Interspecific variation in plant competitive abilities is widespread, can influence coexistence, and remains poorly understood.  Specific traits (e.g. being tall) are commonly associated with enhanced competitive ability, while trait-similarity and phylogenetic relatedness among competitors may also influence competition.  Here we present the results of an experiment in which we disentangle the effects of phylogenetic relatedness, trait-similarity, and character states on the competitive interactions of 23 plant species.  We ask two questions (1) What traits are associated with competitive ability, and are these phylogenetically conserved, and (2) What are the contributions of character states, trait similarity, and relatedness in explaining interspecific variation in competition.  We grew 23 focal species in pots, individually and in combination with two phytometer species at two soil fertilities.  Root and shoot traits were measured on all plants, with trait-similarity measured as the absolute difference in character-states among competing plants. A phylogeny constructed from previously published data provided measures of relatedness.  Competitive ability was measured both as the ability to suppress neighbour growth (competitive effect) and the ability to withstand growth suppression (competitive response).

Results/Conclusions

Interspecific variation in competitive effect ability was explained by variation in size among the focal plants, with bigger plants being better competitors at both soil fertilities (total biomass at low fertility; leaf area at high).  None of the traits associated with competitive effect ability were phylogenetically conserved, nor did the phylogenetic relatedness among competitors influence a species’ competitive effect ability.  The results for competitive response ability were more complicated.  At high soil fertility, being taller increased the ability of a species to withstand growth suppression.  At low fertility, similarity in leaf weight among competing species enhanced competitive response ability, as did large phylogenetic distances between the competiting species.  It is unclear how increased phylogenetic distance among competitors results in enhanced competitive ability, particularly since the traits associated with competition are not phylogenetically conserved.  As a whole, these data suggest increasing a small number of trait characters would result in enhanced ability to competitively suppress ones neighbours, while the mechanisms which result in the ability to withstand competition are more complex.  There was no evidence that root traits were primarily responsible for competitive ability, even under low soil fertility.  These findings have important implications for our understanding of the evolution of plant competitive ability, mechanisms of competitive exclusion, and expected trait dispersion and phylogenetic signals in communities in which competition is occurring.