Uniform crops
You’ll get across this document some bigleaf or layered canopies, as well as some lumped and sunlit and shaded leaf fractions… So let’s clarify first what all of this is about.
Bigleaf and layered canopies
Fig. 1 Wheat as an example of a crop having horizontally-uniform canopy. The prism represents leaf area variation across \(x\), \(y\) and \(z\) axis.
Crops such as wheat have canopies that vary weakly horizontally (Fig. 1, left) and may hence be assumed uniform on the horizontal basis whereby the only change in leaf area is assumed to occur over the vertical axis (Fig. 1, right).
In some cases, the vertical variability of leaf area is not of big concern the modelers. For these cases, the canopy is presented as it where a bigleaf (Fig. 2 - center). In contrast, when the vertical variability in leaf area matters, crop canopies are represented as a succession of leaf layers (Fig. 2 - right), that’s when we use the term layered canopy.
Fig. 2 Illustration of how a crop canopy may be represented following bigleaf and layered structures.
Sunlit-shaded and lumped canopies
Fig. 3 Example of sunlit, shaded and lumped leaves.
With a crop canopy, there are leaves that are subject to the direct (beam) solar irradiance, that are called sunlit, while there are leaves that are casted by those sunlit leaves, that are called shaded (Fig. 3, left).
In some cases, crop modelers are interested in distinguishing between these two leaf categories. This is particularly the case when photosynthesis is simulated over an hourly basis (take a look at de Pury and Farquhar (1997) for example). Therefore, henceforth, we call a canopy sunlit-shaded when the latter leaf categories (or fractions) are explicitly accounted for when simulating irradiance absorption by that canopy. In contrast, a canopy is called lumped, when no distinction is made between its leaf fractions.
Fig. 4 The 4 possible uniform canopy representations considered in our package.
Combining the leaf scale (bigleaf vs layered) and the leaf fraction (sunlit-shaded vs lumped) we obtain 4 possible canopy representations as illustrated in Fig. 4.