| Fraxinus mandshurica is an important precious timber species in the northeast forest area.At present,the resources of this species are on the verge of exhaustion.Most of the natural forests of Fraxinus mandshurica are facing problems such as the decline of forest quality,poor natural regeneration and declining ecological service functions.In the past,there were relatively few studies on the effects of thinning on Fraxinus mandshurica natural forest,and there was no support data for sustainable management.So,this paper takes the Fraxinus mandshurica natural forest in Chonghe Forest Farm,Wuchang City and Zhuanshan Experimental Forest Farm,Harbin City,Heilongjiang Province as the research object to explore the effects of thinning intensity on stand growth,community structure,regeneration status and undergrowth so as to provide reference for the protection and sustainable management of Fraxinus mandshurica natural forest.Research indicates:(1)Thinning significantly promoted the growth of the average diameter at breast height(DBH)of the stand(P<0.05),and the impact on the main dominant species varied depending on the environmental conditions of the land and the thinning intensity.The weak thinning of the Chonghe forest farm significantly promoted(P<0.05)the the average DBH of the forest,but did not have a significant impact on the growth indicators of the population of Fraxinus mandshurica.The influence of severe thinning reached a significant level(P<0.05)in the overall average DBH,tree height,crown width,crown length and canopy rate of the stands,and significantly(P<0.05)promoted the growth of the main dominant species;the effect of moderate thinning was relatively small,but the overall rate of canopy shows a significant increase.(2)Thinning had little effect on the structure of forest tree species.Different thinning intensity had different effects on the diameter classes of the community and main tree species,which failed to change the decline trend of the Fraxinus mandshurica population,and the spatial pattern and interspecific relationship of tree species were affected to different degrees.The weak thinning of the Chonghe forest farm changed the community's diameter structure and height structure,which significantly(P<0.05)reduced the proportion of small-scale individuals in the community,the proportion(accounting for 60%of the total)of medium-diameter grade increased significantly(P<0.05),and the ability of community regeneration decreased.The moderate thinning in the forest farm has little effect on the diameter-level structure of the community,and the severe thinning has a greater impact.The communities are still dominated by Fraxinus mandshurica in a certain period of time.But as time goes on,the reference plots and the moderate thinned plots may eventually form a community with the main dominant species of Acer mono and Tilia mandshurica.With the increase of the intensity of thinning,the main dominant species of the stand are more inclined to change from the aggregate distribution on a partial scale to the random distribution on a larger scale.Thinning makes the negative correlation between Fraxinus mandshurica with Acer mono and Tilia mandshurica and the positive correlation between Acer mono and Tilia mandshurica weakened,and the relationship between the three tree species tends to be stable.(3)With the increase of thinning intensity,the number of speices and quantity of updated seedlings decreased,and the growth indexes of each tree species did not change consistently.There was not obvious change of height classes of seedlings,and the height classes of main tree species was incomplete.The spatial distribution pattern and spatial correlation between seedlings and aldult trees of different species were effect differently.With the increase of thinning intensity,the number of seedling speices and the density were 13,10,11 and 2592,1775,892/trees·hm-2,respectively.Among them,Fraxinus mandshurica has occupied the dominant position of the renewed layer in different treatment plots,but the number decreased by 9%and 258%respectively with the increase of thinning intensity.The growth indexes of seedlings in moderately thinned land was smaller than that in the control plot,and the effect of intensity thinning was not obvious.The height classes of the main tree species seedlings in different treatment plots are incomplete,and there is a lack of low-level(lack of the first height class of Ulmus japonica and Tilia mandshurica)or high-level(lack of the V-height class of Fraxinus mandshurica)height class.The spatial pattern of the main tree species seedlings and their spatial relationship with the mother tree have their own characteristics due to the different intensity of thinning.Thinning has a similar influence on Fraxinus mandshurica and Juglans mandshurica.Moderate thinning transforms their aggregate distribution on a larger scale into random distribution,but the two species seedlings were still aggregated on most scales after severe thinning.The spatial distribution pattern of seedlings of Acer mono,Ulmus japonica and Tilia mandshurica were significantly affected by thinning,but the difference between different thinning intensities was small.(4)Thinning reduced shrub height,coverage and diversity index,increased herb density,coverage and diversity index and richness index as a whole,as well as increased the number of shrubs and herbaceous species.Shrubby height and coverage were significantly reduced(P<0.05)after thinning.The shrubby coverage and density of moderately thinned plots were significantly greater(P<0.05)than severe thinning plots.The density and coverage of herb increased significantly(P<0.05).The difference between moderate and severe thinning is not significant.Thinning promoted the invasion of new shrub and herb species,and the number of herbaceous speices increased(compared with the reference plot,the shrubs and the herbaceous in the Chonghe Forest Farm were increased by 3 species and 9 species respectively;it was redused 2 species and 10 species of shrubs,increased 7 species and 13 species of herbage respectively in moderate thinned and severe thinned plots in the Zhangshan Forest Farm).And there was a certain change in dominant species and important values after thinning.Shrubby diversity was reduced,while herbaceous diversity and richness were increased after thinning.The change of evenness index of shrub and herb were not obvious.(5)The density and growth of renewed seedlings in the Fraxinus mandshurica natural forest were mainly promoted by shrub layer,however they were significantly inhibited by the herb layer(P<0.05).In addition to being significantly affected by shrubs and herb layers,density and height of Fraxinus mandshurica seedlings were significantly positively correlated with the density of Fraxinus mandshurica audlt trees.In summary,when the Fraxinus mandshurica natural forest was managed,a large thinning intensity could be adopted to promote the growth of forest trees,improve the composition of understory vegetation and then promot the economic and ecological benefits.At the same time,forestry workers should pay more attention to the natural regeneration of the forest and could replant major dominant species and manage understory more reasonable to create a micro-habitat suitable for seed germination and seedling growth.It is recommended to carry out long-term dynamic tracking so as to understand the response mechanism of various groups to different disturbances more deeply,and to realize the sustainable management of Fraxinus mandshurica natural forest. |
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