Abstract: To explore tree species composition and diversity of secondary mixed forests of Juglans mandshurica in Northeast China, a total of 123 plots with a size of 0.1 hm² each were set up in 23 typical distribution sites in the central and eastern regions of Heilongjiang, Jilin and Liaoning provinces. We investigated species composition, flora characteristics, importance values, richness and diversity of tree species (DBH≥5 cm). A total of 48 tree species were recorded, belonging to 17 families and 31 genera. 30 species were recorded in Heilongjiang Province, belonging to 14 families and 21 genera. 37 species were recorded in Jilin Province, belonging to 14 families and 25 genera. 35 species were recorded in Liaoning Province, belonging to 16 families and 25 genera. Tree species in these plots  mainly belonged to the flora of North Temperate Zone type. Tree species richness showed a hump pattern with increasing latitude and elevation across different provinces. Tree species richness was most abundant between 300-700 m elevation. The common tree species with the top six importance values wereJ. mandshurica, Ulmus pumila, Acer mono, Quercus mongolica, and Fraxinus mandshurica in the plots among the three provinces. The non-common tree species were Pinus koraiensis (Heilongjiang), Tilia amurensis (Jilin), and Fraxinus rhynchophylla (Liaoning). The diversity indices in the plots were high. Shannon diversity index in Jilin was significantly higher than in Heilongjiang and Liaoning (P<0.05). There was no significant difference in Simpson diversity index and Pielou evenness index among the three provinces. Tree communities shared moderate similarity among the three provinces. Our results can provide a basis for better understanding of species diversity and for forest management of the secondary mixed forests of J. mandshurica in the central and eastern regions in Northeast China.

Key words: Juglans mandshurica, secondary mixed forest, species composition, species diversity.