本研究目的在分析研究西勢水庫集水區林分組成和森林結構，研究地區海拔高約65～729m間，總計設立10×l0平方公尺樣區87處，記錄維管束植物計100科196屬305種。降趨對應分析(DCA)的結果顯示海拔高度和地形是影響植群分布最重要的因素。植群經雙向列表比較法(TWINSPAN)分析後，分為六個植群型及五個亞型，分別為：Ⅰ柳杉型，Ⅰ-A大青－柳杉亞型、Ⅰ-B黑星櫻－柳杉亞型、Ⅰ-C刺杜密－柳杉亞型；Ⅱ相思樹型；Ⅲ茜草樹－小葉樹杞型；Ⅳ山紅柿－紅楠型；Ⅴ大葉楠型；Ⅵ山香圓型，Ⅵ-A蒲桃－山香圓亞型、Ⅵ-B水同木－山香圓亞型。根據調查分析本地區之木本植物胸高直徑大於或等於1cm的種類之歧異度，Simpson氏歧異度指數平均為0.8057，Shannon氏歧異度指數平均為2.1729及均勻度指數平均為0.7838。本研究分析集水區內之植群族群構造，大部份之優勢種均呈反J形分佈，表示此植群具有自行更新能力。若不再受干擾，未來有可能發展成楠櫧林帶，下層以楠木類為優勢之闊葉樹森林，惟該地區常發生颱風及地震等自然干擾因子。

The aim of this study was to analyze the composition and structure of the forest at the watershed of Hsi-Shih Reservoir. Keelung, northern Taiwan The altitude of the research area ranged from 65 to 729 m above sea level. 87 plots were designed in this study, each one of 10×10m^2 in size. A total of 305 species, belonging to 196 genera and 100 families, were recorded in this study Detrended Correspondence Analysis (DCA) showed that altitude and topography were the most important factors for the composition of vegetation. Six main types and five subtypes of the vegetation were recognized by the TWINSPAN method: Ⅰ Cryptomeria japonica type, three subtypes were recognized: Ⅰ-A Clerodendrum cyrtophyllum-Cryptomeria japonica subtype Ⅰ-B Prunus phaeosticta-Cryptomeria japonica subtype Ⅰ-C Bridelia balansae-Cryptomeria japonica subtype Ⅱ Acacia confusa type Ⅲ Randia cochinchinensis-Ardisia quinquegona type Ⅳ Diospyros-morrisiana Machilus thunbergii type Ⅴ Machilus kusanoi type Ⅵ Turpinia formosana type, two subtypes were recognized: Ⅵ-A Syzygium jambas-Turpinia formosana subtype Ⅵ-B Ficus fistulosa-Turpinia formosana subtype Simpson's diversity index, Shannon's diversity index, and evenness index of the woody plants with DBH≧1cm were 0.8057, 2.1729, and 0.7838, respectively. Population structure analyses revealed that most of the dominant species in study site were with inversely J-shaped form, which implies that the forests have the ability to self-regenerate. If there were no disturbance, such as typhoon and earthquake, the forest would possibly change toward the Machilus forest type.