Seed Demography and Seedling Survival in a Population of Atriplex triangularis Willd

Abstract

Survival of different cohorts of Atriplex triangularis in an inland saline environment was observed. Seedlings developing early in the growing season had a higher probability of surviving to fruit than those from later cohorts. In highly saline habitats no plants survived to maturity. Seed rain in the Atriplex zone averaged 32,902 seeds m2 and the seed bank containerd an average of 21,496 seeds m2 in January. Seed germinability remained high, averaging 76% in the seed bank throughout the growing season. Seed germination was observed by monitoring the seed bank and germination in field plots on a monthly basis. Seed bank studies indicated that large seeds of A. triangularis germinated by May, whereas about 10% of the small seeds persisted in the seed bank after May. INTRODUCTION The objectives of this study were to investigate the components of the life cycle of an Atriplex triangularis Willd. population in the field and to assess the conditions that influence individual survivorship and reproductive success. The time of germination, its influence on seedling survival, growth and reproductive effort were examined. Rabinowitz and Rapp (1980) recognized the importance of recording community seed rain because of its significance in understanding recruitment and regeneration in plant populations. Comparisons of the seed rain and the soil seed bank often reveal a difference between aboveand belowground composition (Kellman, 1974; van der Valk and Davis, 1976; Ungar and Riehl, 1980; Rabinowitz and Rapp, 1980). Studies of the seed banks of various habitats (Milton, 1939; Oosting and Humphreys, 1940; Major and Pyott, 1966; van der Valk and Davis, 1976; Roberts and Neilson, 1980; Ungar and Riehl, 1980; Keddy and Reznicek, 1982) have related soil content of buried viable seed to the aboveground flora. The seed bank is a source of new populations, should the existing stand be destroyed, and acts to store genetic information over several generations (Templeton and Levin, 1979). Seed banks have been categorized as transient or persistent (Thompson and Grime, 1979). In a transient seed bank, the seed output is not viable in the soil longer than 1 year. A persistent seed bank is one in which some of the viable seeds are older than 1 year. Atriplex triangularis (Chenopodiaceae) is an herbaceous annual halophyte, common in inland and coastal salt marshes throughout North America, Asia, Europe, Africa and Australia (Osmond et al., 1980). Its habitats vary in salinity from .003% -3.62% total salts (Ungar, 1965, 1973). Seed dimorphism is characteristic of a number of species of Atriplex (Becker, 1913; Kadman-Zahavi, 1955; Koller, 1957; Ward, 1967; Taschereau, 1972; Drysdale, 1973; Osmond et al., 1980). The presence of two different seed morphologies in A. triangularis was reported by Ungar (1971). Large seeds (1.4-2.5 mm diam) have a thin yellowish-brown testa; small seeds (1.1-1.6 mm diam) have a hard black testa. Yellowish-brown seeds germinate readily upon inbibition of water, but the hard black seeds require scarification to break dormancy. Large seeds may represent a short-term means of reproduction, whereas small seeds may represent a storage mechanism (Ungar, 1984). Atriplex triangularis seeds can remain viable in the soil for at least 5 years (Roberts and Neilson, 1980). MATERIALS AND METHODS The study area, approximately 10 m x 30 m, is located in Alexander Township,