Is a plant truly plastic? Nutrients and neighbours induce trait‐specific responses, but performance depends on response direction

Plants live in a heterogeneous world, where nutrient and neighbour distributions vary in space and time. Plants can respond to this variation through plastic responses in individual organs, which are assumed to be coordinated among traits to support a coherent, adaptive strategy, maintaining plant growth in varying environments. However, this assumption remains understudied. Here, we test coordination in trait plasticity and its influence on plant performance. We do so among 27 naturally co‐occurring species under four different neighbour and soil fertility contexts using data from two mesocosm experiments manipulating nutrients and neighbour presence. We focus on commonly‐studied resource‐acquisitive traits: specific leaf area (SLA), root tissue density (RTD), and specific root length (SRL). We found species exhibited plasticity in SLA, RTD and SRL 50–75% of the time and that responses to neighbours were stronger than nutrients for SLA and RTD, but not SRL. However, plastic responses were highly modular, correlated < 10% of the time, and had little coordination consistent with the trait economics paradigm. Consequences of plasticity for plant performance depended on the trait, the response direction, and whether plants were responding to nutrients or neighbours. Plasticity in RTD was unrelated to performance, but species that increased their SRL under low nutrient conditions performed worse than those showing no SRL plasticity, while SRL responses to neighbours had no effect. Similarly, species that increased SLA under low nutrients or with neighbours showed reduced performance compared to those that decreased SLA in those same conditions. Overall, plasticity in SLA, RTD and SRL rarely manifested as a correlated or coordinated response promoting growth maintenance in our study. Instead, plasticity's consequences hinged on the direction of trait shifts, sometimes incurring costs. Future studies should consider plasticity a directional phenomenon.