Diversity and density of the desert seed bank: Interplays between cacti and nurse shrub species

Abstract

Woody plants can modify microclimate, nutrients, and seed availability in the soil. We studied the influence of two shrub species on species diversity, and seed density in the northern Monte desert soils. We hypothesized that the presence of shrubs affects species richness and seed density. We predicted a higher richness and seed density under shrubs than open spaces between shrubs, and differences between shrubs species (Bulnesia retama and Zuccagnia punctata) influenced by variations in their attributes (crown and nutrients supply to the soil). We obtained seeds and seedlings from 20 soil samples from shrub species and open spaces. The seed bank was composed of 21 species and was dominated by perennial species (62%), mainly Cactaceae. The species richness and density were significantly higher under shrubs than open spaces, without significant differences between shrub species. The variations found among sites, samples, and years is consistent with the spatial heterogeneity of seed banks in deserts. Our study suggests a combination of the spatial heterogeneity hypothesis and the life-form hypothesis would explain the plant diversity dynamics in deserts because, for the maintenance of soil seed bank diversity, shrubs function as nurse islands among open spaces, and perennial cacti dominate the soil seed bank.