Abstract: Aims Plant functional traits are stable and measurable morphological and physiological characteristics formed by plants through long-term natural selection and evolution. Forest fires, as an important influencing factor in forest ecosystems, have a significant impact on plant functional traits. Plants adapt to changes in the external environment by altering their functional traits and forming specific survival strategies. Therefore, clarifying the response mechanism of plant leaf and petiole traits to fire is of great significance for revealing the adaptation strategies of plants after fire. Methods In this study, Betula platyphylla, a pioneer tree species in Daxing'an Mountains, was taken as the research object. The forest areas that have experienced severe fires in the past four years were selected as the burned areas, while the adjacent forest stands with consistent environmental background conditions and no fire disturbance were chosen as the control areas, that is, the unburned areas. To analyze the changes in leaf traits, petiole traits, leaf traits and the coupling patterns between petiole and leaf traits of B. platyphylla after fire, and to explore the influence of soil properties on the coupling patterns among traits. Important findings The results showed that: (1) After fire, leaf area and specific leaf area both significantly decreased, while leaf dry matter content and leaf tissue density both significantly increased; meanwhile, both the petiole length and the petiole length were significantly reduced, while there were no significant changes in the dry weight of petioles and the proportion of petiole biomass allocation. (2) After fire, the negative coupling pattern between specific leaf area and leaf dry matter content weakened, while the positive coupling pattern between leaf dry matter content and leaf tissue density strengthened, indicating that fire enhanced the structural defense of plants and the plant resource utilization strategy shifted towards a conservative type; after fire, specific leaf area and leaf tissue density showed an isokinetic growth relationship, reflecting a fixed trade-off between resource acquisition and defense construction in plant leaves. here was no significant difference in the coupling patterns of petiole length-leaf area, petiole dry weight-leaf area and specific petiole length-leaf area before and after fire. However, the interceptions of the three coupling patterns after fire were all lower than those before fire, indicating that after fire, plants reduced biomass input to photosynthetic organs and enhanced support and defense structures. (3) Fire increased the regulatory effect of soil properties on the coupling pattern between petiole and leaf traits, transforming it from being driven solely by total soil phosphorus before fire to being jointly influenced by multiple soil properties such as total soil phosphorus, organic carbon, and soil water content after fire. The results of this study show that after fire, B. platyphylla achieves a "conservative" survival strategy by forming small and thick leaves and short and thick petioles, revealing the growth adaptation mechanism of plants in the early stage of vegetation recovery in the burned area.

Key words: severe fire, trait relationship, allometric growth strategy, soil properties, Daxing'an Mountains