Mycorrhizal associations, symbiotic relationships between plants and fungi, can influence the success of invasive plants. Comparing the net mycorrhizal benefits received by native and exotic host plants may help predict the success of potential invaders in novel habitats. We conducted three studies using Taraxacum officinale (common dandelion) and T. ceratophorum (native alpine dandelion) to explore this hypothesis.

1) Do mycorrhizal associations affect the success of T. officinale in novel alpine habitats? We transplanted T. officinale into alpine communities. Native dandelion density was used to estimate site quality for Taraxacum. We treated half of the transplants with a systemic fungicide to reduce mycorrhizal colonization. 2) Do net mycorrhizal benefits differ between T. ceratophorum and T. officinale? We grew seedlings and adults of both species with and without mycorrhizae in the greenhouse. Adults were exposed to full sun while seedlings were exposed to either full sun or partial shade. 3) Does the difference in net mycorrhizal benefits predict the probability of invasion by T. officinale in novel alpine habitats? We developed a conceptual model of the relationship between the difference in net mycorrhizal benefits and the probability of invasion. We tested this model using greenhouse data on the mycorrhizal benefits received by T. officinale and T. ceratophorum under full sun and partial shade conditions. We plotted the light-specific difference against the mean mycorrhizal abundance in open meadow and understory alpine habitats. We tested the model prediction by transplanting T. officinale into open meadow and understory plots.

Results/Conclusions

1) After one month in the field T. officinale plants were larger when they were colonized by fewer mycorrhizae, particularly in high-quality plots. This suggests that mycorrhizal associations negatively affect the establishment of T. officinale in novel alpine communities. 2) Mycorrhizae increased shoot biomass in T. ceratophorum adults, but not T. officinale adults. Mycorrhizae also increased water-use efficiency in T. ceratophorum seedlings and adults, but decreased water-use efficiency in T. officinale seedlings. These results indicate that mycorrhizal benefits are greater for T. ceratophorum than T. officinale. 3) The model predicted that T. officinale is more likely to invade understory habitats where mycorrhizal abundance is reduced. As predicted T. officinale seedlings had higher growth and survival in understory plots.

Overall, these experiments support the hypothesis that mycorrhizal associations can influence plant invasions. Comparing net mycorrhizal benefits in native and exotic hosts may help predict species likely to become invasive and communities vulnerable to those invasives.