Dynamics And Mechanism Of Biodiversity Following Mountain Closure In Deciduous Broad-leaved Forest

Subtropical forest ecosystems are important habitats for biodiversity.Ecological restoration of damaged and degraded forest ecosystems through closure measures can improve biodiversity and ecological function.However,what remains controversial is how long it takes for degraded ecosystems to recover to their original levels.The dynamic changes and impacts of biodiversity in subtropical forests following closure are not clear.Consequently,in this study,we employed old-growth forests(> 100 years following cessation of anthropogenic disturbances)as a control and selected degraded subtropical broadleaved forests with 0-1,5-6,11-12,20-24,and 28-34 years since restoration(via mountain closure)in Eastern China.We used ANOVA to test the differences in resources(e.g.,light availability and heterogeneity,substrate diversity,soil properties),plant diversity,and soil microbial diversity in the different years since restoration.Non-metric multidimensional scale analysis was used to test the community composition of plants and soil microorganisms in different years since restoration.Regression and redundancy analyses were used to reveal the coevolution between resources,plants,and soil microorganisms.A structural equation model was employed to explore several regulatory pathways and mechanisms of biodiversity maintenance.The main conclusions were as follows:(1)Plant and soil microbial diversity show distinct recovery Patterns during restorationPlant diversity varied significantly between different years since restoration.The diversity of overstorey layer recovered rapidly at the initial stage of closure,and reached the peak at 5-6 years and 20-24 years,respectively.The shrub and herb cover were the highest in year 0 and decreased in subsequent years since restoration.The shrub richness was the highest and the herb evenness was the lowest in the 28-34 year since closure group.The results show that the dynamics of plant diversity is the result of the change of resources,vegetation recovered significantly in a certain enclosure period,but the long enclosure period was not conducive to vegetation restoration.Soil microbial and carbon metabolism function diversity were affected by the time since restoration.The diversity of bacteria and fungi exhibited different change patterns.The Observed OTUs,Shannon index,Simpson index,and Chao1 index of bacteria increased with the restoration duration.However,the observed otu,Shannon index and Chao1 index of fungi showed a "Hump" change with increased restoration times,which initially increased at the beginning of enclosure,attained a peak at 5-6 years and 20-24 years,and then gradually decreased.The functional diversity of microbial carbon metabolism increased with longer restoration times.(2)Community varied significantly in different years since restorationThe species composition of vegetation varied significantly between different years since restoration.There were significant differences in the species composition and proportion between the overstorey layer,shrub layer,and herb layer in different year since closure groups.In the early stage of closure,the indicator species were sun-loving plants,which transformed to shade tolerant and low nutrient requiring species in 28-34 years.Dominant tree species changed from Liquidambar formosana(IV = 0.55)at the early stage of closure to Cornus kousa subsp.Chinensis(IV = 0.24)and Liquidambar formosana(IV = 0.35)at the late stage of closure.The soil community composition varied variably between different years since restoration.The composition of soil bacterial communities in the old-growth forest was significantly different from that in the 0-1,5-6,11-12 and 20-24 year groups,but was similar to that of the 28-34 year group.The species composition of soil fungi in the 0-1year since closure group was significantly different from that of the 5-6 year,11-12 year,and 20-24 year groups,but was similar to that of the 28-34 year group.There were some indicator species of bacteria and fungi that emerged at different times since restoration.(3)Resource availability and heterogeneity showed distinct recovery patterns during restorationLight resources,substrate diversity,and soil properties varied significantly between different years since restoration.In 0-10 cm,the soil carbon,nitrogen,microbial biomass carbon,microbial biomass nitrogen,carbon:nitrogen ratio,organic matter and water content increased;the soil pH,bulk density,and soil phosphorus content decreased;the microbial carbon nitrogen ratio,available nitrogen,and available phosphorus content initially increased and then decreased;the soil substrate diversity initially decreased and then increased;and the light availability and heterogeneity exhibited a decreasing trend overall.(4)Key factors driving the changes of plant and microbial community structures during restorationSoil carbon and nitrogen are important driving factors of vegetation community composition.Soil properties,plant diversity,and community compositions are important factors that drive microbial communities.Tree basal area at breast height,shrub cover,herb cover,tree species composition,soil carbon,nitrogen,microbial biomass carbon,microbial biomass nitrogen,organic matter,carbon nitrogen ratio,soil microbial biomass carbon nitrogen ratio,soil water content,bulk density,and pH were the driving factors that affected species composition changes at the soil bacteria phylum level.Tree richness,evenness,soil substrate diversity,soil available phosphorus and nitrogen were the driving factors at the phylum level.(5)Co-evolution and coupling mechanisms of plant-soil-microorganismsPlants,soil microbl and environmental resources interact with each other.Overstorey layer diversity is an important factor driving understory vegetation and light resources.Soil properties and substrate diversity were affected by tree basal area at breast height,understorey vegetation cover,plant diversity,and the composition of dominant tree species.Substrate diversity is influenced by species diversity and the composition of tree species in the overstorey layer.The shrub cover,richness,evenness,and herb cover were significantly affected by soil properties,while the shrub evenness was related only to soil bulk density and available potassium,and herb evenness was linearly related only to substrate diversity.Soil microbial diversity was significantly affected by the diversity of woody plants and soil matrices.The Shannon index of bacteria increased with higher shrub richness and overstorey evenness.The Shannon index of fungi increased with higher overstorey richness and evenness,which initially increased and then decreased with higher shrub richness.The fungi Chao1 index increased with greater overstorey richness.The AWCD of soil microbial carbon source metabolic activity was significantly affected by overstorey richness and overstorey evenness.The key factors that impacted microbial metabolism of 31 carbon sources were bacterial diversity,fungal diversity,overstorey richness,overstorey evenness,overstorey Shannon index,and shrub cover.(6)Regulation path of biodiversity maintenanceThe diversity of upper plants not only directly affects the diversity of herbaceous plants,but also indirectly affects the diversity of herbaceous plants through light availability and matrix diversity.The effect of trees on soil microbial diversity is low,but it will have an indirect impact through the environment.Overstorey layer species diversity,light availability,and substrate diversity could explain 36.3% of the factors affecting herb evenness.The soil C/N ratio,basal area of tree breast height,and shrub coverage could explain 74.8% of the factors driving the changes in bacterial diversity,while the soil pH,overstorey layer species diversity,and soil available nitrogen content could explain 63.2% of the factors driving changes in fungal diversity.To summarize,the ecological restoration measures of mountain closure affect biodiversity.Resource availability and heterogeneity are the main limiting factors of biodiversity,and there are complex coupling relationships between resource plant microorganisms.The diversity of upper plants not only directly affects the diversity of herbaceous plants,but also indirectly affects the diversity of herbaceous plants through light availability and matrix diversity.The effect of trees on soil microbial diversity is low,but it will have an indirect impact through the environment.