Short-term nutrient dynamics in Islands of fertility created by Ambrosia dumosa (Asteraceae) and invaded by the annual grass Schismus barbatus (Poaceae)

Background/Question/Methods In deserts, small-scale spatial heterogeneity of nutrients is strongly associated with location of perennial shrubs. Numerous studies have documented higher concentration of nitrogen (N), phosphorus (P), and potassium (K) around shrubs than in open spaces. Although the phenomenon of “Islands of fertility” is well documented, we know less about nutrient dynamics around shrubs, especially in the context of grass invasions. Due to their abundance and ability to grow close to and away from shrubs, exotic grasses may alter the small-scale spatial distribution of nutrients associated with desert shrubs. The objectives of this study are to characterize the short-term spatiotemporal changes in N and P around shrubs during the invasion of the exotic grass Schismus barbatus, and to estimate the importance of those changes for maintaining spatial heterogeneity associated with desert shrubs. During 2004 and 2005 we established seed-addition field experiments to evaluate performance of Schismus at different proximities to the native shrub Ambrosia dumosa. Given that size of and distance away from shrubs control nutrients levels around shrubs, we used two seed mixtures (Schismus in monocultures and a mixture of Schismus and Ambrosia seeds), spread at four different distances from large and small Ambrosia. Pots receiving seeds at each distance contained either natural soil or sterile sand. We also included as controls pots with natural soil or sterile sand that did not receive seeds. We quantified extractable NH4+, NO3- and available P in soil samples taken in March 2007 and compared these values with pre-experimental nutrient values taken in 2004. We also analyzed differences in nutrient levels among pots receiving different treatments. Results/Conclusions We found that pre-experimental nutrient levels were significantly higher around large Ambrosia shrubs than around small shrubs; although for both groups, this trend was restricted to areas below the canopy. For all treatments, nutrient depletion was similar among shrub size classes but higher below shrubs than at open, inter-shrubs areas. In pots dominated by Schismus, we detected (1) net nutrient enrichment and (2) higher nutrient levels in areas close to shrubs, although this trend was not significant. Because every growing season we collected above and belowground biomass, we hypothesized that low nutrient requirements of Schismus rather than short-term litter decomposition explains the detected patterns. We concluded that in the long run, this effect of Schismus would act synergistically with fungal and microbial processes that concentrate around shrubs, and are essential for maintaining Islands of fertility in deserts.