High-light gaps and carbon allocation regulate regeneration of understory Fraxinus mandshurica seedlings.

Against the backdrop of climate warming, understanding the regeneration mechanisms of seedlings under forest canopies is crucial for the sustainable development of temperate forests. Broad-leaved Korean pine forests serve as the zonally climax vegetation in the Changbai Mountains, however, the causes of regeneration barriers for their primary tree species remain unclear. In particular, while shade-tolerant Fraxinus mandshurica seedlings struggle to regenerate in primary forests, they regenerate well in secondary poplar-birch forests. Nevertheless, the underlying mechanism responsible for this phenomenon remains unclear. This study employed a combined approach of understory light environment monitoring and controlled nursery experiments to observe seedling survival rates, growth characteristics, seasonal dynamics of nonstructural carbohydrate (NSC)., and carbon assimilation over two consecutive years. The results indicated that F. mandshurica seedlings in secondary forests adopt a strategy of investing more in specific leaf area and above ground biomass during summer, allowing them to survive longer during the canopy closure period. High light availability in early spring and late autumn and the concomitant positive carbon gain drive carbon accumulation in secondary forest seedlings, with primary forest seedlings attaining lower annual carbon gain. Throughout the growing season, NSC concentration in seedlings showed a trend of decreasing in summer and increasing in autumn, with most NSC being stored in the root system before dormancy. Nursery control experiments confirmed that simulating the light environment of secondary forests can improve seedling growth, survival rate, and NSC pools. In summary, this study demonstrates that F. mandshurica seedlings in secondary forests realize net carbon accumulation through phenological escape under high light in early spring and late autumn. During summer canopy closure, they maintain positive carbon gain by increasing aboveground biomass allocation and specific leaf area, and accumulate abundant NSC in perennial branches in autumn for overwintering.