Facilitation in plants shifts along a recovery gradient induced by long-term overgrazing in a temperate steppe community.

INNER Mongolia (China); CHINA

Questions: Plant facilitation often dominates over competition in stressful environments. However, to our knowledge few studies have examined plant facilitation within the context of herbivore pressure. Herbivory-induced is a classic example of indirect facilitation. For example, in the temperate steppe region of Inner Mongolia, China, the intensive grazing by domestic animals has induced widespread grassland degradation. Here, we ask whether such long-term overgrazing leads to facilitation in plants. Location: Xilingol steppe region, Inner Mongolia, China. Methods: We examined the spatial structure of three dominant plant species, including two species that are palatable to sheep (Leymus chinensis and Agropyron michnoi) and one that is unpalatable (Stipa grandis). We examined whether the spatial aggregation of these three species provided evidence for facilitation by comparing their spatial patterns along a recovery gradient: high grazing pressure (HG), an eight-years grazing exclusion (early-stage recovery from grazing; ER), and a 21-years grazing exclusion (late-stage recovery; LR). To accomplish this, we used the bivariate paircorrelation function, g12(r), to assess the spatial association between unpalatable and palatable species. A positive spatial association indicates that facilitation dominates over competition (or dispersal limitation, habitat heterogeneity). We used a null model to assess the significance of spatial associations. Furthermore, we evaluated intraspecific spatial aggregation with the g11(r) by combining to three null models at fine spatial scales. Results: The spatial association between palatable and unpalatable species suggests a hump-shaped relationship along the gradient of the grazing exclusion time. Specifically, Leymus chinensis and Stipa grandis showed a neutral, positive and negative association across the three stages of natural recovery from 21 years of grazing exclusion to current grazing. Similarly, Agropyron michnoi and Stipa grandis showed a negative, very positive and positive spatial association along the same gradient. Moreover, intraspecific spatial aggregation was higher in the current grazing treatment than in the grazing exclusion treatment. This was particularly true at the finest spatial scales (i.e., <0.2 m). Conclusions: Our results suggest that facilitation between and within species along the recovery gradient in steppe communities is related to herbivore pressure induced by long-term overgrazing. First, we observed no evidence of indirect facilitation between species in current grazing treatment, while direct facilitation between plant species appears strongest at early stages of natural recovery along the gradient of the grazing exclusion time. Moreover, we observed indirect intraspecific facilitation dominated in the current grazing treatment, especially at small scales across the same gradient. [ABSTRACT FROM AUTHOR]