Community Characteristics And Dominated Plant Species Relationship In A Dubtropical Secondary Forest In Huitong, Hunan Province

Secondary subtropical broadleaved forest in Huitong County, southwest of Hunan Province, southern China, is at the central of Sino-Japan bryoflora. The flora in secondary subtropical broadleaved forest is characterized with abundant plant species containing many ancient and relict plants, complex and wide connected geographical elements, and abundant endemic species. The floristic elements at the family and genus levels are mainly pan tropical with a certain proportion of temperate elements, whereas at the species level, species endemic to China and species of East Asia are the dominants with larger proportion of temperate elements and certain proportion tropical elements. Plant species diversity in the forest area is richer than that in other secondary forests in the same region, which reveals that the community is disturbed moderately before. Plant species in Huitong subtropical secondary forest are high.Tree species in overstory was dominated by Cunninghamia lanceolata, Pinus massoniana, Alniphyllum fortunei, Litsea coreana var. lanuginose, Machilus rehderii, M. pauhoi, Myrica rubra, Choerospondias axillaris, Cyclobalanopsis glauca, and C. myrsinaefolia. Shrub plant in understory was mainly dominated by Camellia oleifera, Loropetalum chinense, Photinia parvifolia, Rhododendron ovatum, Rh. Mariesii, Eurya loquiana, and Quercus serrata var. brevipetiolata.A 9600 m2 rectangular plot was established to map the location of all trees greater than 4 cm in diameter at breast height (DBH), identify plant species and measure DBH, and total tree height (H). For the forest community, Margalef species richness index, Shannon-Wiener diversity index and Pielou evenness index was 10.28,5.11 and 0.82, respectively, indicating that secondary forest in Huitong had rich and evenness floristic composition. According to dominance value (72.62%), A fortunei, C. lanceolata, C.s axillaris, P. massoniana, Liquidambar formosana, C. glauca, L. chinense, Machilus rehderii, and M. pauhoi were the main dominant tree species for the secondary forest in Huitong.A reverse-J shaped DBH classes distribution was observed for all stems and trees of later serial species while trees of earlier successional species distributed irregularly. The structures of the height distribution were normal-left distribution for the stand and it's main dominant populations of the Huitong subtropical secondary forest community, and indirectly indicated that both intra- and inter- specific competitions were exist for light resources.Hegyi's competition index model was used to investigate intraspecific and interspecific competition intensity for dominant species trees. The results showed that competition intensity of the top 13 dominant plant species was ranked in the order of P. massoniana< M. pauhoi< Styrax suberifolius< L. coreana var. lanuginose< A. fortunei< Clethra fargesii< L. formosana< C. glauca< M. rehderii< C. lanceolata < L. chinense< C. axillaries< Engelhardtia roxburghiana. The tree species whose intraspecific competition was more intense than that of interspecific competition were M. rehderii, C. lanceolata, C. axillaris and E. roxburghiana. On the contrary, the tree species with lower intraspecific competition than that of interspecific competition were P. massoniana, M.s pauho, S. suberifolius, L. coreana var. lanuginosa, C. fargesii, L. formosana, C. glauca. The tree species that had an approximate intraspecific and interspecific competition index were L. chinense and A. fortuneii. The dominant and higher trees showed a strong competition intense compared to the lower trees. Relationship between the competition index and the diameter at breast height of objective tree could be expressed as power function and competition intensity appeared to reduce as the diameter at breast height of objective tree increased.Competition is common natural phenomena. Generally, competition between tree species could be divided into aboveground competition for light and belowground competition for water and nutrient resources in soil. Environmental factors in soil and competition would shift fine root distribution, morphology and architecture. A. fortunei, L. formosan and C. glauca were selected as target to investigate root distribution and morphology in response to abovegroud competition. Competition index (CI) of A. fortunei, L. acakycina and C. glauca was 1.69,0.97 and 2.16, respectively. Total fine root biomass of A. fortunei, L. formosan and C. glauca within 0-45cm soil depth accouted for 104.1g·m-2,122.4 g·m-2 and 54.4 g·m-2, most of which distributed at top 0-30cm soil depth. Fine root biomas of A. fortunei and L. formosan were even distributed in soil depth, but there were soil depth distribution differences between C. glauca and A. fortunei, L. formosana. For horizontal distribution, there were no significant differences among fine root biomass of A. fortunei, L. formosan and C. glauca. (p>0.05) Fine roots of C. glauca generally occupied the places nearby trees, while fine roots of A. fortunei and L. acakycina were not.Vertical and horizontal distribution of root length density (RLD) and specific root area (SRA) of three tree species was similar to that of fine root biomass. RLD and SRA were significantly decreased with increasing soil depth. RLD and SRA of A. fortunei and L. formosan were similar in different horizontal distance, while RLD and SRA of C. glauca were higher closed to tree species. In terms of vertical distribution, specific root length (SRL), specific root area (SRA) and specific root tips (SRT) of L. formosan which decreased significantly with increased soil depth were higher than that of A. fortunei and C. glauca in surface 0-15cm. In contrast, SRL, SRA and SRT increased with increasing soil depth. In terms of horizontal distribution, SRL, SRA and SRT of L. formosan and C. glauca were higher than other tree species in different horizontal distances, while those of A. fortunei were not predominant in each soil layers and each horizontal distance. Although C. glauca was subjected to strong competition and competitive index was high, its fine roots had great adaptability to changed environment (differential distribution, higher SRL, SRA and SRT), and it can coexist with L. formosan for a long time. On the contrary, suffered from competition from aboveground and belowground, A. fortunei is disadvantageous when competed with L. formosan and C. glauca.To test the following two hypotheses whether the spatial distribution of dominant tree species is aggregated in subtropical secondary forests, and whether this spatial pattern can be used to infer causal factors for the coexistence of diverse tree species and predict forest succession trends, we employed Ripley's K(t) function analyze spatial patterns and associations. All trees within the forest showed a significant aggregated spatial pattern as did trees of each dominant species. Negative spatial association was detected for trees of C. axillaris versus A. fortunei, L. formosan or Pinus massoniana, and trees of Cyclobalanopsis glauca against A. fortunei, L. formosan, C. axillaris or P. massoniana. The apparent aggregation of spatial distribution for trees of each dominant species is indicative for mosaic patches that partition resources and maintain diverse species coexistence. Repulsive or independent spatial associations between trees of later and earlier serial species demonstrate the gap regeneration process in forest dynamics. Pioneer deciduous species will eventually be replaced by late serial evergreen species within the secondary forest.