Abstract
Shrub encroachment, i.e., shrub emergence or an increase in woody plant cover, has been widely observed in arid and semiarid grasslands and savannas worldwide since the 2000s. However, until now, there has been a clear division of opinion regarding its ecological implications. One view is that shrub encroachment is an indicator of ecological degradation, and the other is that shrub encroachment is a sign of the restoration of degraded ecosystems. This division leads to completely different judgments about the states and transition phases of shrub-encroached ecosystems, which further affects decisions about their conservation and management. To determine whether ecosystems experiencing shrub encroachment are degrading or are in a postdegraded restoration stage, the spatial distributions and interactions of woody plants after shrub encroachment were investigated in this study. An Ulmus pumila-dominated temperate savanna-like ecosystem with significant shrub encroachment in the Otindag Sandy Land, Inner Mongolia, China, was selected as the research area, and woody plants were surveyed within a 25-hectare (500 × 500 m) plot. Spatial point pattern analysis was employed to analyze the distribution patterns of the woody plants. The results indicated different patterns for U. pumila trees, i.e., a random distribution pattern for old trees (with a diameter at breast height (DBH) of more than 20 cm) and aggregated distribution patterns for medium (5 cm ≤ DBH < 20 cm) and juvenile trees (DBH < 5 cm) at scales of 0–9 and 0–12 m, respectively. For most shrubs, there was significant aggregation at a scale of 0–6 m. However, there were significant negative relationships between old U. pumila trees (DBH ≥ 20 cm) and most shrub species, such as Caragana microphylla and Spiraea aquilegifolia. In contrast, there were positive relationships between juvenile trees (DBH < 5 cm) and most shrub species. These results suggest that, to some extent, shrub encroachment may have disrupted the normal succession pattern in the U. pumila community in this area, and without conservation, the original tree-dominated temperate savanna-like ecosystem may continue to deteriorate and eventually become a shrub-dominated temperate savanna-like ecosystem.
Highlights
Shrub encroachment in grasslands and the densification of woody plant cover in savannas have been widely documented across many arid and semiarid areas of the world [1,2,3,4], including in Forests 2020, 11, 1248; doi:10.3390/f11121248 www.mdpi.com/journal/forestsSouth America, Australia, and the warm deserts of the Southwestern United States
Previous studies have focused mainly on the vegetation distribution, possible encroachment mechanisms, vegetation characteristics, species composition, and carbon budget in the U. pumila-dominated temperate savanna-like ecosystem after shrub encroachment [25,26]
U. pumila-dominated temperate savanna-like ecosystem, which is distributed widely throughout the forest-steppe ecotone on the Mongolian Plateau, China, is quite different from North American prairies and African savannas in terms of climate, soils, and dominant plant functional types; there are some taxonomical similarities among these ecosystems at the genus and family levels [37,38]
Summary
Shrub encroachment in grasslands and the densification of woody plant cover in savannas have been widely documented across many arid and semiarid areas of the world [1,2,3,4], including in Forests 2020, 11, 1248; doi:10.3390/f11121248 www.mdpi.com/journal/forestsSouth America, Australia, and the warm deserts of the Southwestern United States. One view is that shrub encroachment is an indicator of land degradation that is often associated with ecosystem degradation [5], such as declines in forage productivity, biodiversity, and socioeconomic potential, as well as increased erosion [6]. An alternative viewpoint proposing that shrub emergence is a sign of the restoration of degraded ecosystems has emerged recently [7,8], considered to support the biodiversity and a variety of ecosystem services [9]. Shrub encroachment has been described as an alternative stable state occurring several times during the last two millennia in African savannas [10]. Some studies found that the role of shrub species was important in the rehabilitation of degraded sandy land ecosystems [11]. Different opinions exist as to the causes of shrub encroachment in different types of ecosystems [12]: some studies found that underuse leads to shrub and subsequent tree encroachment and, conversion to forest [12]; overgrazing usually causes shrub encroachment in Africa and the Mongolia Plateau [10]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
