The Nature of Growth Forms in the Salt Marsh Grass Spartina alterniflora

Abstract

Increases in nutrient supply convert swards of the short form of Spartina alterniflora into plants tending to resemble the tall form, in biomass/m2 and in general morphology. Competition for light is probably responsible for the vertically elongate, widely dispersed growth habit of tall form. Provision of added nutrients led to larger δ13C values, probably by the incorporation of heavier carbon from the internal air spaces of S. alterniflora during high tide, when exchange of gases with the atmosphere is prevented. The amount of interstitial water replenished at each tide is greater in areas supporting tall-form vegetation than where short form grows. There are also greater densities of mussels near creek banks, and their feeding and defecation lead to larger accumulations of particulate matter and nutrients where tall form is found. The amount of nitrogen delivered by algal nitrogen fixation does not differ greatly under the two forms. Seedling establishment usually produces tall-form plants, and a higher percentage of tall-form plants than short form produce seed. However, since electrophoretic results show no genetic differences, the form of seedlings is probably principally the result of lack of competition for nutrients and is modified by other factors. The probable sequence of events is for seedlings to become established at the edge of the banks and to grow into tall plants. If sedimentation leads to elevated marsh surfaces, as it usually does, competition by the extant plants reproducing vegetatively eventually results in short plants. This succession may be reversed by an increase in nutrient supply. This pattern over time and space is probably found in a variety of other aquatic plants.