Both vacant niches and competition-trait hierarchy are useful for explaining the invasion of Caragana microphylla into the semi-arid grassland.

Aims: Trait-based coexistence theories are useful to explain community processes. The aim was to distinguish which theory played a more important role in explaining the invasion mechanism of Caragana microphylla into communities in the Eurasian semi-arid grassland region. Method: We conducted a water condition (WC) × neighbouring plant diversity (NSD) experiment, measured functional dispersion (FDis) and biomass of neighbours, absolute and hierarchical distances between neighbours and C. microphylla for plant height, specific leaf area and leaf dry matter content (LDMC), and explored the relative effect of each predictor on neighbour-effect intensity index (NIntA) on C. microphylla. Results: Significant effect of NSD was direct or mediated by cascading effects between FDis and biomass of neighbours, significant effect of WC was direct or mediated by LDMC hierarchical distance, shoot biomass of neighbours mediated indirect effects of both NSD and WC on the NIntA on C. microphylla. None of the trait absolute distances was employed to explain the variability of the NIntA on C. microphylla. Conclusions: The reduction of NSD or water could decrease the inhibitory effects of neighbours on C. microphylla by increasing vacant niches or increasing the LDMC hierarchical distance between neighbours and C. microphylla, respectively, suggesting that vacant niches and competition-trait hierarchy played more important roles than competition-trait similarity in explaining shrub invasion processes in the Eurasian semi-arid grassland region. [ABSTRACT FROM AUTHOR]