灌木入侵与放牧干扰对中亚白草种群大小及其分布的影响

Effects of shrub invasion and grazing disturbance on population size and distribution of Pennisetum centrasiaticum

为了解荒漠草原草地灌木入侵与放牧干扰对中亚白草(Pennisetum centrasiaticum)种群大小及分布的影响,在宁夏盐池县杨寨子自然村自由放牧状态下的人工柠条(Caragana intermedia)林地内部由西向东设置3个分区,分别代表3个连续分布的植被类型区,即保留的原始草地、林草过渡带和人工林地。在人工林地围栏封育处理基础上,采用样方法,分灌丛内外,调查并分析中亚白草种群特征与土壤养分、地表羊粪量、林下或林间生境条件的关系。结果表明:(1)从草地分区到林地分区,白草种群的数量持续下降,在群落中的地位不断降低,由建群种过渡为伴生种,直至成为林地中的偶见种。(2)放牧主要通过直接采食作用降低林间白草的盖度和多度,间接驱动了白草向林下的聚集;但在剔除放牧作用后,白草在林下和林间的分布关系发生逆转。(3)柠条密度差异、围栏内外放牧强度差异以及灌丛内外保护作用的不同是决定白草种群大小与分布的潜在重要因素。(4)对于以白草为优势种和建群种的沙化草地而言,适度放牧可以维持相对稳定的多样性和生产力结构功能;持续放牧干扰下,人工柠条的进入,可以通过直接保育效应和资源效应,在局部空间上起到对白草的保护作用,但柠条对环境的改造作用(旱化、土壤质地和结皮等),使生境越来越不适宜白草种群发展,种群破碎化加剧(不连续分布),其对放牧干扰的弹性也不断降低,最终导致了柠条和放牧综合作用下白草种群在柠条林地中地位、分布的下降,直至消失。

To understand the effect of shrub invasion and grazing disturbance on the population size and distribution of Pennisetum centrasiaticum.Three sites comprising original grassland,transition zone of shrub-grassland and artificial shrub forest were selected under free grazed shrub forest in Yang zhaizi village within Yanchi country,Ningxia,northern China.The population features of Pennisetum centrasiaticum,soil nutrient,faeces counts and habitat conditions were surveyed at each site in grazed and fenced plots.We analyzed the relationship between population characteristic and variables environment factors.The results showed that：（1） From grassland to woodland,the number of Pennisetum centrasiaticum individuals declined,indicating alteration of its status within community from the dominant species to the associated species or rare species.（2） The coverage and abundance decreased in the area without shrub cover by the direct sheep feeding,this was an indirect process driving the aggregation of Pennisetum centrasiaticum individuals under the coverage of shrub.However,this distribution pattern changed under the fenced area in comparison to that in grazed area.（3） Potential factors in determining the population size and distribution pattern were density of shrub,intensity of grazing and protective effect of shrub.（4） For the desertified grassland dominated by Pennisetum centrasiaticum.The relatively stable diversity and production would be maintained under the moderate grazing condition.When the shrub invasion succeeded,the population of Pennisetum centrasiaticum could be protected by shrub through effect of conservation and resources enriching under a continuous grazing disturbance condition.However,due to the severe soil drought and occurring of soil crust associated with the development stage of shrub,the habitat would be harmful for the development of Pennisetum centrasiaticum population.Finally,fragmentation of population and declining in resistance of disturbance made the alteration of Pennisetum centrasiaticum status with community.