Abstract: We aimed to verify the density dependence effect in multi-year-old seedlings and explore the effects of biotic and abiotic factors on the composition and quantitative characteristics of the seedlings in a mixed coniferousbroadleaved forest in Jiaohe, Jilin Province, China. We installed 128 subplots (5 m × 5 m) in a 21.12hm² dynamic plot. With multi-year-old seedlings of tree species (height H>30 cm, diameter at breast height (DBH) <1 cm) as the research objects, we examined the species composition and quantity characteristics. Multiple linear regression was used to quantify the relationship between seedlings and the neighboring trees with DBH of >1 cm within a radius of 10 m or 20 m and the abiotic factors. The results showed that: (1) A total of 871 seedlings were recorded, belonging to 13 families, 16 genera, and 25 species, all of which were the seedlings of the main tree species in the plot. The Simpson diversity index was 0.91. The top three species according to the importance value were Acer mono,Acer mandshuricum, and Pinus koraiensis. (2) Biotic factors affecting the number of seedlings varied among species at two scales (10 and 20 m). There was a significant positive correlation between the number of seedlings of A. mandshuricum and the number of its conspecific neighboring trees at both scales. The number of seedlings of P. koraiensis was positively correlated with the sum of basal area of its conspecific trees at both scales and was significantly negatively correlated with the relative basal area of its conspecific trees at the 20 m scale. There was a significant positive correlation between the number of seedlings of A. mono and the sum of basal area of its conspecific trees at the 20 m scale and a significant negative correlation with the relative basal area of its conspecific trees at the 20 m scale, while a significant positive correlation with the number of its conspecific trees at the 10 m scale. The conspecific trees did not show a significant density dependant effect on the number of the seedlings of the three species. (3) The results of regression analysis showed that the overall abundance of the seedlings of the three species were significantly positively correlated with the sum of basal area, leaf area index, and soil pH, but negatively correlated with soil moisture. There was a great difference in the correlation between 10 factors and A. mono, A. mandshuricum andP. koraiensis. The abundance of the seedlings of the three species had different relationships with 10 factors, indicating that there was a great difference in the selectivity of different species to different habitats.

Key words: root exudate, belowground ecology, culture substrate, collection technique