Ions in mangroves

Abstract

Halophytes are plants which complete their life cycle in an environment which has a high salt content and may be contrasted with glycophytes which are plants that are intolerant of such high salt levels. Mangroves are classed as halophytes as they are trees and shrubs that grow primarily in saline habitats. The definition is vague in two important aspects, namely the nature of the salts involved and their concentrations. For species such as Rhizophora stylosa which grows in seawater, the concentration of salt water content is set by the composition of sea water (Table 1). Higher concentrations will naturally occur during periods of high insolation between tides and lower concentrations during periods of rainfall. Other species, such as Aegiceras corniculatum, appear to prefer less saline habitats. Table 2 shows some data on the effect of salinity on growth of five species of mangrove. It is clear that there are some discrepancies in this data but that for these species at least there is some evidence for salt mediated growth. This does not mean that these mangroves require salt for successful growth and most mangroves probably grow reasonably well in freshwater. Indeed, it has been suggested by Snedaker (1979) that freshwater is a physiological requirement and salt water is an ecological requirement. The former prevents excess respiratory losses and the latter prevents invasion and competition of non-halophytes. It may be reasonable therefore to assume that most mangroves are not obligate halophytes.