Plant structure creates a highly heterogeneous environment in space and time in which vegetative elements exist, which results in corresponding heterogeneity in physiological function. Faithfully capturing this heterogeneity through direct measurement is difficult because of the inability to resolve sharp gradients; representing this heterogeneity in models is complicated by computational cost, as well as the difficulty in obtaining necessary data for model parameterization, inputs, and validation.
In this work, we have developed new modeling and measurement techniques that allow for detailed, leaf-level measurements of canopy structure that can be directly fed into a new 3D modeling framework to explore interactions between canopy structure and function. Novel algorithms were developed to use terrestrial LiDAR scanning data to measure the distribution of leaf orientation, leaf area, leaf size, and ultimately to produce robust leaf-by-leaf reconstructions of entire canopies. These reconstructions are directly assimilated into a new 3D biophysical modeling framework to predict leaf-scale radiation absorption, microclimate, transpiration, and photosynthesis. The modeling framework utilizes graphics processing unit (GPU) acceleration to be able to efficiently simulate heterogeneity at scales ranging from leaves to canopies. The above approach was used to examine how structure-induced heterogeneity in biophysical processes shapes emergent collective behavior of plant systems.
Results/Conclusions
The temporal evolution of the probability distributions of leaf temperature, radiation absorption, transpiration rate, and photosynthetic assimilation rate were examined, along with their corresponding response to canopy structure. Although visual observation of plant systems suggests two relatively distinct regimes - sunlit and shaded – the modeled distributions were generally continuous and monotonic with only one clear peak. It was found that in a typical dense canopy only 10% of the leaves were responsible for up to 60% of the total daily radiation absorption, assimilated 45% of the total carbon, and were responsible for 40% of the total transpiration.
Varying canopy structure significantly altered leaf probability distributions. Reducing leaf size or leaf density created a more homogeneous environment for transpiration and photosynthesis, and reduced the importance of leaf angle. Varying structure such that overall light interception was increased potentially increased or decreased water use efficiency depending on the physiological state of the plants (namely hydraulic capacity).
Overall, this work provided insights into how canopy structure augments leaf-scale heterogeneity in the microenvironment in which leaves exist, and revealed emergent behavior that can help to guide simplified model development that can represent the most important aspects of this heterogeneity with reasonable accuracy.