Abstract
<title>Abstract</title> <p> Background and aims Biological soil crusts (biocrusts) and vascular plants represent critical biotic components of arid and semi-arid ecosystems, with their interactions playing a pivotal role in sustaining ecological stability. The direct mechanistic effect of how biocrusts regulate plant growth and development after seedling establishment remains unclear. Methods Indoor pot and field experiments were conducted to investigate the post-establishment growth performance of <italic>Stipa. bungeana</italic> under biocrust cover. Results The results show that biocrusts significantly enhanced soil nutrient availability, enzyme activities and the content of extracellular polymeric substance (EPS). More importantly, biocrusts induced root morphological responses of S. bungeana, manifested as significant increases in root length, root surface area, and root volume. Compared with bare soil, root hair length increased by 51.28% and 46.76% while the root activity decreased significantly by 60.38% and 66.69% under both conditions. Meanwhile, biocrusts drove contrasting carbon allocation patterns: the root-to-shoot ratio declined by 51.28% under indoor conditions but increased by 150.00% in the field. Nevertheless, both the biomass increased significantly. Biocrusts positively modulated the growth performance of <italic>S.bungeana</italic> primarily through soil nutrients, enzymes and optimized root traits. Biocrusts exerted direct promotive effects on S. bungeana growth ranging from 0.1828 to 0.7375, with this direct effect being more pronounced under field conditions. Conclusion This study provided a root-centered mechanistic framework for understanding biocrust-plant interactions, highlighting the critical role of root trait plasticity in mediating plant growth. These findings offer a theoretical basis for maintaining the sustainable growth of vascular plants and improving productivity in dryland ecosystems. </p>