Competitive hierarchy for four common old-field plant species depends on resource identity and availability

Competition is an important factor structuring plant communities. Competitive hierarchies may facilitate prediction of the outcome of interactions among multi-species mixtures, but their utility depends upon invariance along environmental gradients. We examined interactions among four common old-field herbaceous dicots (Dactylis glomerata, Festuca elatior, Trifolium pratense, Plantago lanceolata) with pairwise monoculture and polyculture competition experiments at two levels each of light and water. We constructed competitive hierarchies for unique treatment combinations based on competitive effect and response scores. Biomass production and relative yield were affected by presence and identity of neighbor species and inter- and intraspecific competition were typically not distinguishable. Competitive effect and response scores were inversely correlated, but depended on treatment factor as well as levels within factors. Competitive rankings among these four common plant species varied along resource gradients, although the extreme ends of the hierarchy were relatively stable: under most conditions P. lanceolata was most competitive whereas F. elatior was least competitive. Results lay the groundwork for assessments of multispecies mixtures under field conditions along broader environmental gradients.