Understanding clonal plant competition for space over time: a fine-scale spatial approach based on experimental communities

Aims: Competitive interactions are a determinant driver of plant community structure in temperate grasslands. In such dense vegetation cover, competition mostly occurs over free space, which conditions niche availability. Clonal growth determines how plants exploit horizontal space, by (1) exploring and colonizing free patches (guerilla form) or (2) resisting competitors through space consolidation (phalanx form), with possible intermediate strategies. Few studies have explored the dynamics of co-existing plants belonging to different clonal strategies. Models predict that guerilla forms may be advantageous during the early stages of succession, whereas phalanx forms are expected to be better com- petitors over time. We investigated whether these predictions are true under experimental conditions and explored possible mechanisms that promote clonal plant co-existence.Location: Experimental garden of the University of Rennes 1, France.Methods: We set up a large-scale mesocosm, in which we manipulated differ- ent mixtures of three clonal growth forms (guerilla, intermediate and phalanx) over 5 yr (2009–2014), which impeded all types of reproduction other than clonal spread of the initial planted individuals. We analysed the spatio-temporal dynamics of the communities using linear mixed models to compare the change in relative cover and spatial association (differentiating between associations with similar and different species) for each clonal form.Results: Guerilla forms dominated the early community, but decreased in dom- inance at later stages, at which point intermediate forms benefitted. Further- more, guerilla forms were more likely to co-occur with other guerrillas, with this pattern remaining consistent over time. In contrast, and contrary to our expectations, phalanx forms had the lowest cover throughout the experiment, and exhibited clear segregation from intermediate or other phalanx species. Intermediate growth forms between guerilla and phalanx were the most vari- able over time, displaying both consolidative and explorative patterns depending on the clonal forms of the other species which they coexisted with, suggesting plastic variation in their traits.Conclusions: Our results highlight the key role of clonal forms in driving com- petitive interactions and, hence, determining the spatio-temporal dynamics of grassland communities