Decoupling of aboveground and belowground intraspecific trait variation along richness gradients in plants

How intraspecific trait variation (ITV) responds to species richness remains an unresolved question in community ecology, with two competing expectations: the niche packing hypothesis predicts ITV convergence, while the competitive hierarchy hypothesis predicts divergence. Given distinct competitive mechanisms for light versus soil resources, aboveground and belowground ITV likely exhibit differential responses to species richness. However, empirical research has predominantly focused on aboveground traits, leaving a critical gap in understanding simultaneous aboveground-belowground responses.

This study addresses these gaps through a three-year controlled seedling experiment in Tiantong, Zhejiang, China, constructing experimental communities across species richness gradients (1, 2, 4, 8 species) and systematically measuring 11 key aboveground and belowground functional traits.

Our results reveal an aboveground-belowground decoupling pattern in which plants responded to increasing species richness through greater ITV in traits associated with the aboveground leaf-economics axis, but exhibited more complex belowground responses. Aboveground ITV of mean leaf area and specific leaf area increased significantly with richness, supporting the competitive hierarchy hypothesis. In contrast, ITV of root diameter decreased significantly with richness, indicating trait-specific rather than general support for the niche packing hypothesis. Species richness also drove directional shifts in mean trait values, with aboveground traits shifting toward greater resource acquisition and belowground traits shifting toward thicker-root resource-use strategies. This decoupling indicates plants simultaneously employ divergent strategies in different organ dimensions rather than uniform responses.

The study reconciles long-standing theoretical conflicts by demonstrating that both competitive hierarchy and niche packing operate concurrently but in distinct plant compartments. These findings emphasize the necessity of integrated aboveground-belowground perspectives in explaining species coexistence in diverse communities.