EFFECT OF OXYGEN AVAILABILITY AND SALINITY ON EARLY-LIFE HISTORY STAGES OF SALT-MARSH PLANTS .1. DIFFERENT GERMINATION STRATEGIES OF SPARTINA-ALTERNIFLORA AND PHRAGMITES-AUSTRALIS (POACEAEI)

Gradients in oxygen availability and salinity are among the most impor tant environmental parameters influencing zonation in salt marsh commu nities. The combined effects of oxygen and salinity on the germination of two salt marsh grasses, Spartina alterniflora and Phragmites austr alis, were studied in growth chamber experiments. Germination of both species was initiated by emergence of the shoot and completed by root emergence. Percentage S. alterniflora germination was reduced at high salinity (40 g NaCl/L) and in decreased oxygen (5 and 2.5%). In 0% oxy gen shoots emerged, but roots did not. P. australis germination was re duced at a lower salinity (25 g NaCl/L) than S. alterniflora, and inhi bited at 40 g NaCl/L and in anoxia. However, a combination of hypoxia (10 and 5% O-2) and moderate salinity (5 and 10 g NaCl/L) increased P. australis germination. When bare areas in the salt marsh are colonize d, the different germination responses of these two species to combina tions of oxygen and salt concentrations are important in establishing their initial zonation. In high salinity wetlands S. alterniflora a po pulates the lower marsh and P. australis occupies the high marsh at th e upland boundary.