Nurse plant shading is more important than soil fertility for dryland plant recruitment and diversity

Patches of plants in drylands can arise when larger plants facilitate recruitment via ‘nurse-plant’ effects. Simultaneously, plant–soil feedback processes can increase soil nutrients beneath shrubs leading to spatial patterns known as ‘islands of fertility’ that may also promote plant performance. These phenomena shape dryland plant communities; however, their comparative relative influences on plant density and diversity remain poorly quantified. Using reciprocal soil transplants among pairs of unvegetated interspaces and neighbouring shrubs in combination with cover treatments—open interspace, shaded interspace, intact shrub canopy and trimmed shrub canopy—this study aimed to disentangle the influences of shading from soil properties in native plant recruitment under the foundation species of shrubs Larrea tridentata and Neltuma glandulosa in the Chihuahuan Desert, NM, USA. Seedling density varied significantly across years in response to interannual climate variability and was consistently higher under L. tridentata than N. glandulosa. Cover treatments, soil provenance, shrub species and spatial factors (study blocks) significantly affected seedling density, species richness and community structure. Shading from intact shrubs or shade clothes had a stronger effect on density and richness than soil provenance or shrub species. Seedling density and richness were also higher in soil from beneath shrubs regardless of shrub species, location (native or transplanted locations) or time (study year) since initial translocation. The density of seedlings emerging from shaded interspace soil was initially similar to that observed under intact shrubs indicating the importance of shading by nurse plants for recruitment. However, the effect of shade-cloth canopy surrogates on seedling density in interspaces switched from positive to negative by the third year of the study, likely due to seed bank depletion. Synthesis. The significant influence of shading highlights the role of environmental factors in shaping plant communities of this dryland system, while the evidence of seed depletion combined with the influence of plot location suggests a concurrent influence of stochastic community assembly processes. Understanding how these processes interact to shape spatial and temporal patterns of vegetation community assembly and potentially reinforce alternate ecological states remains an important focus in dryland ecosystems.