Background/Question/Methods In recent years, extensive areas of the Chihuahuan Desert have undergone dramatic shifts in vegetation structure and composition due to the invasion of non-native plant species.  Our 20-year dataset has revealed a similar invasion of a previously rich community of winter annual plants in the San Simon Valley of southeastern Arizona.  This community has declined in diversity and abundance of native annuals through a sustained irruption of an exotic Eurasian species, Erodium cicutarium.  To fully understand the impacts of E. cicutarium on the native annual plants, we initiated a multi-year E. cicutarium removal experiment in 2006.  A split-plot design with twenty 240 cm x 120 cm plots were divided into 120 x 120 subplots with removal of E. cicutarium from one subplot, with the other subplot being a control.  The design included removals starting at different times, with starting times in November, December, January and February.  Removals with different starting times were from separate plots arranged in a randomized-block split-plot design.  Once a removal was initiated, complete removal was conducted once a month for all subsequent months until February.
Results/Conclusions Treatment effects on diversity, abundance and size of native annual plants were at best weak in the first year (2006-07), but preliminary results of the 2008 winter season reveal strong effects of the removal, leading to an increase in total native annual plant abundance.  Surprisingly, one native species, Astragalus nuttallianus, was not affected by the removal and maintained abundance levels as high as E. cicutarium.  The number of species was significantly higher in removal plots during mid-season (December).  These preliminary results show strong potential for influencing plant size of the native annual plants at the end of the season and may indicate long-term recovery of the native species in the absence of the invasive, E. cicutarium.  This experiment will allow us to assess the effects on germination and growth of natives and ultimately determine the stages in the life histories of the native plants most impacted by E. cicutarium.