Photosynthetic profiles in tree canopies: an evaluation of optimisation theories

Forest canopies display extensive structural and physiological modifications among their leaves as a result of biotic (competition and leaf turnover strategies) and environmental (dramatic within-canopy variation in light availability, interactions between light, temperature, humidity and wind) factors that vary within canopies. The most important environmental factor with respect to photosynthesis is light, in both quantity and quality. Reponses to light gradients are well characterized empirically but the underlying processes are poorly understood.

Concepts of optimality yield predictions of optimal behavior that can provide useful means for analysing the functional relationship between driving variables responsible for generating an adaptive trait or pattern of acclimation. However, discrepancies between measured and optimally predicted photosynthetic profiles abound in the literature. Various hypotheses have been put forward to explain these discrepancies, including plant economics, missing structural and physiological constraints and/or incorrect objective functions. Our work aims to first synthetize the main optimality theories, their objective functions, constraints, the scale at which they have been applied and tested (individual leaf, canopies or community) and classify correlated traits/constraints/functions and limitations. Our second aim is to highlight their complementarities and/or differences and advocate that photosynthetic canopy profiles cannot be understood in isolation. We show that viewing plant function as an integrated whole and testing multiple hypotheses that shape foliage structure, chemistry and physiological function simultaneously enables a more realistic evaluation of competing theories of optimal function in photosynthetic canopy profiles.