Structure Characteristics And Influencing Factors Of Lithocarp Us Glaber-Cyclobalanopsis Glauca Community In Subtropical Evergreen Broad-leaved Forest

Subtropical evergreen broadleaved forest in Southeast China is the most typical forest with high biodiversity and productivity as well as complex floristic composition and structure in the world. The forest plays an important role in biodiversity conservation and maintenance of regional ecological security. However, due to long-term inappropriate activities, the area of evergreen broadleaved forest has been reduced extremely, which led to a series of ecological problems, such as simple community structure, poor ecosystem functions, and environmental deterioration. Restoration and rehabilitation of subtropical evergreen broadleaved forest community become very urgent issues to be addressed. Investigation and analysis of community structure, spatial pattern, regeneration characteristics and their influencing factors of evergreen broad-leaved forest at typical regions, which could help to understand the biological characteristics of species and basic rules of interaction with environmental factors, master regeneration and succession trends, reveal the formation and maintenance mechanisms of spatial patterns, and provide theory bases for the rebuilding and recovery of subtropical vegetation zones, conservation and sustainable management of natural forests. Based on the data collected from1hm2permanent sampling plot of Lithocarpus glaber-Cyclobalanopsis glauca forest in Dashanchong Forest Farm, Changsha County, Hunan Province, we aimed to investigate:(1) tree species composition, the structure and flora of the evergreen broadleaved forest community;(2) the species spatial distribution patterns and their driving mechanisms in the evergreen broadleaved forest.(3) the influence and relative contribution of topographic and soil factors to community spatial pattern;(4) the characteristics of understorey regeneration and its influence factors. The main results showed that:(1) The results showed that trees with diameter at breast height (DBH) larger than1cm in the plot, totaled4797stems belonging to73woody species,55genera, and38families, with their total cross-area at breast height (BA) being22.78m2/hm2. Evergreen and deciduous species shared a similar proportion of composition (i.e. evergreen50.68%and deciduous49.32%), and no large woody vines were found in the forest community. For the stems in DBH larger than4cm, there were1831individuals belonging to4families,34genera and45species, and their total BA were21.51m2/hm2. The most dominant species with important values larger than5%were L. glaber, C. glauca, Pinus massoniana, Cunninghamia lanceolata, Cleyera japonica, Choerospondias axillaris. Fagaceae had the highest important value and was the most dominant family in the community. The floristic elements of family and genera were mainly tropical elements with strong translation characteristics from tropic to temperate. The analysis on geographical elements of species indicated the studied location was at the core region of Japan-China forest floristic kingdom. The community was one of subtropical widespread community and jointly affected by the East Asia, tropical Asia and Chinese geological and climatic conditions. The flora had the typical characteristics of subtropical flora with extreme similarity to forest community in Japan and belonged to L. glaber-C. glauca formation group of typical evergreen broadleaved forest in the eastern region, China. The origin time should not be later than the tertiary period.(2) Plant life form spectrum reflected that the communities absolutely dominated by phanerophytes (Ph) with no Chamaephytes (Ch) and annual herb (T), and affected by the typical subtropical climate. The law of frequency was A>B<C>D=E, and inconsistent with the standard Raunkiaer’s law. The significant higher proportion of species with low frequency indicated the community had high species richness with a few dominant species and showed high stability. While C>B and the E-class was significantly lower indicated that the evenness of community was slightly low, and the community had been disturbed by human in history or had a potential succession trend.(3) The distribution of DBH classes and height classes indicated there were obvious vertical differentiation, good understory regeneration and stable structures in this forest community. All individuals and evergreen trees showed "inverted J-shaped" in DBH classes and evergreen shrub species was "L-type", which indicated these trees were growth-type. The "unimodal-type" distribution pattern of deciduous trees and conifers indicated these species were long term recession species with bad regeneration. There was a decreasing trend for the number of species, individuals of all trees with height increasing, which indicated that the dominant populations regulated individual survival and development and maintained the stable development of communities by r strategies.