under LRD, while MRD and SRD increasingly restricted growth and metabolic coordination. These findings highlight the integrative role of the carbon-nutrient-ion network in early stress adaptation and provide a physiological basis for karst ecosystem restoration.