围封禁牧对小叶锦鸡儿灌丛化草原群落组成和结构的影响

Abstract

典型草原向灌丛化草原的转变是过度放牧引起的重要结果之一。为研究内蒙古地区小叶锦鸡儿灌丛化草原对围封禁牧的响应，在内蒙古锡林郭勒典型草原退化区选取小叶锦鸡儿成片分布的典型地段，自2003年，分别设置禁牧样区和放牧样区。于2008-2011年连续四年调查禁牧样区小叶锦鸡儿种群生长和生理生化指标及灌丛间群落基本特征，并将2011年禁牧样区与放牧样区小叶锦鸡儿种群与灌丛间群落进行比较。结果显示：（1）围封后小叶锦鸡儿种群开始衰退，主要体现在种群盖度的降低和叶片氮磷含量（特别是磷含量）有所降低，而个体构件生长状况无显著差异；（2）灌丛间群落的物种组成随围封年限增加发生显著变化：禁牧后1年生植物逐渐退出群落，多年生丛生禾草重要值随围封年限增加而显著增加，即围封禁牧对小叶锦鸡儿灌丛化草原的群落恢复有积极作用；（3）小叶锦鸡儿种群盖度与多年生丛生禾草的重要值显著负相关，说明两者之间存在竞争关系，推测多年生丛生禾草的竞争是禁牧后小叶锦鸡儿衰退的重要原因。;Grassland ecosystem is one of the most important terrestrial ecosystems in the world which may impose an impact upon global environmental change. The dynamics of a grassland ecosystem can be affected by disturbance in terms of promoting community succession and changing community structure. Among the disturbing factors, grazing is considered one of the key factors which affect both above-and below-ground processes in the ecosystem. One of the most prominent consequences of overgrazing is the transformation of grassland to shrub-land. In order to protect grassland ecosystem and restore vegetation productivity, a number of measures have been put into practice, among which fencing an area to exclude livestock is often considered to be a simple but effective way. The aim of the present study was to explore the response of the <em>Caragana microphylla</em>-dominated community to fencing and to predict when the degraded community would restore to its normal status after grazing exclusion. The study site was located in Xilingol degraded grassland of Inner Mongolia, China. Long-time grazing resulted in the expansion of <em>C. microphylla</em> populations in this site, and we set different treatments since 2003. Animals were excluded by fencing in the plot of grazing exclusion (EG), while the grazing intensity was controlled at 3 sheep/hm² in the plot of grazing (G). The two plots were adjacent and had similar environment until 2003. We investigated the growth and physiological traits of <em>C. microphylla</em> and the basic characteristics of the herbaceous communities, and compared the temporal changes in plot EG after 5-8 years of fencing and the spatial differences between these two plots after eight years of different utilization regimes. We analyzed the response of the traits of <em>C. microphylla</em> and the composition of herbaceous community to grazing and fencing, and also the relationship between the cover of <em>C. microphylla </em>and the ‘Importance Value’ of different functional groups. (1) The coverage of <em>C. microphylla</em> decreased in the plot EG during the four years while the morphological traits showed no significant variation, indicating that <em>C. microphylla</em> population declined after grazing exclusion. The decline of <em>C. microphylla </em>reflected a decrease of population coverage instead of the growth status of individual modules. Moreover, leaf nitrogen and phosphorus content, especially the latter, decreased after grazing exclusion, which might be associated with the decline of <em>C. microphylla</em> population. (2) Herbaceous community composition showed significant change after eight years of grazing exclusion. Annuals disappeared from the community, while the Important Value of perennial bunch grasses increased significantly in the plot EG both in temporal and spatial comparison. The change of community structure indicated that grazing exclusion had a positive effect on the restoration of <em>C. microphylla</em>-dominated grassland. (3) The coverage of <em>C. microphylla</em> decreased with the increasing Importance Value of perennial bunch grasses, indicating that there existed a competition between <em>C. microphylla</em> and the perennial bunch grasses. The decline of <em>C. microphylla</em> population might be caused by the expansion of well-grown perennial bunch grasses. It is suggested that grazing exclusion impose an effect on the restoration of <em>C. microphylla</em>-dominated community, but re-establishment of the typical steppe with climax community needs at least eight years; in other words, the restoration of <em>C. microphylla</em>-dominated grassland to its original status is a long-term process.