Determinants of whole-plant light interception: Linking physiological function to canopy structure

Background/Question/Methods

Plants vary widely in branching architecture, leaf size, leaf angle and leaf display, but it is unclear how individual plant architectural traits affect whole-plant light interception (Qint). Because total photosynthesis is strongly related to Qint, efficient investment of biomass to maximize Qint is crucial to survival in the shade, and competition with neighbour plants.  It has been shown by our previous work that light interception efficiency (LIE, intercepted light per unit leaf area) can be explained by two traits: crown density (the ratio of leaf area to total lateral crown surface area) and leaf dispersion (a measure for the randomness of the leaf distribution in 3D). However, that work was applied to isotropic diffuse radiation, which is relevant only to isolated plants. Here, I extend the previous model to account for the angular distribution of available light, as in the case of small plants growing under canopy gaps of varying sizes. To do this, I use simulations with a new implementation of a detailed 3D model (YplantQMC) on over 1800 digitized plants (the largest digitized plant database to date) combined with simplified modelling. Intensive simulations using these realistic digital mock-ups of real plants were used to evaluate alternative simplified model formulations. 

Results/Conclusions

In addition to crown density and leaf dispersion, which together explain Qint of isolated plants, crown shape and leaf angle were found to explain substantial additional variation in Qint across all simulated plants (For canopy gaps of 10 degrees wide, R2 of simple model with four variables = 0.85, with only the first two R2 = 0.5). I also found an interaction with crown density and crown shape and leaf angle : for sparse crowns (low crown density and low self-shading), leaf angle was much more important than crown shape. For dense crowns, the opposite was true. Because most of the plants in the database had very low crown density, it can be concluded that leaf angle is more important than crown shape in determining Qint and hence understorey success.  These results can be used to evaluate the efficiency of alternative plant architecture designs, and suggest that few simple metrics of canopy structure can explain whole-plant rates of light interception and photosynthesis.