A comparison of methods to estimate photosynthetic light absorption in leaves with contrasting morphology

Accurate temporal and spatial measurements of leaf optical traits (i.e., absorption, reflectance and transmittance) are paramount to photosynthetic studies. These optical traits are also needed to couple radiative transfer and physiological models to facilitate the interpretation of optical data. However, estimating leaf optical traits in leaves with complex morphologies remains a challenge. Leaf optical traits can be measured using integrating spheres, either by placing the leaf sample in one of the measuring ports (External Method) or by placing the sample inside the sphere (Internal Method). However, in leaves with complex morphology (e.g., needles), the External Method presents limitations associated with gaps between the leaves, and the Internal Method presents uncertainties related to the estimation of total leaf area. We introduce a modified version of the Internal Method, which bypasses the effect of gaps and the need to estimate total leaf area, by painting the leaves black and measuring them before and after painting. We assess and compare the new method with the External Method using a broadleaf and two conifer species. Both methods yielded similar leaf absorption estimates for the broadleaf, but absorption estimates were higher with the External Method for the conifer species. Factors explaining the differences between methods, their trade-offs and their advantages and limitations are also discussed. We suggest that the new method can be used to estimate leaf absorption in any type of leaf independently of its morphology, and be used to study further the impact of gap fraction in the External Method.