Differentiation And Maintenance Of Relict Deciduous Broad-leaved Forest Patterns Along Micro-topographic Gradient In Subtropical Area, East China

The subtropical area of East China is one of the most important refugia for survivors of Tertiary relict flora, therefore contains large number of relict deciduous broad-leaved trees. The capabilities of population regeneration and community maintenance were reduced due to the natural and anthropogenic impact. Hence, the coexistence mechanism of relict deciduous broad-leaved trees in local area should be further studied to provide practical guidance for conservation and restoration of relict deciduous broad-leaved trees especially the rare and endangered species, and enrich the research on theory of mountain plant diversity and conservation ecology in the subtropical evergreen forests area of China.The distribution type, habitat characteristics, floristic composition, community structure and plant leaf traits of relict deciduous broad-leaved forests have been studied in this paper. It could reveal differentiation pattern of relict deciduous broad-leaved trees and communities along micro-topographic gradient. Furthermore, it would clarify the cause of differentiation pattern and their maintenance mechanism by the analysis of habitat heterogeneity, response of plant trait, inter-specific interaction and population regeneration pattern. The main conclusions are as follows:(1) Differentiation pattern of relict deciduous broad-leaved forest along micro-topographic gradient.The results of cluster analysis and characteristic depiction of communities indicated that spatial distribution of relict deciduous broad-leaved trees was different among micro-topographic units. They were dominated at each site and formed topographic climax community. Cyclocarya paliurus was widely distributed and dominated on river bed and valley slope between 600 to 800 m a.s.l.. The abundance and dominance of Liquidambar formosana was higher on sideslope and river bed below 800 m a.s.l.. Above 800 m a.s.l., L. acalycina was dominanted on sideslope, valley slope and crest slope, Nyssa sinensis was on head hollow and sideslope, and Emmenoptery henryi was on river bed and head hollow. Euptelea pleiospermum was only located on river bed between 1000 to 1200 m a.s.l.. Compared with the altitudal zonation of mountain vegetation, the spatial distribution, floristic composition, community structure and species diversity of relict deciduous broad-leaved forests were significantly different. They were mainly distributed on specific unstable habitat and supplied for bare habitat that evergreen broad-leaved forest was hardly occupied.(2) Altitude and micro-topographic units were the main factors that determed the community differentiation pattern.The result of CCA analysis indicated that the community distribution pattern was significantly affected by altitude and micro-topographic units. The effects of slope and aspect were not significant. The floristic compostion and dominant species changed significantly along the altitude. Low-altitude dominant species L. formosana was replaced by L. acalycina, E. henryi, N. sinensis at high altitude. The main companion species Cyclobalanopsis gracilis and Cyclobalanopsis myrsinifolia were replaced by Daphniphyllum macropodum, and Quercus acutissima was replaced by Quercus serrata var. brevipetiolata. Micro-topographic units have their own surface characteristics which contributed to different disturbance regimes. The disturbance regimes would hinder the process of vegetation succession. Slope was the most important factor that affects landslide and soil erosion, and an important characteristic of each micro-topographic unit as well. This could give the opportunity for population regeneration and maintenance of relict deciduous broad-leaved tree due to the landslide.(3) Niche differentiation among relict deciduous broad-leaved trees and other trees at spatial and temporal scales.The topographic characteristics and spatial distribution pattern of relict deciduous broad-leaved trees were different. They had a specific habitat preference and were mostly distributed at unstable habitat. E. henryi and E pleiospermum were located on valley habitat while N. sinensis and E. acalycina mainly located on crest slope and side slope. Their niche breadth was narrow and their niche overlap was smaller so that the differentiation pattern was significant. Hence, it reflected the exclusion of species spatial distribution, which resulted from species'long-term adaptation to micro-environment and utilization to different resources. N. sinensis, L. acalycina, L. formosana and E. henryi were a long-lived pioneer species. They could invade into the bare habitat after disturbance that accompanied with short-lived trees. If the habitat were long-term stable after once disturbance, long-lived relict deciduous broad-leaved trees would become dominant species when the short-lived pioneer species such as Carpinus viminea, Platycarya strobilacea disappeared gradually. The relationship among these different pioneer species was significantly negative. It indicated that their niche was a significant difference and represented the niche complement of species on the process of community succession. Besides, long-lived relict deciduous broad-leaved trees N. sinensis and L. acalycina would provide a shelter for evergreen trees during the process of community succession. They were coexisted with evergreen tree due to its tall stature and long life span. Their relationship appeared significant positive relationship. This reflected that the vertical niche differentiation between relict species and evergreen trees.