Medusahead (Taeniatherum caput-medusae) is an exotic annual grass which is invading the Intermountain West of North America. At some sites in Central Oregon, USA, medusahead stands persist as dense monocultures, and are known to outcompete native perennials in high clay soils. However, intact native bunchgrass stands dominated by bluebunch wheatgrass (Pseudoregneria spicata) show at least some levels of resistance to medusahead invasion, especially on steep north facing slopes.
Although some aspects of the invasability of medusahead and resistance of bunchgrass steppe are known, questions still remain concerning the exact mechanisms of the interactions. Mycorrhizal fungal associations affect plant competition and community assemblage, and could potentially influence the above ground interactions observed in this system.
We collected root specimens from the Clarno Unit of the John Day Fossil Beds National Monument in Central Oregon and observed the colonization of medusahead and bluebunch wheatgrass roots by mycorrhizal fungi. Bluebunch wheatgrass roots were separated into groups based on species identity of neighboring plants as well as root morphological type. Clearing and staining of the roots with KOH and trypan blue allowed us to quantify percent root colonization by mycorrhizal fungi.
While vertically extending bluebunch wheatgrass roots were colonized by AMF at an average of 60 percent, shallow, horizontally extending roots were rarely colonized. No evidence of AMF colonization was found in the roots of medusahead. Dark septate endophytes colonized the roots of both species. Our results suggest that medusahead may be influencing the soil fungi community in neighboring bunchgrasses, including an increase in percent colonization by dark septate fungi and slight decrease in percent colonization by arbuscular mycorrhizal fungi. This alteration of the root associated fungi community in bluebunch wheatgrass could potentially alter the outcome of the plant-fungi relationship, leading to altered competitive interactions between plants.