(4) The dominant populations and all individuals of common species in community primarily were aggregation distributions across different vertical layers, their aggregation strength at most increased with spatial distance increasing. In the same scale, the aggregation strength at overstory layer was weaker than the midstory and understory layer at community level. Shade-intolerance species such as P. massoniana and C. axillaris gathered weaker at overstory than at their midstory layers. However, shade-tolerant evergreen trees such as L. glaber, C. glauca, C. japonica and C. laceolata showed aggregation strength increasing from understory to overtory layers. The spatial associations among different vertical layers of community or dominant species were independent consistently or positively correlation each other at close distances, which indicated the community had good spatial differentiation and the majority of species occupied their own suitable spaces. The seed dispersal limitation, biological characteristics and microhabitat heterogeneity might be the main driving forces to aggregation distribution formation.(5)Topographic and soil factors belonged to moderate spatial variability with close relation each other and played decisive roles in determining the species composition and spatial distribution of community. RDA analysis showed that the contribution of topographic and soil variables decreased from understory to overstory layers at community level. Habitat filtering was one of important processes for spatial distribution patterns formation of inter-and intra-species with different growth stages. Topography explained more than soil to dominant species, community level and each vertical layer of them. And at least53.4%contribution of the two factors came from joint effects of them. Altitude, convexity and aspect were the most important topographic factors and AP, TK and TP were the most important soil factors. However, there was over70%variability unexplained and showed that clustering processes (biological characteristics, dispersal limitation, etc.) with no correlation to habitats played important roles.(6) Based on statistical analysis about the trees density with different functional types at regeneration and succession layers, the results showed that:There was a well regeneration in general and seedlings and saplings were mainly consisted by evergreen species. And they would maintain and consolidate their absolute dominant role in the community in the future for high species richness and plenty of individuals. However, pioneer deciduous trees and conifers would gradually be disappeared due to lack of seedlings recruitment.(7) The dead standing trees with DBH≥1cm belong to32species with total278stems/hm, shared5.08%of total stands in community and mainly consisted by small diameter classes individuals. There were178stems/hm2sharing64.03%of total individuals of dead standing trees and L. glaber was most with44.96%. Deciduous shrubs and trees were smaller populations and had higher mortality than evergreen trees and shrubs. The results indicated that intra-and inter-specific competition were intense in the understory layer of community and pioneer species such as P. massoniana, were be gradually replaced by evergreen broad-leaved trees at late stage of succession. However, few large-size pioneer species was dead, which indicated the pioneer species would not be excluded in a short period of time for the current stability of species composition and structure of community.(8) There were seed propagation and spouting stems regeneration ways. Evergreen trees had higher proportion of sprouting stems with the ration between seedlings and sprouting stems below1.32, which showed they had excellent recruitment ability with two functional traits of seed and sprouting propagation owning the characteristic of climax species. Deciduous trees recruited mainly by seedlings with the ratio of3.5. Shrub species commonly regenerated by seedlings that shared70%of total individuals of the same functional type. Sprouting regeneration ability determined the status and roles of different functional trees in community regeneration process.(9) Based on the correlation and regression analyses between topographical, soil nutrient, moisture, properties of the main storey, density of dead standing trees and density of all species or different functional types, the result showed that:the spatial heterogeneities of light, soil moisture and soil nutrient affected the growth of different functional-type trees and became the main influencing factors on understorey regeneration. The main influencing factors were very different from the regeneration to the succession layer. The factors relating to soil moisture and light were the most important factors influencing the density of regeneration layers, and richness of the main storey and the factors relating to soil nutrients determined the spatial pattern of succession layer. There was obvious preference of soil nutrients and habitat to trees with different functional types from regeneration layer to succession layer. Competition, habitat heterogeneity, life-history strategies and seed dispersal etc. were the main determinant processes of understorey regeneration in L. glaber-C. glauca community.