Nurse shrubs influence plant biodiversity across environmental gradients in rocky outcrops and surrounding rangelands

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 in preserving 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 rocky outcrops, and compared to those in the surrounding rangelands. Therefore, we sought to assess the effects of two dominant shrub species (Artemisia aucheri and Prunus pseudoprostrata) 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 on six sites located in rocky outcrops and their surrounding rangelands across a precipitation gradient (160-910 mm) and under different soil fertility levels in northern Iran. Methods: We established 80 1-m(2) (1 m x 1 m) plots in each site with respect to the presence of nurse shrubs, and measured biodiversity components within the plots using Rao Q entropies (q(0), q(1) and q(2)) and mean pairwise distance (MPD) indices. The relative interaction index (RII) was also calculated to quantify the interaction type and intensity. Finally, variation partitioning analysis was performed to determine the relative importance of biotic interactions and abiotic factors such as precipitation, electrical conductivity (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 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 for maintaining plant biodiversity in rocky outcrops depending on their growth form and the biodiversity component considered. In this case, nurse shrubs could reduce double environmental filtering, consistent with establishing a functional paradox trade-off to support a wide range of plant species with functionally divergent relatedness.

Automated summary

The presence of nurse plants plays a critical role in preserving plant biodiversity in rocky outcrops, outweighing the effects of abiotic factors. However, in surrounding rangelands, nurse plants have a negative impact on plant biodiversity. These findings highlight the importance of nurse plants in maintaining plant biodiversity in rocky outcrops.