| In-depth study of the dynamics of vegetation composition and structure is one of the hot spots of ecological research.The evergreen and deciduous broad-leaved mixed forest is one of the most abundant and complex vegetation types in the subtropical region of China,which is highly susceptible to climate change.Study on its vegetation composition and structural dynamics is significant.This study used a 1 hm2 sample plot in the evergreen and deciduous broad-leaved mixed forest in the Tianmu Mountain.Based on data obtained from three surveys(1996、2012,and 2017),the characteristics of the species composition,community structure,and dominant population structure in the study area over the past 21 years were analyzed.At the same time,the historical meteorological data of the meteorological stations in the study area were selected based.The trend of climate change in the region in the past 70 years was analyzed to discuss the dynamic changes in vegetation in the evergreen and deciduous broad-leaved mixed forest.The results showed the following:(1)In 1996,40 species with DBH of ≥10 cm were recorded belonging to 34 genera and 24 families.In 2017,49 species of 42 genera and 31 families were recorded.In the past 21 years,there were 3 families,4 genera,and 6 species in the tree layer.At the same time,there were 9 families,12 genera,and 15 species recruited.Twenty-one species were dead or recruited in the community with a variation range of 52.5%.The species composition changed dramatically.The recruitment and mortality of the rare species are the main reasons for the change of tree species in the tree layer.The plants in the study area were changed from a temperate to tropical distribution.(2)The analysis of the number and importance valve,diversity index,recruit and dead individuals of different life forms in the study area shows that:① The proportion and dominant of evergreen life form increased significantly.The appearance of the tree layer of the community is developing from a relatively balanced state of evergreen and deciduous trees to a state dominated by the evergreen trees.② The Simpson diversity index,Shannon-Wiener index,and richness index of the two life forms showed an increasing trend,and the species were more abundant.Both the life forms underwent dramatic dynamic changes.③ The evergreen life form was significantly improved owing to its extremely high rate of recruitment and low mortality.The cumulative recruitment rate of the deciduous life form was slightly higher than the cumulative death rate.The number of trees decreased slightly but the changes in tree species were significant.(3)From the aspect of DBH and height structure,the number of deciduous trees species with small diameter was large and the proportion of evergreen life form was increased.With an increase in the diameter,the competition among the trees with medium diameter was intensified.As a result,the number of trees with medium diameter was reduced and the number of trees with large diameter was leveled off.Therefore,the diameter structure of the community was inverted "J" type during the 21 years.In terms of space,the evergreen life form was updated well and it was better than the deciduous life form.The advantages of the evergreen life form in the forest layer were constantly extended to the upper layer and the advantages of the main forest layer were maintained The deciduous life form only maintained the advantages of the upper layer of the forest owing to a large number of trees in the large diameter grade(4)According to the analysis of diameter scale structure,static life table and survival of the Daphniphyllum macropodum species,the age structure of the Daphniphyllum macropodum species maintained a broad base and a narrow top indicating that the species was in a stable growth stage in the last five years.This species is rich in small-scale reserves and has strong renewal ability.The life expectancy of the population was decreasing with an increase in the diameter step.The maximum value appeared in the age class Ⅰ and the survival curve of the population were nearly linear.In 2017,it was closer to the Deevey Ⅱ type than in 2012.These results show that the natural regeneration ability of the cingulate wood population was strong.Further,the current growth condition is good and the population is growing.Thus,the results show that the distribution of flora in the study area is close to a temperate distribution than a tropical distribution.Furthermore,the community structure and appearance developed from a balanced state of evergreen and deciduous trees to a state of being dominated by the evergreen trees.The distribution of Daphniphyllum macropodum is increasing,the environmental competitiveness is strong,and the population is more matured and stable in the backdrop of climate change. |
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