Abstract
Competition is a major factor structuring plant communities and controlling their productivity. The functional similarity between the interacting species and the context resource availability are assumed to be most critical factors that modulate the strength, sign, and outcome of plant competition, yet their roles and interactions are subjected to debate. In a glasshouse experiment, we constructed monocultures and bi-specific cultures of three common perennial grasses of Mediterranean drylands, the short grass Brachypodium retusum and the tussock grasses Stipa tenacissima and Lygeum spartum, and investigated how the functional similarity between these species modulate their interactions and culture productivity under contrasting levels of water availability. Regardless the degree of functional similarity between the interacting species, B. retusum consistently exhibited a greater competitive ability than the other two species, followed by L. spartum, and with S. tenacissima behaving as the weakest competitor. Bi-specific cultures of B. retusum and either L. spartum or S. tenacissima produced higher biomass than the average biomass of the respective monocultures (i.e. overyielding), whereas the combination of the most similar species, L. spartum—S. tenacissima, which exhibited the highest competition symmetry (i.e., the more similar mutual impact), did not show any significant overyielding. Higher water availability increased productivity and promoted transgressive overyielding for the most dissimilar species, B. retusum and L. spartum, which however exhibited intermediate competition asymmetry. This study calls attention to the thin line between differences in functional traits and competition asymmetry that could eventually lead to either competitive exclusion or resource partitioning and coexistence.
Highlights
Competition has been since long considered the most significant interaction structuring plant communities [1,2,3] and thereby controlling ecosystem function [4]
The difference in final height between the plants growing in the bi-specific cultures and in the respective monospecific cultures did not significantly vary from zero (Fig 1, panel B), except for S. tenacissima under frequent watering, which showed a negative effect of the interaction with L. spartum, and for B. retusum under reduced watering, which showed a positive effect of the interaction with S. tenacissima, as compared with the respective intraspecific interaction effect
The competitive effect of L. spartum was higher on S. tenacissima than on B. retusum, and the competitive response of B. retusum was stronger against S. tenacissima than against L. spartum
Summary
Competition has been since long considered the most significant interaction structuring plant communities [1,2,3] and thereby controlling ecosystem function [4]. Functional similarity and competitive symmetry control productivity in grass mixtures collection and analysis, decision to publish, or preparation of the manuscript
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