本研究於惠蓀林場杉木人工林設置30個15m×15m的樣區進行林分密度指數與物種多樣性指數關係探討；共調查78科153屬216種之維管束植物，其中蕨類植物佔17科34屬47種，蕨類商數高達6.95，顯見杉木人工林下環境相對濕潤，可提供地生和附生蕨類的棲息地。地被植群分群結果可分為芒萁型、九節木型及柏拉木型等3型，降趨對應分析結果大致與植群分類相符，而典型對應分析發現海拔、水分梯度、土壤有機質和有效磷等為影響植群分化的重要環境因子。使用3種林分密度與物種數關係發現，喬木層物種數隨杉木的胸高斷面積上升而下降，地被層物種數則無明顯關係，但在適中的林分密度下有較多的物種組成。我們推測此現象是過度疏開的林分致使芒萁快速生長並佔據生育地，與其他生物發生競爭；而林分密度過高則造成林分過於鬱閉，只有耐陰性植物可以存活。因而在適度的林分密度下，物種多樣性較高。

The relationship between stand density and species diversity of 30 plots in Chinese fir (Cunninghamia lanceolate) plantations were studied at Hui-Sun Forest Station. A total of 216 vascular plants that belonged to 153 genera and 78 families were recorded. Among them, 47 fern species were found, and the Pteridophyte-Quotient was as high as 6.95, suggested that the environments of the Chinese fir plantation were relatively humid that provided suitable habitats for terrestrial and epiphytic ferns. According to the cluster analysis results, the understory plant community could be divided into three dominant types: Dicranopteris linearis type, Psychotria rubra type and Blastus cochinchinensis type. The results of detrend correspondence analysis were generally consistent with the classification of the vegetation type. Canonical correspondence analysis revealed that the altitude, moisture gradient, soil organic matter, and soil available phosphorus were the main environmental factors that could effectively differentiate the vegetation types. The relationships between stand density and species diversity were differed for tree layer and understory. The species diversity of tree layer declined along with basal area of Chinese firs, but not with understory. We found that high species number appeared in the medium stand density, and the dominant understory species, Dicranopteris linearis, successfully competed with other species and occupied the habitat of low stand density. On the other hand, high stand density would restrict the colonization of species, and allow only shade-tolerant plants to appear. Therefore, a suitable stand density of China fir plantation provided various niches for species to coexist and maintain high species diversity.