中文摘要：

贺兰山针叶林是我国西北地区珍贵的森林生态系统，本文通过研究其结构特征和植物种类组成，为森林保育提供建议。基于森林结构特征，贺兰山针叶林可分为4类，分别为乔木径级组成以小于lOcm为主，以10～20cm为主，径级组成较均衡和以大于30cm为主。分类的结果主要反映了砍伐后恢复时间的长短和海拔高度的影响。根据样方的物种组成分析，共得6个群丛，主要反映了海拔的影响。物种密度以云杉藓类林最低。云杉藓类林向上与高山灌丛草甸交错，物种密度增加。海拔降低，云杉林类型改变，物种密度增加。海拔最低的油松林，物种最为丰富。两种分类反映了贺兰山针叶林不同方面的特征，但也存在联系。海拔高度对于森林结构和物种组成的影响都是明显的，而当森林结构特征类似时，包含的群丛较多，但某一群丛则往往仅属于一种或两种结构类型。

英文摘要：

Coniferous forest in Helan Mountain is the relatively rare forest ecosystem in northwestern China. For the conservation of forest, the structure and floristic composition of coniferous forest have been studied. Based on principal components analysis of forest structure, mainly four kinds of forest could be obtained, that is diameter at breast height （DBH） of trees mostly less than 10 cm, DBH of trees mostly between 10 and 20 cm, DBH of trees distributed approximally equally and DBH of trees mostly greater than 30 cm. The result mainly reflected the times after prohibition of logging, and also indictated the effects imposed by altitude. Based on TWINSPAN analysis of species composition, six plant associations could be found, which were mainly influenced by altitude. The species density in spruce-moss forest was the minimum in the whole coniferous forest. Ascending from spruce-moss forest, the forest disappeared gradually and overlapped with alpine bushes and meadow, with species density increasing. Decreasing with altitude, the floristic composition changed and species density increased. Down to the lowest pine forest, the species was most abundant. The comparison of the classification of forest structure with that of species composition indicated that, though the criteria of analyses were different and the features of forest were viewed from different sides, there existed some inherent relations between foreset structure and floristic composition. The forest structure and species composition were all mainly influenced by altitude. When the forest structures were similar, several plant associations could be found, but one assicoation always belonged to single forest structural type.