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

Aggregations of plants surrounded by sparsely vegetated areas in drylands can arise when larger plants facilitate the recruitment of smaller "protégé" plants-a phenomenon referred to as the "nurse plant" effect. Numerous drivers can generate a nurse plant effect; efforts to simultaneously quantify multiple drivers are rare. Given higher densities of protégés beneath the foundational shrubs Larrea tridentata and Neltuma glandulosa, multiple potential mechanisms underlying the nurse plant effect were quantified in the Chihuahuan Desert, New Mexico, USA. As expected, there was a greater concentration of soil nutrients and lower photosynthetically active radiation and soil temperatures beneath shrubs. Throughout the study, however, soil moisture was consistently higher in interspaces despite the greater water holding capacity of soils beneath shrubs. Nutrient concentrations were greater beneath N. glandulosa than L. tridentata, while protégé numbers did not significantly differ among the species. The canopy size of both species was positively related to understory shading, and the size of N. glandulosa was positively related to soil nitrogen and microbial biomass. The results of this study suggest that much of the abiotic nurse plant effect of this low-latitude system is explained by radiation interception and concomitant reductions in temperatures experienced by protégé plants as opposed to the direct effects of shrubs on soil water availability. As global change pressures intensify in drylands, a loss of perennial plant cover could have negative effects on soil biogeochemical pools and plant diversity. Quantification of the mechanisms driving the nurse plant effect across environmental and climatic gradients could improve our understanding of plant community dynamics in drylands.