| Summary: | 博士 === 國立臺灣大學 === 森林學研究所 === 90 === The aim of this study was to analyze the species diversity of Cryptomeria japonica plantations in Machilus-Castanopsis forest zone of northern Taiwan. The sample consists of a series of different-aged stands, 35 plots and 55 plots for plantation and natural or semi-natural vegetation respectively. For alpha diversity, there is no significant difference between 23 and 39 years old plantations at altitude below 750m for all species group, while in plantation of 23 and 46 years old stands above750m, the woody story, woody species and understory species, show a significant difference. Beta diversities among plots of same age show an increasing trend with age at altitude above 750m. Similarities of species composition among successional sere become increasingly higher, and species turnover rates gradually decrease toward later stages as stand aged. However, beta diversity indices of plots below 750m, show a reverse trend.
The result of tree size structure analysis reveals a clear demarcation between small size classes from woody understory species and medium size classes from Cryptomeria japonica populations. After 40 years of succession in the understory of Cryptomeria japonica stands, species diversity and abundance of species of Lauraceae dominated the understory, while species of Fagaceae was poorly represented. Though woody story species richness tended to recover across various sites, it was still less than the average number in adjacent evergreen broad-leaved forests. The shrub-herb layer species richness almost remained constant after canopy closure. In twenty-three to forty-six years old plantations of Cryptomeria japonica stands, the woody understory is replenished with canopy tree saplings from mature forests, thus the stands have entered the undersory reinitiation stage.
Spatial distance measures between Cryptomeria japonica plantations and nearest natural stands across landscape were made. Only herb species are significantly influenced by the distance on the species diversity pattern. Woody story and shrub-herb layer species show a good recovery rate up to 93﹪and 90﹪species richness level as compared to the adjacent evergreen broad-leaved forests, but epiphytic ferns and orchids are still limited in occurrence. There are about 30 vulnerability rank species, which are not easy or need more time to recover in studied plantations. The results of this study suggests that mosaics structure of landscape patterns between natural and artificial stands as those found around the Ta-tong Mt. can be a better model to maintain or promote diversity of forest species.
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