‘Let the concept of trait be functional'? Revisiting the individual‐level foundation of plant functional ecology

Plant functional ecology posits that functional traits affect individual fitness indirectly by affecting performance, and that the effects at the individual level scale up to the community and ecosystem levels. Within this framework, a set of ‘economic traits' has been related to resource use strategy at the interspecific level. However, few studies have assessed these patterns at the individual level. This gap of empirical knowledge hinders the integration across organization levels. We conducted a systematic review and meta‐analysis to evaluate the correlations of leaf, stem and root economic traits with growth rates at the individual level while controlling for growing conditions. Traits that exhibited the expected correlation – where a more acquisitive trait syndrome corresponded to faster growth rates – were stem‐specific density (in adult individuals), photosynthetic rate per unit leaf area, chlorophyll content per unit leaf area, and the leaf carbon‐to‐nitrogen ratio. Contrary to expectations, specific leaf area was negatively correlated with individual growth rate. Most of the leaf traits and all root traits assessed, however, did not show any correlation with the individual growth rate. The present work calls into question the common assumption of universal functionality of 'economics traits' and highlights that the functional traits framework lacks a mechanistic explanation for how patterns observed at the interspecific level scale up from the individual level.