The ability of an introduced plant to successfully spread and become abundant may depend in part on its interactions with soil microbes. As nearly ubiquitous members of the soil microbial community, arbuscular mycorrhizal (AM) fungi form associations with the roots of 80% of terrestrial plants and confer varying degrees of benefit to their plant hosts, often by providing plants with resources that their roots cannot otherwise access.  Species-specific associations between plants and AM fungi are known to form under certain conditions, and invasive plants may gain a competitive advantage over native plants by cultivating particular species of AM fungi.  While knowledge of how plants, soils, and AM fungi interact is therefore essential to a full understanding of the causes and consequences of plant invasion, the role that AM fungi play in modifying the competitive ability of invasive species is not well understood.  As a first step towards understanding the role of AM fungi in influencing plant composition and restoration in California grasslands, we conducted a comparative study of fungal colonization on the roots of Nassella pulchra (a native grass) and Bromus diandrus (an invasive grass).  Using field samples taken from two sites at the UC Sedgwick Reserve, we measured the frequency of AM fungal colonization in the roots of each species of grass.   

Results/Conclusions

We found that infection was higher on the invasive Bromus diandrus than on native Nassella pulchra, and that differences in infection between the grasses were greatest under low soil phosphorus conditions.  In addition, higher infection rates were found in nitrogen rich sites known to favor the growth of invasive annual grasses.  Finally, infection rates also varied seasonally, being higher in the fall than either spring or winter.  Invasive grass persistence may be due in part to the preferential release from phosphorus limitation conferred by AM fungi.  Nitrogen rich sites may favor the growth of invasive grasses in part because AM fungal infection rates are highest in these areas.  The fact that AM fungal infection is greatest in the fall may also favor invasive annuals, which have an earlier phenology, and grow quickly from seed after the first fall rains.