Optimization of foliage photosynthetic capacity in tree canopies: towards identifying missing constraints

Plant canopies are characterized by huge within-canopy gradients in environmental and physiological characteristics, but distributions of foliage photosynthetic capacity in plant canopies do not follow simple optimality criteria. Various hypotheses have been put forward to explain the lack of "optimality", including missing physiological and structural constraints and/or missing fundamental relationships. In this issue, Dewar et al. (2012) highlight important optimality constraints due to finite minimum leaf dry mass per unit area the plants can achieve, while Peltoniemi et al. (2012) proposes that limited acclimation of hydraulic conductance to within-canopy light conditions is a key missing factor in canopy optimality. This commentary advocates that future large-scale model exercises need to introduce more realism in model predictions both by including hydraulic controls as well as consider additional constraints shaping foliage structure, chemistry and physiological activity.