Effect Of Thinning On The Ecological Function Of Natural Mixed Broadleaf-conifer Secondary Forest In Xiaoxing'an Mountains

As an important timber forest base,Xiaoxing'an Mountain area plays an irreplaceable ecological role.Due to the high intensity cutting and unreasonable management in the past,a large number of secondary forests and a small number of over-mature forests have been formed in the forest area.Since the implementation of the natural forest protection project,the forest resources have been effectively restored and improved,but the overall quality of forest resources is low,the ecological function is still weak.By analyzing the ecological function of the secondary forest after thinning,the results can accurately understand the process of ecological function after thinning about the natural mixed broadleaf-conifer secondary forest,and provide theoretical basis for the sustainable management of secondary forests,so as to improve the forest quality and stand productivity,and restore forest ecological functions.This study was taking the natural mixed broadleaf-conifer secondary forest in Xiaoxing' anling area as the research object,that transformed through different thinning intensities,including 10%(A),15%(B),20%(C),25%(D),30%(E),35%(F),and the unthinned plot as the contrast(CK).The soil physical and chemical properties,the litter water-holding capacity,canopy structure,photosynthesis,stand structure,species biodiversity,the growth of retained and renewed tree and other indicators have been observed in various plots after thinning for consecutive years.The spatial-temporal variation process and rule of all indexes are analyzed.Based on the survey data at the end of the period,the comprehensive evaluation model was established to evaluate the effects on ecological functions of different thinning intensities.The main contributions and conclusion are as follows:(1)The soil physical and chemical properties and the litter water-holding capacity have not recovered from the initial state of investigation in the sixth year after thinning.The comprehensive analysis showed that high thinning intensity had the destructive effect on soil physical and chemical properties,and the litter water-holding capacity was better under low thinning intensity.In the sixth year after thinning,the change process of litter water-holding capacity and immersing time conformed to the logarithmic relationship.The fitting equation of litter absorbing speed and immersing time accorded with power function.(2)The leaf area index and the ability to capture photosynthetic effective radiation were improved when the thinning intensity was 20%,and the canopy structure characteristics could be improved effectively.Transpiration rate,stomatal conductance and net photosynthetic rate were all higher in the medium thinning intensity,which promoted the tree photosynthesis.The leaf area index was negatively correlated with the gap fraction and positively correlated with the interception of photosynthetic effective radiation.(3)The diameter distribution and tree height distribution of the stand after thinning showed a unimodal type and left deviation,it is concluded that the Weibull distribution function was suitable for all diameter distribution and height distribution.After the 25%thinning intensity,the uniform angle index was optimal for the random distribution.The low thinning intensity was beneficial to improve the dominance of the forest,and the high thinning intensity promoted the mingling,the storey differentiation after thinning is generally lower than that of the control plot.According to the comprehensive goodness value,the natural mixed broadleaf-conifer secondary forest is suitable for the improvement of the forest spatial structure with 35%thinning intensity.(4)Species diversity increased in all areas after thinning.In the medium thinning intensity,the shrub layer and herb layer were the best in terms of species richness and diversity,while the growth of retained and renewed trees was good.The growth rate of regeneration trees(Pinus koraiensis,Picea asperata and Larix gmelinii)in the first year or the second year after thinning were higher in most of the plots,and then showed a trend of decrease and then increase.The seedling survival rate decreased gradually over time in the thinning plots,and the decline rate decreased year by year,and gradually became stable.(5)This study picked out the 53 indexes including the soil physical and chemical properties,litter water-holding performance,canopy structure and light environment,biodiversity stand spatial structure,Species diversity and vegetation growth and so on.The index weights were obtained by combining subjective and objective methods.A comprehensive evaluation model was established by using principal component analysis(PCA)for the effect of ecological function of secondary forests after different thinning intensities.The ecological function in the sixth year after its thinning was comprehensively evaluated,it is concluded that the ecological function of natural mixed broadleaf-conifer secondary forest is optimal when the thinning intensity is 25%.