Grazing Altered the Pattern of Woody Plants and Shrub Encroachment in a Temperate Savanna Ecosystem

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.

Does Shrub Encroachment Indicate Ecosystem Degradation? A Perspective Based on the Spatial Patterns of Woody Plants in a Temperate Savanna-Like Ecosystem of Inner Mongolia, China

“…This deduction can also be indirectly supported by the small amount and high variability of the annual precipitation in this area, which is typically 200-350 mm [37,38]. Drought has been increasing continuously in this area since 1960, and the most severe drought in recorded history in this area occurred during 1999-2011 [51]. In addition, U. pumila seedlings often suffer from severe water stress during dry summers that is caused by repeated cycles of drying in the upper soil layers [52].…”

“…This situation is likely due to the influence of grazing. Historically, the U. pumila-dominated savanna-like ecosystem of the Otindag Sand Land was an important pasture source, providing a large amount of forage for livestock [51,56]. Although precipitation and soil moisture have been the main limiting factors affecting population regeneration of woody plants in arid and semiarid areas [57], grazing has become an important factor affecting population regeneration; long-term continuous heavy grazing has resulted in the destruction of topsoil and degradation of the rangeland [58].…”

“…In this region, shrubs have occupied the grassland habitat. This encroachment has led to a decrease in grass coverage and a decline in pasture quality [51]. This change is likely to become more serious with the increase in grazing pressure and the impacts of global warming.…”

Response of soil microbial compositional and functional heterogeneity to grazing exclusion in alpine shrub and meadows in the Qinghai–Tibet Plateau

“…In addition, shrubs provide hedges for animals, causing livestock to aggregate and walk in the matrix, thereby increasing microbial inputs outside of canopies. In more harsh habitats, shrubs provide shade for animals (Zhang et al, 2019). Moreover, the impact of large farm animals (cattle, sheep, camels, and horses) on shrubs and soil microorganisms differs from that of small 10.3389/fmicb.2022.1038805 animals (wild chickens, lizards, ants, and beetles) (Liu et al, 2015;Stanton et al, 2018).…”

Multi-Temporal Evaluation of Quantitative and Phenological Vegetation Dynamics Using Sentinel-2 Images in North Horr (Kenya)

“…Due to the presence of these alien species, the general greening observed can be associated to desertification processes linked to changes in species composition and shrub encroachment [75][76][77]. The pauperization of native-species is an indicator of weak health [8] in the ecosystem, caused by the reduction of species richness and by the decrease of woody plants in favor of bush and shrub [76,78,79]. Biodiversity reduction causes the loss of different ecosystem services and, thus, is considered a form of land degradation [2,80,81].…”