Linking Canopy Structure and Water-Use Efficiency in Heterogenous and Anisotropic Canopies

Although our understanding of plant biophysical processes such as transpiration and photosynthesis at the leaf level has advanced rapidly due to portable infrared gas analyzers, these instruments are low throughput and produce instantaneous measurements for single leaves. Thus, it is difficult to determine how these measurements scale to the canopy level, especially in heterogeneous and anisotropic canopies. A better understanding of canopy water-use efficiency (WUE) is needed to elucidate how canopies differ in terms of carbon and water process sensitivity to changes in plant traits, management practices, and changing climate.

Models have been used to scale up leaf-level processes to the canopy level for many decades. However, the canopy is usually represented through simplified equations based on assumptions of horizontal homogeneity and often leaf isotropy. In this work, we used a detailed 3D leaf-resolving canopy model, Helios, to independently study the effect of interacting plant architectural traits on WUE. The nature of the 3D model allowed for examination of WUE in response to variation of leaf inclination independent of leaf azimuth, canopy heterogeneity, and canopy density.

The canopy response of photosynthesis and WUE to changes in leaf angle in homogenous canopies was significant in dense canopies with a high leaf area index. At midday, canopies with high leaf area index and leaves tending toward horizontal resulted in up to ~60% less photosynthesis and WUE in the lower part of the canopy compared to a spherical angle distribution. Moreover, daily integrated photosynthesis and WUE were reduced by up to ~40% and ~25%, respectively, relative to spherical leaves. Azimuthally anisotropic leaves, which are rarely accounted for, can further affect WUE. When primarily vertical leaves are biased toward E-W or N-S directions, the WUE was reduced by ~15% or increased by ~30%, respectively, compared to the azimuthally isotropic case. Furthermore, heterogeneity increased WUE as plant spacing increased, for instance, by 7% when the plant spacing was twice the plant height. Results demonstrated that leaf anisotropy can have significant impacts on both instantaneous and daily integrated WUE at the canopy level, which is enhanced when canopy density, leaf azimuthal anisotropy, or heterogeneity are increased.