Inferring trait-based mechanisms and variance consequences of abiotic secondary dispersal from the spatio-temporal distribution of a desert soil seed bank

Abstract

Questions: Quantifying the combined effects of seed traits, microsite types, and abiotic dispersal agents on the mean and variance of seed bank density and composition will help understand vegetation dynamics in deserts. In a site with NE slope and mainly southerly winds, we asked whether large and oblong seeds were more likely to be transported and deposited in water obstructions and underneath the northern part of shrub understories. We also asked whether seed standing crop was less variable in water obstructions than in other microsites less affected by seed deposition. Location: A sand field in San Juan province, Argentina. Methods: We sampled the soil seed banks of four microsite types: water obstructions where debris/litter accumulates, southern and northern shrub understories, and open spaces among shrubs. Sampling was carried out in 11 plots and replicated at six moments between 2010 and 2012. Results: Debris microsites showed the highest levels of seed density, species richness and standing crop; seed density and standing crop were also least variable in these microsites. Debris microsites also contained high seed abundance of stress-intolerant, shrub-dependent species. Seed density, species richness, seed standing crop, and the proportions of both large and oblong seeds were all higher in northern than in southern understories. The relationship between seed mass and the proportion of seeds that were apparently retained (i.e., not washed away and deposited in water obstructions) was more strongly negative in southern than in northern understories. Conclusions: Northern and southern understories appeared to act as water run-on (i.e., seed recipients) and run-off zones (i.e., seed donors), respectively. Seed accumulation under the shelter provided by shrubs may promote seed predation in these microsites. Abiotic dispersal would therefore contribute to the maintenance of vegetation patches when seed arrival underneath shrub canopies saturates granivore consumption.