| Eucalyptus has been planted on a large scale in the south of China for its fast-growing qualities.While it has filled the gap in the timber market and brought huge economic benefits,there are also a number of problems such as the decline of land strength when planted blindly.Previous studies have shown that site conditions have a significant impact on the growth of Eucalypts.To improve the economic benefits of eucalyptus plantations while maintaining land strength,it is important to determine whether the site conditions are suitable for planting Eucalypts and to plan the planting of eucalyptus plantations in a rational manner.In this study,108 plantations in three regions of Guangxi’s main eucalyptus producing areas were continuously operated for more than 20 years,and the effects of elevation,slope and slope direction on the soil fertility quality of eucalyptus plantations were analyzed through the evaluation of soil fertility quality by principal component analysis and the evaluation of stand quality by adding stand factors.The study also investigated the differences in the growth of eucalyptus plantations between different sites,and explored the suitable site conditions for the growth of eucalyptus plantations,providing a strong scientific basis for the planning of new plantations and a scientific guide for the rational management of existing eucalyptus plantations.The following conclusions were drawn from the study.(1)Based on the 10 standing factor indicators,we found that precipitation and elevation were the most significant standing factor indicators affecting eucalyptus plantations,followed by slope and slope orientation.The results of the stand quality evaluation were analyzed in a linear fit model with the average accumulation,and the accuracy of the fall point was 87.48%.The soil fertility quality of eucalyptus plantations decreased significantly with increasing altitude,especially when the altitude was above 600 m.The soil fertility quality of eucalyptus plantations and the accumulation of trees in the sample plots decreased significantly(p<0.05).Soil fertility quality and stand accumulation were best in the 15-30° sample plots,and slightly lower in the 0-15° slope,but both were significantly better than the above 30° sample plots(p<0.05).(2)The results of the principal component analysis of the 30 physical and chemical indicators show that the most significant indicators affecting soil fertility quality in the minimum data set are:bulk weight,total nitrogen content,effective copper,organic carbon content,meal content and fast-acting potassium content,with weights of 0.198,0.190,0.189,0.176,0.145 and 0.106 respectively.This indicates that the minimum data set can be used as a substitute for the full data set to evaluate soil fertility quality.(3)On the basis of the minimum data set for soil fertility quality evaluation,10 indicators with the most significant influence on the evaluation of stand quality were selected through correlation analysis:altitude,precipitation,bulkiness,powder content,organic carbon,total nitrogen,fast-acting potassium,effective copper,catalase activity and microbial biomass nitrogen,and these 10 indicators formed the minimum data set for stand quality evaluation.The results of the minimum data set of indicators are closer to those of the full data set than those of the soil fertility quality evaluation,and they are better fitted to the woodland stock.(4)The accuracy of the results of the stand quality evaluation combined with soil fertility quality was better than that of the soil fertility quality evaluation alone in all regions,and the coefficients of determination of the linear fit model(R2=0.8598,0.8422,0.8402)of the combined stand quality evaluation with the results of the full data set evaluation were significantly higher than those of the linear fit model(R2=0.7217,0.8347,0.7139)of the soil fertility quality evaluation with the results of the full data set evaluation.0.7217,0.8347,0.7139)and reduced the errors with the full data set(Bias=0.1382,0.0048,0.1263),indicating that the combination of soil fertility quality evaluation and stand quality evaluation is a better fit and can more accurately predict the average accumulation of Eucalyptus plantation forests in Guangxi.(5)Woodland accumulation and stand quality showed a significant positive correlation(p<0.05),but the correlation was not linear.The correlation between stand quality and woodland storage varied among the different stand types,but the overall trend was positive,with the highest coefficient of determination for the linear correlation between stand quality and woodland storage in the East region of Guangxi(R2=0.8893)and a slightly lower coefficient of determination for the linear correlation between stand quality and woodland storage in the South and Central regions of Guangxi(R2=0.8675)than in the the East region of Guangxi,but storage was significantly correlated with stand quality in all three regions.in all three regions was significantly correlated with stand quality. |
