Evolution of rarity and phylogeny determine above- and belowground biomass in plant-plant interactions

Rare species are often considered inferior competitors due to occupancy of small ranges, specific habitats, and small local populations. However, the phylogenetic relatedness and rarity level of interacting species in plant-plant interactions are not often considered when predicting the competitive response of rare plants. We used a common garden of 25 species of Tasmanian Eucalyptus , varying in rarity to allow us to differentiate the competitive abilities of rare versus common species when grown in mixtures varying in phylogenetic relatedness and rarity. We demonstrate increased biomass production of rare plant species when interacting with genetically intermediate neighbors through synergistic non-additive effects not seen in common species. Additionally, we also find that all plants, regardless of rarity status, maintain 47% greater aboveground and 69% greater belowground biomass when interacting with common species compared to the rarest species. However, species-specific interactions with one particular common species, E. globulus , yielded a 97% increase in biomass compared to average biomass yields in other interactions, suggesting the importance of E. globulus integration into rare species restoration plantings. These results are important because they suggest that the evolutionary processes driving species rarity and the phylogenetic divergence of traits interact to drive ecological dynamics of plant-plant interactions in non-additive ways. Through the ecological and evolutionary consideration of performance traits, rarity, and species-specific effects, we can more accurately predict plant-plant interaction dynamics varying in rarity and relatedness across the landscape.