Elton’s diversity resistance hypothesis first advanced the notion that more diverse communities are likely to be more capable of resisting invasion. More recently, increased functional diversity as well as functional similarity of natives to invaders has also been suggested to promote invasion resistance. Teasel (Dipsacus laciniatus) is a common invader of prairies and roadsides. Because teasel is often active in early spring and fall, we hypothesized that cool season prairie grasses and forbs would most effectively suppress Teasel invasion and be stronger competitors because they are functionally similar to Teasel. To test whether functional similarity and diversity have any influence on suppressing Teasel seedling growth or on ability of prairie species to grow in the presence of Teasel, we planted five functional monocultures (warm season forbs, cool season forbs, legumes, C₃ grasses, and C₄ grasses) and three different types of functionally diverse combinations of these groups of prairie plants in a greenhouse pot experiment.  After two teasel seedlings were introduced to half of the pots in each treatment group, they were grown for a total of ten weeks. Teasel plants were harvested and half of each pot was clipped, dried, and weighed to estimate prairie community biomass.

Results/Conclusions

We found that functional monocultures containing C₄ grasses were the most effective at suppressing Teasel growth, while legumes and cool season forbs were the poorest competitors. Prairie biomass where Teasel was absent was consistently higher than in pots where Teasel was present for all 8 community groups, with no difference in the impact of teasel presence across groups. Prairie biomass was highest in communities of C₄ monocultures and mixtures consisting of C₄ grasses, legumes, and warm season forbs. Overall, functional diversity or similarity were not important for resisting Teasel invasion.