Quantifying potential abiotic drivers of the nurse-plant effect in two dominant shrub species of the northern Chihuahuan Desert

Aggregations of plants surrounded by areas without vegetation cover in dryland ecosystems are thought to arise when larger plants facilitate the recruitment and/or performance of smaller "protege" plants--a phenomenon referred to as the "nurse-plant" effect. While numerous drivers can generate a nurse-plant effect, efforts to quantify multiple drivers simultaneously are rare. After verifying a higher density of proteges beneath the foundational shrubs Larrea tridentata and Prosopis glandulosa, multiple potential mechanisms underlying the nurse-plant effect were quantified in the Chihuahuan Desert of southern New Mexico. Comparisons of properties under shrub canopies relative to unvegetated interspaces revealed significantly greater concentrations of soil nutrients and lower photosynthetically active radiation and soil temperatures beneath shrubs but a consistently higher soil moisture in interspaces despite a greater water holding capacity in soils beneath shrubs. Nutrient concentrations were greater, on average, in soils beneath P. glandulosa than L. tridentata but protege plant numbers did not significantly differ among the species. Further, in L. tridentata and P. glandulosa, canopy size was positively related to levels of understory shading, and canopy size of P. glandulosa was also positively related to soil nitrogen and microbial biomass. Results of this study suggest that a majority of the variance in the abiotic nurse-plant effect of this low-latitude system is explained by radiation interception and the concomitant reduction in temperatures experienced by protege plants as opposed to direct effects of shrubs on soil water availability. As global change pressures intensify in drylands, a loss of perennial plant cover through mortality or dieback in canopies could have substantial, negative effects on soil biogeochemical pools and plant diversity. Additional quantification of the spatial and temporal variance in different mechanisms driving the nurse-plant effect across environmental and climatic gradients is needed to improve our understanding of plant community dynamics in dryland ecosystems.