Competition for resources can be a selective force that shapes plant and fungal communities. Closely related species are expected to have more similar ecologically important traits that lead them to co-occur in similar habitats and be in greater competition. We tested whether plants grown with close relatives produce experience greater competition than do those planted with distant relatives. We planted three focal crop species (rye (Secale cereale), bellbean (Vicia faba), radish (Raphanus sativus x Brassica oleracea x Raphanus maritimus)) in monoculture or in two-species combinations with crop species at a range of phylogenetic distances. We expected that above-ground biomass of the focal species would increase with phylogenetic distance to its paired crop. We also expected that the total biomass of crop combinations would be greatest when separated by the largest phylogenetic distance, because distant relatives would be more likely to show complementarity.
All plants are colonized by a great diversity of endophytic fungi, which include pathogenic, commensal, and mutualistic symbionts. Because fungi are competing for nutrients and space within the host, we expect greater competition among fungi from within a particular host species than between fungi from different hosts. We isolated fungi from each of the three focal species and grew them in malt extract agar. Then, we measured mycelial growth when grown in paired cultures with fungi from the same or from a different host species. For both plants and fungi, we calculated the Log Response Ratio (LRR) as ln(growth in mixture/growth in monoculture), where positive values indicate greater competitive ability and negative values indicate reduction in growth from competition.
Results/Conclusions
There was no consistent phylogenetic signal in biomass produced by the focal crop species when paired with other crops. In other words, competition did not seem to be stronger among closer relatives. More distantly related pairs of species did not produce more total biomass (over-yielding) than did pairs of closer relatives. Thus, complementarity also did not show a phylogenetic signal. Fungal colonists of the crops, on the other hand, showed greater reduction in growth when paired with fungi isolated from the same host species than when paired with unfamiliar fungi from distantly related crops. Competition among fungi that regularly co-occur within a host species may be important in regulating the spread and impact of pathogens within hosts, which in turn may affect the competitive ability of the host plant.