Beyond trait distances: Functional distinctiveness captures the outcome of plant competition

Functional trait distances between coexisting organisms reflect not only
complementarity in the way they use resources, but also differences in
their competitive abilities. Accordingly, absolute and relative trait
distances have been widely used to capture the effects of niche
dissimilarity and competitive hierarchies, respectively, on the
performance of plants in competition. However, multiple dimensions of the
plant phenotype are involved in these plant-plant interactions (PPI),
challenging the use of relative trait distances to predict their outcomes.
Furthermore, estimating the effects of competitive hierarchy on the
performance of a group of coexisting plants remains particularly difficult
since relative trait distances relate to the effects of a focal plant on
another. We argue that trait distinctiveness, an emerging facet of
functional diversity that characterizes the eccentric position of a
species (or genotype) in a phenotypic space, can reveal the unique role
played by a given individual plant in a group of competing plants . We
used the model crop species Oryza sativa spp. japonica to evaluate the
ability of trait distances and trait distinctiveness to predict the
outcome of intraspecific PPI on the performance of single genotype and
genotype mixtures. We performed a screening experiment to characterize the
phenotypic space of 49 rice genotypes based on 11 aboveground and root
traits. We selected nine genotypes with contrasting positions in the
phenotypic space and grew them in pots following a complete pairwise
interaction design. Relative distances and distinctiveness based on traits
associated with light competition were by far the best predictors of the
performance of single genotypes - taller genotypes that acquired resource
faster being the best competitors - while absolute trait distances had no
effect. These results indicate that competitive hierarchy for light
dominates PPI in this experiment. Consistently, trait distinctiveness in
plant height and age at flowering had the strongest, positive effects on
mixture performance, confirming that functional distinctiveness captures
the effects of trait hierarchies and asymmetric PPI at this scale. Our
findings shed new light on the role of trait diversity in regulating PPI
and ecosystem processes and call for a greater consideration of functional
distinctiveness in studies of coexistence mechanisms.