Sahara mustard (Brassica tournefortii) is one of the top exotic invasive plants spreading through the southwestern deserts of North America. In the Mojave Desert, it is common on road sides, large washes, sand dunes, and disturbed areas. Sahara mustard germinates before many of the native annuals and as a result it has a competitive advantage over soil nutrients and moisture. As a winter annual, Sahara mustard seeds germinate after winter rainfall and population densities fluctuate as a function of available soil moisture and soil type. To elucidate patterns of Sahara mustard population densities in Mojave Desert habitats, we established 5 replicate field sites in Lake Mead National Recreation Area, Nevada USA, each containing water addition, experimentally disturbed soil, and control plots. Our objectives were to document whether experimental treatments had an effect on seedling emergence, phenology, and plant density. We used a generalized randomized block design and replicated each treatment 15 times. We performed the soil disturbance treatment once, and water additions (7 liters / plot) were applied from November 2007 through January 2008 at a two week interval. In addition, we performed a soil seedbank study to make comparisons between species emerging from the soil seedbank and the above ground community. We collected 60 soil samples of the top 5 cm and placed 360 cm3 of soil for each plot in separate 1-gal plastic pots filled with 300 cm3 of soil mix. Pots were randomly placed on a bench in a temperature-controlled greenhouse, watered daily and monitored emergence for 6 months. We repeated the seedbank experiment in a germination chamber.
Results/Conclusions
At the field sites, seedling emergence was patchy. We recorded low densities in large washes and high densities in disturbed road sides. Experimental manipulations were significant in highly disturbed sites. Seedling density was higher in soil disturbed plots than in other treatments. We found a significant association between phenology and treatment. Sahara mustard did not germinate in the greenhouse or germination chamber. These data suggest that depth of Sahara mustard seed in the soil seed bank may be greater than 5 cm and soil disturbance events may aid germination by bringing seeds to the soil surface where seeds have a better chance of emergence.
