Types and species diversity of Cupressus communities in Southeast Tibet, China.

Cupressus communities are the important parts of natural forests in Southeast Tibet, with significance on plateau biodiversity conservation and ecological security. To clarify their types and plant species diversity, we analyzed species composition and diversity indices, diameter class pattern of constructive species in different communities in Milin County, Lang County and Bomi County, Southeast Tibet. The results showed that Cupressus communities could be classified into four types: C. torulosa-Lonicera lanceolate + Philadelphus tomentosus-Pteris cretica var. nervosa + Digitaria violascens community (Community1), C. gigantea-Desmodium elegans + Sophora moorcroftia-na-Tripogon filiformis + Cymbopogon distans + Orinus thoroldii community (Community2), C. gigantea-Leptodermis purdomii-Chenopodium hybridum + O. thoroldii community (Community 3), and C. gigantea-Ceratostigma minus-A. desertorum + O. thoroldii + Tribulus terrester community (Community 4). Across those four community types, Margalef index, Shannon index, Pielou index and interspecies encounter probability index all ranked with the decreasing order of Community1, Community2, Community 3 and Community 4. Diameter class pattern of the constructive species showed that many young and few middle/old individuals were within the four communities. High mortality rate occurred during the interim period from young to middle age classes. In general, populations of Cupressus plants showed a declining trend. 西藏东南部柏木属植物群落是青藏高原天然林资源的重要组分,在维持高原生物多样性与生态安全方面作用显著.为明确其群落类型及物种多样性,本研究通过在米林县、朗县和波密县设置典型样地和划分群落类型,分析了不同群落物种组成、物种多样性指数及建群种径级结构.结果表明: 柏木属植物群落可划分为4个群落类型:西藏柏木-柳叶忍冬+绒毛山梅花-凤尾蕨+紫马唐群落(群落Ⅰ)、巨柏-圆锥山蚂蝗+砂生槐-线形草沙蚕+芸香草+固沙草群落(群落Ⅱ)、巨柏-甘肃野丁香-杂配藜+固沙草群落(群落Ⅲ)、巨柏-小蓝雪花-藏沙蒿+固沙草+蒺藜群落(群落Ⅳ).各群落Margalef丰富度指数、Shannon多样性指数、Pielou均匀度指数及种间相遇机率指数均表现为:群落Ⅰ＞群落Ⅱ＞群落Ⅲ＞群落Ⅳ.4个群落的建群种幼龄级个体多而中老龄级少,植株由幼龄级向中龄级过渡阶段死亡率较高,柏木属植物种群呈衰退趋势.