Abstract
Ulmus pumila-dominated temperate savanna is an important tree-grass complex ecosystem in the Otindag sand land, northern China. To date, few investigations have been undertaken on the spatial patterns and structure of this ecosystem and its driving factors under different grazing pressures. The objective of our study therefore is to explore whether grazing has affected the population structure/pattern of woody plants and shrub encroachment in a temperate savanna ecosystem. Results indicate that species richness and seedlings decreased with increasing grazing pressure. An increase in grazing pressure did not significantly affect adult-tree density, but it hindered the normal regeneration of U. pumila seedlings, further inducing population decline. U. pumila seedlings had a more significant aggregated distribution than juvenile or adult trees. The adult and juvenile trees had an aggregated distribution at the small scale and a random distribution at the large scale. Shrubs also showed a significant aggregated distribution. No clear effect on the spatial patterns of adult trees was observed; however, there was a noticeable effect for juveniles and seedlings under different grazing pressures. U. pumila seedlings had a positive association with their juveniles and Spiraea aquilegifolia, but a negative association with Caragana microphylla. Shrub encroachment occurred with decreasing grazing pressures. In conclusion, overgrazing led to the decline of U. pumila population, but the decrease in grazing pressure increased shrub encroachment in the temperate savanna ecosystem. Moderate grazing management may be a better way to enhance the stability of U. pumila population and reduce shrub encroachment.
Highlights
Savanna patchy ecosystems are predominantly structured by complex interactions between the dominant trees and their herbaceous plants found beneath or around trees [1,2,3]
Because vegetation in fragile habitats is sensitive to environmental change and prone to degradation, U. pumila-dominated savanna in the Otindag sand land has been seriously degraded due to climate change
An increase in rangeland grazing pressures in recent decades has led to serious land degradation, and even to desertification of areas in this region [51]
Summary
Savanna patchy ecosystems are predominantly structured by complex interactions between the dominant trees (or shrubs) and their herbaceous plants found beneath or around trees [1,2,3]. In China, temperate savanna ecosystems are mainly distributed in the ecotones of arid and semi-arid areas [5]. As a typical temperate savanna ecosystem, Ulmus pumila-dominated savanna is a climate climax community with a characteristic tree-grass two-phase structure that is composed of U. pumila-dominated woody plants and herbaceous understory vegetation. These ecosystems are widely distributed in the Otindag and Horqin sand lands. Public Health 2019, 16, 330; doi:10.3390/ijerph16030330 www.mdpi.com/journal/ijerph
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