Does plant trait plasticity explain the relative competitive ability of plant species along a N:P stoichiometric gradient?

Abstract

The balance between nitrogen and phosphorus is an important determinant of plant species competition and therefore for species diversity. To see whether the competitive advantage of certain species can be explained by plasticity in plant functional traits a greenhouse experiment was conducted. For this experiment two N-specialists, two P-specialists and two opportunistic species were grown at with five N:P supply ratios (i.e. 1,7; 5; 15; 45; 135) at three overall nutrient supply levels (i.e. low, medium, high). The intermediate N:P supply ratio and the slightly N-limited treatment have the highest total biomass. Under extreme nutrient limitation all species produce significantly less biomass, performing better under N-limitation than under P-limited conditions. The results indicate that opportunistic species overall have the lowest success ratio, while the overall success ratio of N-specialists is highest. Plant trait values are significantly affected by nutrient supply level, species and N:P supply ratio. Overall most interactions effects are significant, indicating the importance of inclusion of the effect of nutrient level as well as the N:P ratio. In general all species show plasticity in traits, but not for each trait. Best performing species was Alopecurus pratensis (i.e. a N-specialist), for this species phenotypic plasticity was only found for the leaf length. This, as well as the outcome of the regression analysis, indicates that phenotypic plasticity in this selection of traits (i.e. leaf length, SLA and SRL) does not explain the success ratio of a species.