Objective To investigate the effects of mixed planting of Larix olgensis and Fraxinus mandshurica on the growth, development and defense capacity of the former, and to reveal the dynamic patterns of growth characteristics and defense strategies of L. olgensis under the mixed planting conditions, with the aim of providing scientific theoretical basis and practical references for mixed planting configuration, sustainable management and ecological shelterbelt construction of L. olgensis plantations in Northeast China.

Method Potted saplings were used to simulate mixed planting plots. Three treatments were set: 1∶1 mixed planting (HZ1) and 3∶3 mixed planting (HZ3) of L. olgensis and F. mandshurica, and monoculture control. Growth and development indices (including seedling height, ground diameter, dry weight of various organs and biomass allocation pattern), nutrient contents in leaves (amino acids, soluble sugars, proteins), physical defense indices (leaf mass per area, leaf thickness, leaf density), and chemical defense indices (antioxidant enzyme activity, secondary metabolite content) of L. olgensis were determined. The differences in growth, development, and defense-related characteristics of L. olgensis under different mixed planting patterns were systematically investigated.

Result (1) Compared with the control group, L. olgensis in mixed planting showed significantly greater tree height and dry mass of all organs (P < 0.05), but a significantly smaller ground diameter (P < 0.05). Biomass allocation was biased toward leaves and roots, reflecting the adaptive strategy of plants to prioritize competition for light and nutrients in mixed environments. Meanwhile, leaf mass per area and leaf thickness of needles were significantly higher in the mixed planting treatment (P < 0.05), indicating the enhancement of physical defense barriers. (2) The contents of amino acids, soluble sugars, and proteins in the needles of L. olgensis under mixed planting were generally higher than those in the control group, indicating that mixed planting enhanced the physiological metabolic activity and nutrient reserve levels of the plants. (3) The contents of insect-resistance secondary metabolites (tannins and lignin) in the needles of L. olgensis were higher than those in the control group, with tannin content being significantly higher at most time points (P < 0.05). In contrast, the total phenol content associated with stress resistance and antioxidant activity was significantly lower (P < 0.05), indicating enhanced chemical defense capability against insects. The activities of antioxidant enzymes (CAT, POD, and SOD) were generally lower than those in the control group, reflecting smaller environmental stress pressure in mixed forests and optimized resource allocation by the plants.

Conclusion Mixed planting of L. olgensis and F. mandshurica can synergistically enhance the growth capacity and defense capacity of L. olgensis. The plants achieved an optimized balance between growth and defense by adjusting biomass allocation and defense substance synthesis strategies. The research results verified the dual advantages of coniferous-broadleaved mixed planting patterns in enhancing plantation productivity and stress resistance from the perspective of physiological and ecological mechanisms, providing theoretical basis and practical guidance for structural optimization and sustainable management of L. olgensis plantations in Northeast forest regions.