The relative importance of biotic filtering reduces with aridity and shrub encroachment in Caragana microphylla shrublands

Abstract

Background and aims

Exploring plant community assembly mechanisms is of central interest to infer community dynamics and succession in the context of global change and intensive human activities. Our aim was to understand the grassland community assembly mechanisms and how species’ functional compositions and groups might change under ongoing climate change and shrub encroachment.

Methods

Here, we used standardized effect sizes (SESs) of mean pairwise distance (MPD) to evaluate community functional trait and phylogenetic relatedness patterns and infer how both aridity and shrub cover influence assembly mechanisms in the Inner Mongolia Steppe, China.

Results

Community multi-trait patterns shifted from convergence to stochasticity as aridity increased. Increasing shrub cover directly decreased the convergence of community multi-trait patterns and decreased the presence of perennial grasses as well as CWM_LDMC. Both aridity and shrub cover affected the community LDMC patterns indirectly by negatively regulating the soil nitrogen (N) content. A concave relationship between the soil N content and the community LDMC patterns indicated that abiotic filtering, niche differentiation and biotic filtering dominate community assembly at lower, medium and higher soil N contents, respectively.

Conclusion

Our results imply that the weakening of biotic filtering and enhancement of stochastic processes with the increasing aridity or shrub cover are driven by losing soil nitrogen and perennial grass which includes the dominant competitors.