PS 14-136 - Trait similarity does not influence competition in a grassland plant community

Background/Question/Methods

There are contradictory views about the impact of similarity among plants. Limiting similarity suggests that to minimize competition, a plant should differentiate itself from neighbours to limit niche overlap. However, if environmental conditions filter for certain types of plants, only similar species could possibly coexist in a given community. Here, we directly test the limiting similarity hypothesis by asking how trait similarity with neighbours affects the competition experienced by an individual. We added seedlings of five species to pots of plant communities taken from a local grassland, then followed the performance of the seedlings over the summer. We also fertilized half of the pots to test if competition due to similarity increases at high productivity. We then collected detailed functional trait data from focal plants and calculated community-weighted trait values for neighbours using trait values previously collected at the field site. We calculated similarity between focal plants and communities via (1) the absolute difference between community- and individual traits and (2) as the pairwise multivariate distance between trait values of focal plants and their respective pots using PCA. If similarity influences the strength of competition among plants, we would expect to see a positive relationship between competition and similarity.

Results/Conclusions

Overall similarity between a colonizing plant and the resident community did not impact on the strength of competition experienced at both levels of fertilization. For single traits, there also was no broad support for the concept of limiting similarity. Four of seven traits had no relationship between similarity and the strength of competition. Of the remaining three, the sign of the relationship changed with fertilization. Interestingly, although competition was generally stronger with fertilization, in two of the three models where similarity influenced competition, plants experienced reduced competition with increased similarity when fertilized. We suggest these findings challenge longstanding assumptions about the mechanisms of plant interactions, with implications for modern conceptualizations of community assembly.