| The karst area is characterized by a fragile ecological environment and low economic development.Uncontrolled deforestation has increased the rate of ecosystem degradation.To address this issue,it is crucial to implement measures for natural and managed restoration of vegetation,which can play a significant role in comprehensive control of rocky desertification in the Southwest Karst area.Ecological engineering has proven effective in increasing vegetation coverage and controlling rocky desertification in karst areas,utilizing both natural restoration ability and managed restoration management.In addition to increasing vegetation coverage,different vegetation restoration measures also improve the physical and chemical properties of soil.However,there is still much room for improvement in restoration strategies.This study,conducted at the Huanjiang Karst Ecosystem Observation and Research Station of the Chinese Academy of Sciences,aimed to investigate the effects of different vegetation restoration modes（managed and natural restoration）on vegetation succession and soil retention.The study also explored changes in key indicators related to the phosphorus cycle and the sequestration potential of soil carbon,nitrogen,and phosphorus under different returning farmland modes.Furthermore,the spatio-temporal evolution pattern and process of three representative forest productivity under natural restoration in the southwest karst region were examined,describing the distribution of productivity and vegetation（including community type,species composition,species structure,and diversity）along ecological environment gradients.This information could be valuable in the development of more effective restoration strategies.Utilizing quantified factors including vegetation,soil,and topography,a structural equation model was developed to analyze the evolving patterns of various restoration models over time.The aim was to provide effective adjustment strategies for rapid vegetation restoration and to improve the efficacy of ecological reconstruction efforts in the Southwest Karst region.The primary outcomes of the study are as follows:（1）Among the different modes of farmland restoration under managed restoration,the returning to forest mode had significantly higher contents of different forms of carbon,total nitrogen（TN）,and available nitrogen（AN）than the returning to grass and abandonment modes.There were differences in different forms of phosphorus between the restoration modes and the control（corn）.The returning forest and grass modes had significantly higher contents of total phosphorus（TP）and organic phosphorus（OP）than the control corn,while the inorganic phosphorus（IP）in the control corn soil was higher.Soil multi-functionality,soil carbon multi-functionality,and soil nitrogen multi-functionality were significantly higher in the forestation mode than in the grass-returning mode and the control corn.Pearson correlation analysis revealed a significant positive correlation between different forms of soil carbon,nitrogen,and phosphorus and most of the soil multi-functionality,with different forms of soil carbon being an important factor in explaining soil multi-functionality.Random forest model showed that cation exchange capacity（CEC）,low activity organic carbon（AOCL）,organic carbon（SOC）,total nitrogen（TN）,and total carbon（TC）had a very significant impact on soil multi-functionality,with soil carbon accounting for 32.56%,soil nitrogen accounting for 18.58%,soil phosphorus accounting for 27.64%,and other factors accounting for21.22%.These findings suggest that soil carbon is the most important factor in explaining soil multi-functionality,followed by soil phosphorus.（2）Over a period of 15 years from 2007 to 2022,three vegetation types were observed under the natural restoration model.The shrub forest and primary forest showed a higher number of dead individuals in the first 10 years than newly added ones,while the opposite was true for the next 5 years.The total number of individuals gradually increased in both these vegetation types,whereas it decreased in the secondary forest.Upon analyzing the importance value of species with a value of≥1 in 2007,it was found that40.91%and 47.83%of these species were present in the shrub forest and secondary forest,respectively.Additionally,the species Lin accounted for 64.71%.Spatial analysis of the three vegetation types revealed that the vegetation density decreased in the first 10 years but increased in the next 5 years,while the average diameter at breast height and average tree height showed the opposite trend.Over the past 15 years,the vegetation density of the secondary forest has decreased steadily,but the average diameter at breast height and average tree height have increased significantly faster than those of the shrub and primary forests.The primary forest exhibited the highest species diversity and structure diversity index,followed by the secondary forest,while the shrub forest had the lowest.The shrub forest had the highest functional diversity index,while the primary forest had the smallest.（3）According to the natural restoration model,the three vegetation types had varying biomass and productivity changes.Shrub forest biomass decreased by 8.31 Mg·hm-2and productivity decreased by 0.55 Mg·hm-2·a-1.In contrast,secondary forest biomass increased by 34.8 Mg·hm-2with an increase in productivity by 2.32 Mg·hm-2·a-1.Primary forest biomass increased by 42.91 Mg·hm-2,and productivity increased by 2.86Mg·hm-2·a-1.The growth rate of biomass and productivity followed the order of Primary forest>secondary forest>shrub forest.Secondary forest and primary forest growth rates increased gradually.Species diversity differed across the three vegetation types.Shrub biomass was negatively correlated with species richness,but positively correlated with the Shannon-wiener index,Simpson index,and Pielou evenness index.In contrast,secondary forest biomass was positively correlated with species diversity.However,primary forest biomass and species diversity showed no clear pattern.Structural diversity also varied among the three vegetation types.Shrub biomass was positively correlated with Pielou evenness index but negatively correlated with structural richness,Shannon-wiener index,Simpson index,and most stand density.Secondary forest biomass was positively correlated with structural richness,Shannon-wiener index,and Simpson index,but negatively correlated with Pielou evenness index and stand density.Similarly,primary forest biomass and structural diversity showed no clear pattern.Functional diversity showed different trends across the three vegetation types.The biomass and functional diversity of shrub forests gradually increased over time,while the biomass and functional diversity of secondary forests gradually decreased.Primary forest biomass and functional diversity increased first,then decreased before the trend was enhanced again.However,the productivity of the three vegetation types was mostly negatively correlated with functional diversity.（4）Principal Component Analysis（PCA）results showed that the first two axes of topography contributed 79.58%to the interpretation of secondary forest,while the first two axes of soil contributed 93.53%to the interpretation of primary forest.The first two axes of species diversity contributed 98.13%to the interpretation of shrub forest.The contribution rate of the two axes to the interpretation of primary forest was the highest at 94.92%,while the contribution rate of the first two axes to the interpretation of shrub forest was the highest at 86.71%.The relationship between biomass,ecological multi-functionality,and three variables of topography,soils and biodiversity was analyzed.The order of biomass and ecological multi-functionality was as follows:secondary forest>shrub forest>primary forest.Soil had a significant impact on biomass,and it explained a relatively large portion of the biological quantity.Soil and biodiversity had a relatively large explanation for ecological multi-functionality.Productivity followed the order:shrub forest>secondary forest>primary forest.The structural equation model showed a good fit for the relationship between topography,soil nutrients,biodiversity,biomass,productivity,and ecological multi-functionality.It explained 74%of the ecological multi-functionality of the shrub forest,55%of the ecological multi-functionality of the secondary forest,and 63%of the ecological multi-functionality of the primary forest. |
