Abstract Question: Rocky outcrops generally restrict recruitment and survival of plant species due to their environmental conditions including low soil moisture, nutrient shortages and microclimate harshness. Under such severe conditions, nurse plants may play critical roles to preserve plant biodiversity. Although, the nurses’ effects on plant biodiversity are fully understood in different major biomes, the relative effects of abiotic factors and nurse plants as important factors have rarely been studied in the rocky outcrops, and compared to their surrounding rangelands. Therefore, we sought to assess the effects of two dominant shrub species (Artemisia aucheri Boiss. and Prunus pseudoprostrata Pojark) on all biodiversity components including taxonomic, functional and phylogenetic diversity across some environmental gradients such as precipitation and soil gradients in rocky outcrops and their surrounding rangelands. Location: The study was conducted in six sites located in rocky outcrops and their surrounding rangelands across precipitation gradient (160-910 mm) and under different soil fertility levels in northern Iran. Methods: We established 80 1m2 (1m x 1m) plots in each site with respect to presence the nurse shrubs, and measured biodiversity components within the plots using Rao Q entropies (q0, q1 and q2) and mean pairwise distance (MPD) indices. Relative interaction index (RII) was also calculated to quantify the interaction type and intensity. Finally, variation partitioning analysis was performed to determine relative importance of biotic interactions and abiotic factors such as precipitation, EC and phosphorus on structuring biodiversity components. Results: Plant-plant interactions and abiotic factors consistently influenced different components of biodiversity in rocky outcrops and their surrounding rangelands. However, nurse shrubs showed stronger effects on plant biodiversity than abiotic factors by providing suitable microenvironments to harbor less tolerant plant species under the severe conditions of outcrops. In addition, microhabitats beneath nurse shrubs significantly responded to soil electrical conductivity (EC) and precipitation gradients, with a decrease in competition intensity under intermediate levels of the gradients. In contrast, nurse shrubs negatively affected different components of biodiversity, with a decrease in biodiversity under intermediate levels of environmental gradients in surrounding rangelands. Conclusions: Our findings show critical roles of nurse shrubs to maintain plant biodiversity in rocky outcrops depending on their growth form and biodiversity component considered. In this case, nurse shrubs could reduce double environmental filtering consistent with establishing functional paradox trade-off to support a wide range of plant species with functionally divergent relatedness.

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