Exotic grasses have invaded natural areas, roadways, and urban areas in the western United States.  In the deserts and semidesert grasslands of southeastern Arizona exotic grasses achieve high fuel loads which alter the frequency and intensity of fire regimes causing shifts from natural plant communities to exotic grasslands.  Although the threat of increased fuel load is well known, few studies have examined the direct interactions between exotic and native grasses in the Sonoran Desert; interactions which may influence the dynamics of exotic grass populations as well as the potential for future fires.  Competition between a native perennial bunchgrass (D. californica) and two introduced perennial bunchgrasses (P. ciliare and M. repens) was examined.  In addition, two divergent populations of D. californica (accessions) were included to evaluate the effect of seed source on competitive ability.  
Results/Conclusions

A field study showed that P. ciliare is a superior competitor for resources in relation to D. californica (P < 0.05) and tolerant of depleted resource levels, while M. repens was not found to be a superior competitor (P > 0.05), but was able to tolerate depleted resource levels better (P < 0.05) than the D. californica accessions.  A greenhouse study found that while P. ciliare was able to outcompete D. californica accessions of equal age, at least one accession of D. californica was able to tolerate the presence of P. ciliare neighbors which were 21 days younger, indicating that seed source and time of establishment may play an important role in the outcome of competition.  These studies show that P. ciliare can directly outcompete D. californica when given an equal chance at establishment.  However, the initial establishment of native plants may be beneficial at reducing the effects of highly competitive exotic neighbors during the first growing season.