本研究調查台灣北部楠櫧林帶內之柳杉人工林植群，分析其物種多樣性發展及變異，共設置天然林及次生林樣區35個，以及柳杉人工林樣區55個。以β多樣性來說，在海拔>750m柳杉林樣區，呈現生育地異質性指數h值隨演替序列而下降，物種組成相似性逐漸昇高，物種組成轉換率下降現象；在海拔<750m樣區則呈現相反之趨勢。不同海拔及年齡柳杉樣區之樹木徑級曲線顯示，小徑級有相當數量，和柳杉徑級有明顯區隔。由柳杉林分組成及結構分析結果研判，各年齡級柳杉林分，下層有相當數量之天然林冠層樹種幼樹，皆已進入下層植物再現期。本研究分析結果顯示，大桶山區柳杉林分和天林分所構成之鑲崁地景結構，有利於柳杉林內天然物種多樣性之恢復。

The aim of this study was to analyze the species diversity of understory plants in Cryptomeria japonica plantations within Machilus-Castanopsis forest zone of northern Taiwan. The samples consisted of a series of different-aged stands, 35 plots and 55 plots for natural or semi-natural vegetation and plantation respectively. Beta diversity indices among plots above 750m in elevation showed a decreasing trend with stand age. Similarities of species composition along successional sere became increasingly higher and species turnover rates gradually decreased toward the later stages. However, beta diversity indices among plots below 750m in elevation showed a reverse trend with stand age. The tree size structure analysis showed a clear demarcation between small size classes of woody understory species and medium size classes of Cryptomeria japonica. In Ctyptomeria japonica plantations aged from 23 to 46 years, the woody understory was replenished with canopy tree saplings from natural forests, suggesting the Cryptomeria japonica stands had entered the understory reinitiation stage. The results of this study suggest the landscape mosaics around Mt. Ta-tong, mixing with natural forests and Cryptomeria japonica plantations, promote the recovery of native tree species diversity in Ctyplomeria japonica plantations.