A six-year grazing exclusion changed plant species diversity of a Stipa breviflora desert steppe community, northern China

Abstract

Excluding grazers is one of most efficient ways to restore degraded grasslands in desert-steppe communities, but may negatively affect the recovery of plant species diversity. However, diversity differences between grazed and fenced grasslands in desert-steppe are poorly known. In a Stipa breviflora desert steppe community in Northern China, we established six plots to examine spatial patterns of plant species diversity under grazed and fenced conditions, respectively. We addressed three aspects of species diversity: (1) The logistic, exponential and power models were used to describe the species-area curve (SAR). Species richness, abundance and Shannon diversity values change differently with increasing sampling areas inside and outside of the fence. The best fitted model for SAR was the logistic model. Excluding grazers had a significant impact on the shape of SAR. (2) Variograms was applied to examine the spatial characteristics of plant species diversity. We found strong spatial autocorrelations in the diversity variables both inside and outside the fence. After grazing exclusion, the spatial heterogeneity decreased in species richness, increased in abundance and did not change in Shannon diversity. (3) We used variance partitioning to determine the relative contributions of spatial and environmental factors to plant species diversity patterns. Environmental factors explained the largest proportion of variation in species diversity, while spatial factors contributed little. Our results suggest that grazing enclosures decreased species diversity patterns and the spatial pattern of the S. breviflora desert steppe community was predictable.

Keywords: Diversity patterns; Spatial heterogeneity; Species-area curve; Variance partitioning.

Figure 1. The landscape of study sites (The middle picture was taken using Unmanned aerial vehicle (DJIS1000 + Canon 5D Mark III)).

The fence line is clearly visible with a sharp contrast between vegetation density and cover inside and outside. Photos by Lei Wang. (A) Inside the fence; (B) the whole study site; (C) outside the fence.

Figure 2. Variograms of species richness, Shannon diversity and abundance outside and inside the fence.

The horizontal line indicates the overall variance of the variables. (A) Spatial heterogeneity of plant species richness; (B) spatial heterogeneity of plant Shannon diversity; (C) spatial heterogeneity of plant species abundance.