Study in the mechanisms of species diversity in temperate hardwood forests and the restoration programs

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1. Analysis of the mechanisms of species-diversity in temperate deciduous broad-leaved forests.To investigate the relative importance of several biotic and abiotic environmental factors in determining species diversity, we investigated number of tree species, nutrient and water contents, light, coverage of understory vegetation (e.g., shrubs, dwarf bamboo), depth of leaf litter, and distance from conspecific adults in each of 600 individual quadrats (10 m x 10m) in a 6ha plot. In the analysis of sequential equation model (SEM), we found that distance of conspecific adult had a strong effects on abundance and diversity of tree species in seedling stage, but the effect decreased with increasing plant size. For saplings, the abundance was strongly affected by the abiotic factor such as nitrogen contents, light level and water availability. We also analyzed the causes of distance-dependent mortality of tree seedlings for 8 tree species in a hardwood forests by sowing thei seeds at two distances (i.e. far and near from conspecific adults). In the six of the eight species studied, we found distance and/or density-dependent seedling mortality which was mainly caused by pathogenic fungi. In the analysis of pathogenic fungi, we observed speies-preference, which preferentially attack conspecific seedlings compared to hetero-specific seedlings in Prunus grayana.2. Analysis of the recovery mechanisms of species diversity in coniferous plantation and the restoration programs. In the Criptomeria japonica plantation, heavy-thinning (67% of total basal area) had a greater effects on species diversity rather than light-thinning (33%). Distance from neibouring hardwood forests also strong effects on the species diversity.