Nitrogen distribution and leaf area indices in relation to photosynthetic nitrogen use efficiency in savanna grasses

Leaf photosynthetic characteristics, distribution patterns of nitrogen content per unit leaf area (n(L)) and leaf area production per unit n(L) were measured in natural stands of a C-4 grass (Hyparrhenia rufa) from the seasonal savannas and of a C-4 grass (Paspalum fasciculatum) and two C-3 grasses (Leersia hexandra and Hymenachne amplexicaulis) from the flooded savannas in central Venezuela. Daily rates of canopy photosynthesis (P-cD) as well as the optimal leaf area production per unit n(L) at which P-cD for a given total amount of nitrogen in the canopy (i.e., canopy-PNUE) is maximized were also calculated. The C-3 and C-4 species from the flooded savannas had similar light saturated rates of photosynthesis per unit n(L) (i.e. leaf-PNUE) and similar canopy-PNUEs which was in strong contrast with previous studies. Especially H. rufa but also L. hexandra and H. amplexicaulis had leaf- and canopy-PNUEs which were considerably higher than the values calculated for most other species with the same photosynthetic pathway (i.e., C-3 or C-4). In contrast to previous studies, differences in the light gradient in the canopy between stands only partially explained differences in N distribution. Measured leaf area indices were greater and the average n(L) values were consequently smaller than the calculated optima. There was, however, a very strong linear correlation between the optimal and actual average nL indicating that even though the model overestimated average n(L), it did predict the differences in leaf area production per unit nitrogen - the inverse average n(L) - very well. This result strongly indicates that leaf area production per unit of leaf nitrogen increases with leaf-PNUE and decreases with the extinction coefficient for light. Grass species from seasonal savannas have extremely high leaf-PNUEs and thus optimally produce large amounts of leaf area per unit n(L). This helps explain how stands of these species may have high leaf area indices and achieve high photosynthetic productivity despite the very low nutrient availability at which they grow.