Positioning root chemical defence functions in the root economics space across subtropical tree species

Plant roots face multiple environmental stresses in natural ecosystems, including nutrient limitation and biotic attacks, driving strategic allocation of limited resources between defence and acquisition. Yet, coordination between these two fundamental root functions remains poorly understood. Here, we quantified chemical diversity and metabolomic traits of root secondary metabolites to infer root chemical defence traits from untargeted metabolomic data, and measured classical root traits across 13 coexisting subtropical tree species. Our analyses revealed that root chemical defence varied along both dimensions of the root economics space, with coordinated changes in defence-related metabolite diversity, evenness, and metabolome aromaticity. Conservative and do-it-yourself roots had more diverse and evenly distributed defence portfolios with greater aromaticity, whereas acquisitive and outsourcing roots showed the opposite pattern. Additionally, a greater number of terpenoid and nitrogen-containing alkaloid metabolites increased in abundance in acquisitive roots. Moreover, reduced pathogen colonization in do-it-yourself and conservative roots was linked to greater richness and evenness of defence-related metabolites, independent of their cumulative abundance. These findings reveal that root defensive functions are intricately integrated with acquisition strategies, providing new insights into the multiple ecological dimensions of plant belowground strategies that affect species coexistence and ecosystem functioning.