Seasonal patterns of CO2 and water vapor exchange of the tall and short height forms of Spartina alterniflora Loisel in a Georgia salt marsh.

Seasonal patterns of the responses of net photosynthesis, transpiration, leaf diffusive conductance, water-use efficiency and respiration to temperature, light and CO2 concentration were determined on intact plants of the short and tall height forms of Spartina alterniflora. The studies were conducted on in situ plants in an undisturbed marsh community on Sapelo Island, Ga. Net photosynthesis of the tall form at full sunlight was significantly higher than the short form except during the winter months. Tall S. alterniflora did not light saturate during any season, whereas the short form tended to saturate during all seasons except the summer. The temperature optima of photosynthesis of both forms were similar and showed acclimation to prevailing seasonal temperatures. Leaf conductances to water vapor decreased with increasing temperature and were significantly different between the height forms only at higher temperatures. Dark respiration was relatively low at seasonal temperatures, but increased with temperature. Dark respiration and the respiratory Q10 of the short form tended to be slightly higher than those of the tall form during all seasons. Transpiration rates and water-use efficiency of the tall form were generally higher than the short form.The seasonal response patterns showed intrinsic differences in the capacities of the height forms to metabolize CO2 and respond to prevailing environmental parameters. Analyses of the components of the CO2 diffusion pathway suggested that metabolic or internal components were more important than stomatal factors in determining the photosynthetic patterns of the short height form. It is suggested that the observed differences in the physiological responses of the height forms of the C4 species are due to micro-habitat differences between the low and high marsh. Higher salinity, lower nitrogen availability and other soil factors may limit the CO2 and water vapor exchange capacity of the short form compared to the tall.