Kin recognition is density-dependent and uncommon among temperate grassland plants

Some plant species may frequently encounter neighbourhoods composed of genetically closely related individuals. Such species should benefit from an ability to recognize kin and to display cooperative behaviour towards closely related neighbours. While kin recognition has been demonstrated in plants, previous studies have only examined single species in single environmental settings, making it impossible to draw conclusions about the prevalence of this behaviour in plants and its consequences in different competitive environments. We investigated the ability of eight co-occurring plant species to distinguish between sibling and nonsibling neighbours and to avoid competition with close relatives at different neighbour densities. Apparent sibling recognition and cooperative behaviour towards siblings were only observed in Trifolium repens: with increasing neighbour density, plants grown among siblings significantly increased investment to seed production at the expense of allocation to competitive organs, while plants grown among unrelated individuals did not display such a response. One more species (Lychnis flos-cuculi) exhibited differences in specific leaf area in sibling and nonsibling groups, but other traits remained unchanged. The effect of genetic relatedness on plant traits always depended on neighbour density. Our results suggest that kin recognition may not be a common phenomenon in temperate grasslands and that it is more likely to occur in species experiencing frequent sibling interactions because of localized seed dispersal and/or clonal propagation.