The distribution of leaf photosynthetic activity in a mixed grass-legume pasture canopy

The distribution of net photosynthetic activity of leaves was measured in a mixed grass (Setaria sphacelata var. sericea)-legume (Desmodium intortum) pasture stand using a method based on concurrent measurement of the rate of CO2 exchange, and (14)CO2 dosing followed by rapid harvesting according to height strata. Comparisons were also made between plots which differed in the period of regrowth following defoliation.The usual superiority of leaf net photosynthetic rates of a C4 grass, compared with C3 legume leaves, was found in the upper, well illuminated strata. These rates were, however, much lower than those usually described for horizontally exposed leaves, primarily because leaves in the pasture stand were inclined to the horizontal. At greater depth in the canopies, the superiority of rates in the grass was less evident, and consequently the relative contributions of grass and legume to canopy photosynthesis became more dependent on their leaf area indices.Attention is drawn to the relative simplicity of the method for examining the contribution of leaves, which may differ according to species or position in the canopy, to productivity of the whole stand.