Summary 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.

SummaryPlant functional traits can vary widely as a result of phenotypic plasticity to abiotic conditions. Trait variation may also reflect responses to the identity of neighbours, although not all species are equally responsive to their biotic surroundings. We hypothesized that responses to neighbours are shaped by spatial community patterns and resulting variability in neighbour composition. More precisely, we tested the theoretical prediction that plasticity is most likely to evolve if alternative environments (in this case, different neighbour species) are common and encountered at similar frequencies.We estimated the frequencies of encountering different neighbour species in the field for 27 grassland species and measured the aboveground morphological responses of each species to conspecific vs heterospecific neighbours in a common garden.Responses to neighbour identity were dependent on how frequently the experimental neighbours were encountered by the focal species in their home community, with the greatest plasticity observed in species that encountered both neighbours (conspecific and heterospecific) with high and even frequency.Biotic interactions with neighbouring species can impose selection on plasticity in functional traits, which may feed back through trait divergence and niche differentiation to influence species coexistence and community structure.

Summary1. Previous studies have concluded that spatial aggregation of conspecifics should benefit weak competitors and put stronger competitors at a disadvantage, thus promoting plant species coexistence. However, if competitive ability is viewed as a behavioural trait, it becomes evident that traits determining spatial patterns and competitive ability could co-evolve, resulting in greater dispersal in stronger competitors and reduced competitive ability in spatially aggregated species. 2. To test this prediction, we combined spatial data from a field survey of seven temperate grassland communities with the results of a common-garden competition experiment involving 28 focal species. 3. We found that species exhibiting strong conspecific aggregation and infrequent heterospecific encounters in the field maintained greater growth in competition with conspecifics than with heterospecifics. In contrast, species that mostly encountered heterospecific neighbours in the field achieved greater growth when surrounded by heterospecific than conspecific neighbours, indicating greater competitive ability. The observed patterns of conspecific aggregation were related to variation in clonal dispersal characteristics and there was a direct positive relationship between clonal dispersal distance and competitive ability. 4. Synthesis. Our study demonstrates that viewing competitive ability as a behavioural trait that imposes different costs and benefits on an individual depending on the identity of its neighbours can provide new insights into the long-debated topic of mechanisms promoting plant species coexistence.

Background and aims Functional traits may underlie differences in niches, which promote plant species coexistence, but also differences in competitive ability, which drive competitive exclusion. Empirical evidence concerning the contribution of different traits to niche differentiation and the ability to supress and tolerate competitors is very limited, particularly when considering belowground interactions. Methods We grew 26 temperate grassland species along a density gradient of interspecific competitors to determine which belowground traits a) explain species' ability to suppress and tolerate neighbours and b) contribute to niche differentiation, such that species with dissimilar trait values experience reduced competition. Results We found that having larger root systems with extensive horizontal spread and lower root tissue density enabled efficient suppression of neighbours but did not significantly contribute to the ability to tolerate competition. Species with deeper root systems, lower specific root length and less branched roots were better at tolerating competition, but these traits did not significantly affect the ability to suppress neighbours. None of the measured traits contributed significantly to niche differentiation, either individually or in combination. Conclusions This study provides little support for belowground traits contributing to species co-existence through niche differentiation. Instead, different sets of weakly correlated traits enable plants to either suppress or tolerate their competitors.

When foraging and competing for below‐ground resources, plants have to coordinate the behaviour of thousands of root tips in a manner similar to that of eusocial animal colonies. While well described in animals, we know little about the spatial behaviour of plants, particularly at the level of individual roots. Here, we employed statistical methods previously used to describe animal ranging behaviour to examine root system overlap and the efficiency of root positioning in eight grassland species grown in monocultures and mixtures along a gradient of neighbour densities. Species varied widely in their ability to distribute roots efficiently, with the majority of species showing significant root aggregation at very fine spatial scales. Extensive root system overlap was observed in species mixtures, indicating a lack of territoriality at the level of whole root systems. However, with increasing density of competitors, several species withdrew roots from the periphery of foraging ranges and increased intraplant root aggregation in the remaining area, which may indicate consolidation of foraging areas under competitive pressure. Several species exhibited responses consistent with resource contest in species mixtures where encounters with competitors’ roots triggered increased root aggregation at the expense of foraging efficiency. Such responses only occurred in mixtures of species with comparable competitive abilities but were absent in asymmetric species combinations. Synthesis. Combining fine‐scale measurement of plant root distributions with spatial statistics yields new insights into plant behavioural strategies with significant potential to impact resource foraging efficiency and productivity.