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