Leaf nitrogen distribution and whole canopy photosynthetic carbon gain in herbaceous stands

The amount of photosynthetically-active photon flux density incident upon a leaf and the nitrogen content of that leaf strongly affect the photosynthetic carbon gain of that leaf. Therefore, the canopy structure of a stand, affecting the light climate in the canopy, and the leaf nitrogen distribution pattern in the canopy, affect the carbon gain of the whole canopy. This review discusses the results of studies directed to this problem and obtained so far in open and in dense canopies of stands of herbaceous, monocotyledonous or dicotyledonous, plants in their growing or flowering stages. It is found that the leaf nitrogen distribution pattern in the canopy is vertically non-uniform, and in dense stands more strongly so than in open stands. The leaf nitrogen distribution pattern in most canopies closely approaches an optimal pattern in that it maximizes whole canopy potential carbon gain as calculated for the actual total leaf nitrogen content and leaf area index of the stand. The resulting increase in potential carbon gain as compared to a uniform leaf nitrogen distribution pattern is considerable and it is larger in dense stands than in open stands. For at least some dense stands simulation studies show that with the available total leaf nitrogen content, whole canopy carbon gains could still be considerable higher had a lower leaf area index been developed.