Abstract: [Objective] To explore the best tending measure,the thinning intensity experiment was carried out in the secondary forest of the mixed coniferous and broad-leaved forest to analyze the effect of different thinning intensities on stand growth and species diversity,and to provide a guidance for rational management of forestation activities in the region.[Method] Four permanent plots with a size of 1 hm² were established for thinning experiment in the secondary forest of the mixed coniferous and broad-leaved forest in Jiaohe Forestry Experimental Bureau of Jilin Province in July 2011.The thinning intensities were set by basal area,which were 0%(control,CK),17.24%(light thinning,LT),34.74%(medium thinning,MT),and 51.87%(heavy thinning,HT).Five sample plots (20 m×20 m) were selected from each standard plot to determine the actual thinning intensity.2 and 4 years after thinning,the DBH,tree height,basal area,stock volume,species diversity and community similarity were measured and analyzed,and the impacts of thinning intensity were studied.[Result] 4 years after thinning,the average DBH increment of the stands increased compared with the control.There was no significant difference between other treatments and the CK (P>0.05),except for the significant increase of LT treatment (P<0.05).The average tree height and growth rate of the stand were lower than that of the CK,but there was no significant difference among the treatments (P>0.05).The total basal area and stock volume of the stand were lower than those of the CK,showing that they decreased with the increase of thinning intensity.However,LT and MT treatments increased the increments of individual basal area and stock volume,and HT treatment was reversed,there were no significant difference among treatments (P>0.05).2 and 4 years after thinning,the shrub layer,the herb layer and the overall abundance increased significantly,but the HT treatment reduced the abundance of the tree layer.Compared with 2 years after thinning,the herb layer was reduced 4 years after thinning.Thinning did not change the species richness,diversity and evenness of the tree layer,but significantly increased the Shannon-wiener index and the Simpson index of shrub layer and herb layer.The MT treatment had the most obvious effect on the increase of species abundance,richness and diversity.The community similarity index was higher among treatments,all the similarities were at moderate or high level.[Conclusion] Thinning had a certain promoting effect on the growth of DBH in short-term,but it has not promoted the growth of stand height.Although thinning has not promoted the increase of total basal area and stock volume of stand,a rational thinning intensity could significantly increase the individual increment of basal area and volume,and promote the species diversity of shrub layer and herb layer.Considering the growth of forest and maintaining species diversity,the most suitable thinning intensity of near-maturing mixed coniferous and broad-leaved forests in Northeast China was 34.74% in this study.

Key words: thinning intensity, stand growth, species diversity, mixed coniferous and broad-leaved forest, short-term effects