Fungal endophytes of cool-season grasses are recognized for the production of compounds that protect their hosts against herbivores aboveground. Furthermore, fungal endophyte effects on soil biotic interactions have also been detected. However, little is known about their role in plant competition in the context of biological invasions. Can cool-season grass endophytes act as a “novel weapon” driving their host’s invasibility, and, if so, what soil biotic interactions may be involved? In a greenhouse study, we measured the effects of Neotyphodium coenophialum, a fungal endophyte of tall fescue (TF), an exotic cool-season grass, on competitive interactions between TF and 12 plant species co-existing in an old-field community in Ontario, Canada. Specifically, using soil from the old-field, we grew all plants either alone or together with TF, either infected (E+) of not (E-) with the endophyte. The 12 competitors were selected in a balanced manner based on functional group (family) and geographic origin. This consisted of two pairs of grasses, forbs and legumes, with each pair including an exotic, from the same range as TF in Europe, and a native species, from North America (NA). We measured biomass, arbuscular mycorrhizal fungal (AMF) colonization and root lesions as well as AMF (18S) and bacteria (16S) diversity using MiSeq.
We found that TF tended to be a stronger competitor against European forbs when the endophyte was present. In contrast, TF was equally competitive against all NA plants, regardless of the endophyte’s presence. The competitive effect of TF on competitors was significantly more variable among NA than European plants, depending on the endophyte and plant functional group. For example, NA legumes experienced stronger growth reductions when competing with E+ TF, compared to their European counterparts. When TF grew alone, the endophyte reduced its host’s mycorrhizal colonization and root lesions by 26% and 12%, respectively. Such negative effect on mycorrhizal colonization was alleviated when E+ TF grew with other plants, irrespective of their geographic origin. However, geographic origin was important because, when competing with E+ TF, mycorrhizal colonization of NA species was significantly more reduced than that of European species. In contrast, E+ TF caused reductions in root lesions among all the competitors, irrespective of geographic origin. In conclusion, our results suggest that a non-native fungal endophyte of a cool-season grass may have significant effects on plant competition, and act as a “novel weapon” capable of reducing mycorrhizal colonization of co-occurring native, but not, exotic species.