The interaction of soil surface gravel content and nitrogen deposition on the competitiveness of the invasive grasses Schismus arabicus and Schismus barbatus in the northwest Sonoran Desert

Background/Question/Methods

The exotic grasses Schismus arabicus and S. barbatus (Schismus) are winter annual invasive species to arid and semi-arid regions that have been shown to increase in cover under nitrogen addition experiments. Schismus can produce enough biomass to carry fire in arid regions when subjected to greater than 5 kg/ha/yr of anthropogenic nitrogen deposition during an average rain year. Secondary observations made during previous studies have noted that Schismus appears to be less viable in areas with higher surface gravel content (particle size 2 mm - 64 mm). We conducted a field experiment measuring the productivity of Schismus relative to native annual vegetation in sites of variable surface gravel/rock cover (0-60%) and spanning a nitrogen deposition gradient (3-9 kg/ha/yr) in the northwestern Sonoran Desert. At each site, 8m x 8m plots were established around the dominant shrub, Larrea tridentata, in areas with varying amounts of surface gravel cover. Vegetation cover and density were measured in quadrats placed on the north and south edges of the dripline of each shrub and species richness was tallied within the entire plot.

Results/Conclusions

Schismus cover and density decreased with increasing surface gravel content. The grass reached a threshold of productivity at a surface gravel cover of 10%. When data was analyzed using gravel cover classes of >10% cover and <10% cover, there was no response by Schismus to nitrogen deposition in the high gravel cover class, but a positive response of density and cover in the low cover class for sites receiving nitrogen deposition above the established critical load (5 kg/ha/yr), indicated by a significant interaction between N and gravel content. Species richness of native annual vegetation decreased with increasing Schismus cover across all sites but did not vary between low and high gravel cover. Mean annual plant cover was consistent across all sites, but sites receiving levels of nitrogen deposition above the critical load had lower species richness and lower native plant cover. This was concomitant with an increase in Schismus cover at these locations. Overall these results exhibit that the productivity of Schismus is influenced by the surface characteristics of the soil and productivity can be enhanced by anthropogenic nitrogen additions. Native annual species richness is reduced in sites with low gravel cover through competition with Schismus and the lack of microsites available to escape the competitive interaction.