Carbon Distribution Patterns And Dynamics Of Vegetation Layers In Mixed Broad-lived Deciduous And Evergreen Forest At Different Recovery Stages In Mulinzi

Against the backdrop of global warming,forest ecosystem,as an important part of terrestrial ecosystem,is irreplaceable in mitigating the increase of atmospheric CO₂concentration through its carbon fixation and carbon storage function.In order to explore the distribution pattern,dynamics and carbon fixation of vegetation in this ecosystem,this thesis takes the subtropical evergreen and deciduous broad-leaved mixed forest in Mulinzi National Nature Reserve in southwestern Hubei Province as the research object.Three types of forests were selected at different locations,including the old-growth forest,the 25-year secondary forest after clear-cutting,and the 40-year secondary forest（calculated from the first re-examination in 2019）,and 42fixed monitoring plots with a size of 20m×20m were established for two surveys conducted in 2014 and 2019;by using the biomass model（for woody plants）and harvest method（for herbaceous plants）,the biomass of trees,subtrees,shrubs,and herbaceous plants in the vegetation layer was estimated,and the carbon density and carbon storage of each layer were calculated based on the carbon content,as well as the carbon distribution pattern of the evergreen and deciduous broad-leaved mixed forest vegetation layer,as well as the short-term and long-term dynamics and differences;it also calculates and analyses the carbon fixation amount and capacity of the vegetation layers,explores the changes in the carbon distribution pattern and carbon sequestration capacity during the restoration and succession process of this forest,and on this basis,analyzes the impact of different community structure factors on carbon distribution and carbon sequestration capacity of the vegetation layer.The research results are presented below:（1）In the sample area,232 species of woody plants were investigated,belonging to 63 families and 126 genera,including 69 evergreen plants and 163 deciduous plants;the species composition changed slightly over the past five years,with the addition or disappearance of species being rare;the dominant species in the 25-year secondary forest were Cyclobalanopsis glauca,Carpinus turczaninowii,and Weigela japonica var.sinica;the dominant species in the 40-year secondary forest were Carpinus turczaninowii,Symplocos sumuntia,and Cyclobalanopsis glauca;while the dominant species in the old-growth forest were Cyclobalanopsis glauca,Eurya alata,and Camellia cuspidata;as the restoration and succession proceed,the forest density gradually decreases,and the average diameter at the breast height gradually increases;the species diversity within the community exhibits an initial increase and subsequent decrease during the process,while it declines over a period of 5 years.（2）As the restoration and succession of the forest,the biomass of vegetation layer gradually increases in the community,but it is not consistent in the vertical distribution.In 25-year secondary forests,the proportion of subtree layer biomass is the largest,while in 40-year secondary forests and old-growth forests,the tree layer biomass dominates;the carbon density of the vegetation layers in the three forest stands is in the order of old-growth forest,40-year secondary forest,25-year secondary forest.The proportion of carbon density in tree layer gradually increases as the restoration and succession progress,while that in subtree layer and shrub layer gradually decreases,and the change in herb layer is not significant.（3）During the 5-year period,the annual growth rate of vegetation carbon density was in the order of 4.03 t·ha^-1·a^-1for 40-year secondary forest,3.31 t·ha^-1·a^-1for old-growth forest,and 2.32 t·ha^-1·a^-1for 25-year secondary forest;the annual growth rate of carbon density in vegetation layer was 9.78%for 40-year secondary forest,8.80%for 25-year secondary forest,and 6.25%for old-growth forest;among the three types,the annual carbon fixation of 40-year secondary forest was the highest,followed by old-growth forest,and 25-year secondary forest was the lowest.The tree layer makes the largest contribution to carbon fixation in the vegetation layer,followed by the subtree layer and the shrub layer,while the herb layer was the lowest.In comparison between the two time dimensions,the carbon fixation of the vegetation layer was faster in the early stage of restoration and succession,with a slower increase rate in the later stage,but the carbon density could still maintain growth.（4）Factors such as forest restoration time,proportion of large diameter individuals,diversity index,average breast height diameter,and forest density have significant effects on vegetation carbon density.The first three factors together accounted for 58.8%of the variation in carbon density in the tree layer;forest density was the main influencing factor for the carbon density variation in the subtree layer,accounting for 23.8%of the variation;forest density and average breast height diameter together accounted for 25.1%of the variation in carbon density in the shrub layer.The carbon fixation of vegetation layer in evergreen and deciduous broad-leaved mixed forest in the region is significantly influenced by the forest density and proportion of large diameter individuals,with no significant relationship with other factors,and as their proportions increase,the annual growth rate of community carbon density also increases significantly.