A Field Study of Photosynthetic Capacity and its Response to Nitrogen Fertilization inSpartina alterniflora

Seasonal maximum leaf photosynthetic rates (Pmax, determined in full sunlight) were compared between populations of tall and shortSpartina alterniflora(smooth cordgrass) on Sapelo Island, Georgia. Little or no differences in Pmax were found between the tall (near or on creekbanks) and the short (20–30 m from the creek) populations. All the leaves measured were light saturated at a photosynthetic photon flux >1000 μmol m−2 s−1. These results differ from those reported previously and indicate that other mechanisms besides photosynthetic capacity should be used to explain the low productivity of the short population. Limited nitrogen, high carbon cost and insufficient photosynthate allocation to build and maintain leaf area may be factors that result in low productivity. Leaf N and maximum photosynthetic rates of both tall and short populations were greatly enhanced after receiving N fertilizer. In the fertilized short population, however, elevated leaf N was diluted by a great expansion of leaf area shortly after the treatment. Expanding leaf area instead of maintaining a very high leaf N inS. alternifloraseems to have selective advantages of both enhancing leaf N use efficiency and discouraging heavy herbivory by not increasing palatability. The nitrogen use efficiency ofS. alterniflora(C4) is higher than many C3species and is strongly correlated with temperature. Each additional millimole of leaf N can increase Pmax up to 0•20 μmol CO2 m−2leaf area s−1in summer, which may explain some differences in Pmax among populations. Nitrogen use efficiency appears to decrease as the leaf N exceeds a possible optimum range of 100–120 mmol m−2leaf area.