中文摘要：

贺兰山位于中国温带草原和温带荒漠的过渡带,是研究干旱区山地物种多样性海拔格局的理想区域。该文通过样方法调查研究区的森林和灌丛群落,并运用广义可加模型分析物种多样性的海拔分布格局。结果表明：1）海拔是物种丰富度的重要影响因子,一般能解释原始数据30%-40%的变差。2）对于森林和灌丛群落,草本植物都是群落中比例最高的物种,而且决定了群落总物种丰富度的海拔分布格局。3）森林群落的乔木层物种丰富度在中海拔区域最高,反映了中海拔区域相对优越的水热条件。灌木层和草本层的物种丰富度明显受到乔木层郁闭度的影响,有随海拔升高而降低的趋势。4）灌丛群落的灌木层和草本层物种丰富度均呈单峰格局,皆因低海拔的干旱和高海拔的寒冷抑制了多数物种的生存,仅气候条件适宜的中海拔区域能够生存丰富的物种。

英文摘要：

Aims Our objective was to reveal the altitudinal pattern of species diversity in woody plant communities of Mountain Helan, which is located on the ecotone between temperate steppe and desert regions in arid northwestern China. Methods We surveyed forests and shrublands with the quadrat method and used Generalized additive model （GAM） to model relationships between species richness of tree, shrub and herbaceous layers with altitude. Important findings Altitude accounted for 30 % - 40 % of the variances of species richness patterns, including tree, shrub, herb and total species richness. Both for forest and shrubland communities, 60% - 80% of the species were herbaceous and patterns of total species richness with altitude were almost determined by herbaceous species. In forest communities, species richness of the tree layer peaked at middle altitude, because of favorable precipitation and temperature. Species richness of shrub and herbaceous layers in forest communities decreased with altitude, showing a significant influence of forest canopy cover. For shrub communi- ties, species richness of shrub and herb layers also peaked at middle altitude. Aridity at low altitude and cold at high altitude limited species＇ survival, and only at middle elevations could coexist large numbers of plant species.