(4) The persistence of population and community depend on different reproduction strategy and regeneration pattern at different micro-topographic unitsSome relict species such as C. paliurus and L. acalycina regenerated discontinuously at safe site via long-distance diffusion. And E. heryi and E. pleiospermum regenerated discontinuously around mature tree under frequent disturbance. After occupied by sexual regeneration, N. sinensis and L. formosana could regenerate and form multi-stems individuals by inherent sprouting ability at relative stable sites. At river bed and valley slope habitat, E. henryi and E. pleiospermum, L. formosana also could be recruited by strong sprouting ability due to sparse seedling and loss incurred by disturbance.At low altitude, evergreen broad-leaved forests were damaged seriously under natural and artificial disturbance so that there were few evergreen seedlings. Therefore, L. formosana would predominate for long-term at each micro-topographic unit by the sporadic regeneration and strong sprouting ability. At middle altitude, N. sinensis, L. acalycina, L. formosana and E. henryi were regenerated discontinuously while the regeneration type of C. gracilis and Lithocarpus harlandii displaced an inverse-J type on the relative stable habitat. The evergreen trees would predominate gradually and long-lived relict species only existed at emergent layer. And N. sinensis and L. acalycina also could occupy new habitat via long-distance diffusion when forest gaps were formed.At lower mountain evergreen broad-leaved forest area, the population of Phoebe sheareri was regenerated well and would replace the first dominant species on river bed and valley slope. However, C. paliurus could regenerate well at mosaic community level by virtue of sporadic regeneration far away from mature trees. On river bed at middle altitude, the size classes of dominant evergreen species C. gracilis and L. harlandii were smaller and some individuals died due to the frequent disturbance. E. henryi would predominate long-term by virtue to sporadic recruitment and strong sprouting ability. The regeneration of E. pleiospermum population depended on frequent disturbance and displaced a sporadic type. Furthermore, the survival rate and sprouting rate of saplings and young trees were higher. Thus E. pleiospermum population could persist for a long period.(5) The response of trees with similar leaf traits to micro-topographic units was unanimous.Micro-topographic units were the greatest factor that affects the leaf traits of main broad-leaved trees. It affected the distribution of tree and their adaption ability via changing the soil nutrition and disturbance regime. The leaf trait characteristics of dominant trees located on river bed and valley slope were larger leaf area and specific leaf area, lower leaf dry matter content, higher nitrogen content. And the dominant trees located on head hollow, crest slope and side slope have smaller leaf area and specific leaf area, higher leaf dry matter content, lower nitrogen content. From the river bed to crest slope, leaf area and nitrogen content showed a decreasing trend, while leaf dry matter content tended to increase.The results of cluster and PCA analysis showed that 34 trees can be classified into 4 types based on the similar leaf traits. Evergreen trees except for Daphniphyllum macropodum had the characteristics of lower leaf area, specific leaf area and nitrogen content, and high leaf dry matter content. They were mainly located on side slope and became the main companion species or dominant species. Deciduous trees mainly adapted on valley habitat and their leaf traits characteristics were larger leaf area, higher specific and nitrogen content and lower leaf dry matter content. C. paliurus has a similar leaf traits pattern with a large number of deciduous trees such as Kalopanax septemlobus, Litsea auriculata, Styrax obassius, Magnolia cylindrical. The leaf traits characteristics of L. formosana, E. henryi, E. pleiospermum, cornus controversum and Alangium chinense were also similar. N. sinensis, L. acalycina, Tilia breviradiata and D. macropodum were the third type. Evergreen tree and smaller leaf area deciduous trees such as Carpinus viminea, Quercus serrata var. brevipetiolata and Acer palmatum were the fourth type. There was a good correspondent relationship between the species group types based on leaf trait characters and their distribution patterns.(6) Ecological species group of relict deciduous broad-leaved tree in subtropical area of East China.The relict deciduous broad-leaved trees could be classified into two ecological species groups based on spatial distribution and strategy of regeneration, adaptation and competition.Type 1 was species that supplement for bare habitat, such as E. pleiospermum, C. paliurus, E. henryi and L. formosana. They were characterized by predominating on valley habitat with frequent disturbance, unimodal distribution type, strong vegetative reproduction ability, and lower interspecific competition.Type 2 was species that repairment for forest gap, such as N. sinensis, L. acalycina, L. formosana. They were characterized by predominating on side slope with long period disturbance, regenerated by sexual and vegetation reproduction, persisted by discontinuously regeneration that far away from mature trees, and the vertical niche differentiation with other trees.