The influence of salinity on the productivity of four plant species occurring or potentially occurring in Suisun March was investigated with laboratory measurement of growth, photosynthetic responses and competitive interactions, and field measurements of water relations, growth and carbohydrate reserves. Special emphasis was placed on the responses of Scirpus robustus and its competitors because of its importance as a waterfowl food source. The results showed that photosynthesis and growth were much more reduced by increased salinity in Scirpus than in Spartina foliosa, Salicornia virginica or Cotuls cornupifolia. However, at low salinities Scirpus had the highest growth rates. Competition studies showed a strong shift in competitive advantage from Scirpus at low salinities to Salicornia at high salinities. Field studies showed that at most sites where Scirpus occurs, there is a strong seasonal change in salinity from nearly fresh water in the spring to high summer salinity levels. The rapid growth of Scirpus at the low salinities present during the spring may give it a competitive advantage over Salicornia. Scirpus also exhibited considerable tolerance to high salinities--defined here as an ability to survive even though growth was completely stopped. This tolerance may be important, since high summer salinities could prevent encroachment of less salt tolerant but highly competitive and less desireable species such as Scirpus acutus or Typha latifolia. We conclude that management plans designed to enhance Scirpus productivity should put as much weight on maintaining an annual cycle of salinity as reducing the yearly average salinity.