Divergent Root and Shoot Trait Strategies Shape Competition–Facilitation Shifts Between Conspecific and Heterospecific Neighbors Under Contrasting Soil Moisture Conditions

ABSTRACT

Soil moisture availability is a key driver of plant growth and competitive interactions among plant species. Trait differences among species are expected to influence competitive outcomes, with stronger competition often occurring among functionally similar species due to niche overlap. However, most studies have focused on aboveground traits, whereas belowground traits involved in water acquisition remain less explored. Here, we investigate how above‐ and belowground traits vary along soil moisture gradients and how they shape plant responses to intra‐ and interspecific competition. Seedlings of four tropical legume species were grown in a glasshouse for 30 weeks under three soil moisture levels (low, medium, and high) and three neighbor treatments: intraspecific competition, interspecific competition, and no‐competition. At harvest, above‐ and belowground morphological and physiological traits, as well as biomass accumulation, were measured using standardized protocols. Plant performance was strongly influenced by competition and soil moisture. Intraspecific competition increased belowground traits, including specific root length, average root diameter, and root–shoot ratio, particularly under low moisture conditions, and resulted in greater belowground biomass. In contrast, interspecific competition enhanced aboveground traits, including leaf area, average leaf mass, and specific leaf area, leading to higher aboveground biomass and total biomass. Nodule number and nodule dry mass increased with soil moisture, whereas antioxidant enzyme activity was highest under low moisture. Relative interaction index analyses further showed stronger belowground competitive effects under intraspecific interactions and greater aboveground facilitative effects under interspecific interactions. These findings highlight that plant–plant interactions in tropical legumes are highly context‐dependent, with neighbor identity and soil moisture together influencing competitive outcomes. Divergent below‐ and aboveground investment strategies reveal how plants adjust resource allocation, shaping growth performance under changing environmental conditions.