Shifts in above‐ and below‐ground trait dissimilarity under competition mediate the future impact of neighbours

Abstract Competition drives shifts in multiple traits in plants, dynamically altering interspecific trait dissimilarity and subsequently reshaping competition intensity. Yet the directionality of trait dissimilarity changes and their effects on competition are unclear In a 3‐year greenhouse experiment with 5818 seedlings of seven tree species grown with and without neighbours, we quantified neighbour‐induced shifts in interspecific trait dissimilarity within separate above‐ and below‐ground multidimensional trait spaces and tested how these shifts influenced competition intensity, quantified using seedling biomass. We found opposing neighbour‐induced responses between above‐ and below‐ground multidimensional trait spaces: species became more similar in above‐ground traits but diverged below‐ground. These contrasting shifts were significantly associated with reduced competition intensity, but their relationships unfolded at different temporal scales. Above‐ground convergence corresponded to lower competition intensity within the same year, whereas below‐ground divergence predicted lower competition intensity in the subsequent year. Synthesis. Our findings reveal complementary above‐ and below‐ground shifts in multidimensional trait dissimilarity in response to neighbours, providing new insight into potential mechanisms by which plants deal with competitive pressure. These findings suggest a dynamic feedback loop between competition and trait expression through coordinated shifts across multiple traits, and indicate that coordinated above‐ and below‐ground trait change may contribute to plant coexistence by reducing interspecific competition.