| This paper takes Eucalyptus urophylla × E.grandis DH32-29 Clonal pure forest in Qipo Forest Farm of Guangxi Zhuang Autonomous Region as the research object,and the process of forming mixed forest of different ages by interplanting high-quality broad-leaved trees after thinning of near mature Eucalyptus was studied.The main contents of the study are stand growth,growth dynamics,stand structure,soil physical and chemical properties,soil enzyme activities.In order to provide an important theoretical basis for Eucalyptus pure forest cultivation.1.Growth of eucalyptus in the modified stand:the diameter order distribution of eucalyptus in the modified stand did not gradually deviate to normal distribution with the increase of thinning intensity.The DBH,tree height and volume per plant increased with the increase of thinning intensity,and were more obvious than that of unmodified stand.The decreasing trend of annual growth of eucalyptus in the modified stand with greater thinning intensity was obviously slower than that in the unmodified stand.The score of each stand membership function is as follows:HT>MT>HT*>MT*>LT>LT*>CK>CK*,the best HT stand after cultivation,and the effect of cultivation was most obvious in Eucalyptus plantation with moderate thining.Compared with MT*,DBH,tree height,volume per plant and total stock volume were increased by 11.70%,14.33%,26.39%and 24.56%,respectively.2.For the growth of native tree species after stand transformation,the growth status and stand structure of Mytilaria laosensis were the best under all treatments.With the increase of thinning intensity,the volume per plant of Mytilaria laosensis was higher than that of Castanopsis hystrix:742.86%,625.64%,262.38%,39.15%,respectively,than that of Michelia macclurei:661.29%,387.93%,302.20%,400.95%;The growth changes of Castanopsis hystrix were the most obvious under different thinning intensity.High thinning intensity of eucalyptus was the most beneficial to the growth of Castanopsis hystrix.The average DBH of Castanopsis hystrix in HT was increased by 194.70%and the average tree height was increased by 117.82%compared with CK.There was no significant difference in skewness among all treatments,but the Michelia macclurei was significantly higher than that of Castanopsis hystrix and Mytilaria laosensis.In general,the change of culture mode with smaller eucalyptus density was more conducive to the growth of interplanting tree species,and the total stock of three species in HT stand with the best performance increased by 267.67%compared with that in CK stand with the worst performance.3.Soil physical properties after stand transformation:saturated water holding capacity,capillary water holding capacity,field water holding capacity,total porosity and water content in each soil layer first increased and then decreased with the increase of thinning intensity of eucalyptus in the stand,while soil bulk density first decreased and then increased.As a whole,CK forest after cultivation was the best,and the effect of cultivation was the most obvious in the deep soil(40～60cm)of the control.From the perspective of soil water holding capacity,soil saturated water holding capacity increased by 142.84%,soil capillary water holding capacity increased by 158.21%,and soil field water holding capacity increased by 154.38%.4.Soil chemical properties after stand thinning:the soil pH value decreased with the increase of soil depth,contrary to the unthinning stand,the soil pH value increased with the increase of thinning intensity in the same soil layer.Compared with the unthinning stand,the increase of pH value was most obvious in the surface soil(0～20cm),which increased by 2.60%,3.10%,6.35%and 7.05%,respectively.The organic matter in the whole soil layer increased first and then decreased with the increase of thinning intensity.The total K in the whole soil layer was superior to that in the control treatment.Soil total N,total P,total K,available N,available P and available K showed higher thinning intensity than lower thinning intensity.Overall,the MT of the modified stand was the best,and the effect of the control stand was the most obvious.Organic matter,soil total nitrogen,total phosphorus,total potassium,available nitrogen,available phosphorus and available potassium in the surface soil of the control stand increased by 30.77%,33.87%,70.57%,97.47%,26.51%and 40.84%,respectively,compared with that of the unmodified stand.5.Soil enzyme activities after stand transformation:the activities of four kinds of soil enzymes decreased with the increase of soil depth.There were some differences in the activity expression of different enzymes in different treatments.The MT stand had the best performance,and the medium thinning had the most obvious effect.Soil phosphatase increased by 11.16%,93.43%and-7.71%,respectively,compared with the unmodified stand with the increase of soil depth.Soil sucrase increased by 114.67%,196.20%and-34.14%,respectively.Soil urease increased by 8.89%,8.81%and-3.16%,respectively.Soil catalase increased by 246.04%,183.81%and 258.31%,respectively.6.Through correlation analysis,it was found that the modified stand had higher correlation differentiation degree than the unmodified stand,and the stand structure was more complex,but the analysis of the interaction between stand management and index was more complex than the unmodified stand.The comprehensive score of MT stand was the highest after principal component analysis of each index,which indicated that the quality of MT stand was the best.Therefore,if the forest manager needed to change the pure eucalyptus forest in the later stage,the density of eucalyptus preserved by thinning should be classified as the standard of MT forest.According to the difference of principal component scores of modified and unmodified stands under different thinning intensities,the comprehensive score of modified and unmodified stands under weak thinning increased by 1.224 compared with that of unmodified stands,and the effect of modified and unmodified stands was the most obvious.The results indicated that the best benefit would be to choose the stand with weak thinning under the existing four thinning intensities. |
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