| Summary: | 博士 === 國立臺灣大學 === 生態學與演化生物學研究所 === 92 === Taiwan yellow false cypress (Chamaecyparis obtusa Sieb. & Zucc. var. formosana (Hayata) Rehder) is a valuable timber species dominated in temperate coniferous forest in Taiwan. For sustainable use of this species, regeneration under natural conditions was examined and studied in Yuanyang Lake Nature Reserve (YYL) located in northern Taiwan. Woody plants with basal diameters larger than 1 cm were recorded at three study sites in order to compare population structure of Taiwan yellow false cypress at different localities and forest types, when compared with that at Szumakuszu (SMKS) of northern Taiwan. Seed germination, seedling survival, spatial pattern of Taiwan yellow false cypress, role of coarse woody debris (CWD), and co-existence of this species with broadleaf species were examined at the lakeside of YYL. An inverse J-shaped size distribution pattern for Taiwan yellow false cypress on CWD at both the lakeside and cliff sites suggested that CWD provides important substrate for the growth of this species. However, this species had independent of inhabited substrate type at SMKS site. At lakeside forest, the projected area of CWD was about 247 m2ha-1. There was no significant difference in seed germinations between substrates such as CWD and forest floor. However, it was revealed that seedlings on CWD had a higher survival rate than on the forest floor. This might be the reason why a clumped distribution of seedlings in the forest. The relationship between clumped seedlings and randomly distributed matures suggested that spatial variation was regulated by density-dependant mortality between seedling and mature stages. Further studies done on fifty down logs showed that the seedlings on down logs of ca. 10 years old might be as high as 19.5 seedlings m-2, while those on older down logs (ca. >30 years old) lowered to 1.6 seedling m-2. On relatively young down logs, less broadleaf seedlings than Taiwan yellow false cypress was revealed. However, there were new invaders of broadleaf species on older down logs, giving rise to an elevation in the density of broadleaf on it, even higher density than Taiwan yellow false cypress. It is assumed that availability of nutrient resources from mosses detritus and organic matters on CWD seems to provide an important substrate for establishment of Taiwan yellow false cypress and broadleaf species on young down logs. Thus, the niche partition hypothesis was employed to explain the co-existence of Taiwan yellow false cypress and broadleaf species in this nature reserve.
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