Microtopography modulates stochastic plant community assembly: links to soil resource heterogeneity and landscape connectivity

In arid ecosystems, environmental filtering is typically assumed to drive community assembly, yet the interplay between local microtopographic heterogeneity and landscape connectivity remains unclear. Here, we evaluated taxonomic, functional, and phylogenetic diversity and applied a functional β-diversity-based null model framework across windward slopes, interdune lowlands, and leeward slopes in the Mu Us Desert. We found that significant spatial variation in soil resources was accompanied by distinct patterns in multidimensional diversity. Nevertheless, stochastic processes consistently dominated community assembly (> 88%) across all microhabitats, primarily driven by homogenizing dispersal (56%–91%), which formed a shared backbone of compositional similarity across the landscape. Specifically, plants in interdune lowlands and leeward slopes were structured largely by dispersal processes, whereas windward slopes retained distinct deterministic signatures (11% homogeneous selection) and elevated contributions from undominated processes (29%). These patterns suggest that, even under environmentally stressful conditions, the observed stochastic dominance is likely supported by the combined influence of resource heterogeneity and landscape connectivity, rather than being driven solely by local filtering. Our results refine the stress-dominance hypothesis by implying that, in high-connectivity landscapes, mass effects can mask local selection pressures, providing ecological insights for integrating connectivity into dryland restoration strategies. • Dune microtopography redistributes soil water and nutrients, producing partially decoupled microsite resource gradients. • Taxonomic, functional and phylogenetic diversity varies across windward slopes, interdune lowlands and leeward slopes. • Trait-based null models indicate stochasticity dominates community assembly along the dune microtopographic sequence (>88%). • Homogenizing dispersal prevails (56–91%), while undominated and selection processes intensifying on windward slopes. • High connectivity masks environmental sorting, highlighting microtopography as a restoration lever alongside connectivity.