The Mangrove Swamp and Salt Marsh Communities of the Sydney District: II. The Holocoenotic Complex with Particular Reference to Physiography

Abstract

The holocoenotic complex operating in the Sydney mangrove and salt marsh communities is extremely intricate, with a multiplicity of inter-relationships between the component factors. The governing factor in the explanation of plant distribution is the tide-elevation-salinity influence in relation to the environmental requirements and tolerance ranges of the plant species. Plant zonation is closely associated with elevation above mean sea level, and very small differences in microtopography are involved. Tidal flooding affects plant zonation by its influence on soil salinity and probably by its action in reducing seedling establishment; its role in seed dispersal; and its effects on metabolism. Soil salinity is a consequence of the interaction of frequency of tidal inundation, evaporation and rainfall. Plant zonation is closely related to the seasonal patterns of soil salinity in each zone. Intensity and range of salinity in each zone are important, but duration of high salinity is more important in explaining zonation. Fluctuation in water table level is a tidal, rather than a seasonal, phenomenon. Water table fluctuation per se does not explain plant zonation, but the salinity of the water table is clearly related to it. Drainage and aeration are also chiefly controlled by physiography, and do not per se explain plant zonation. The dynamics of these communities are therefore largely controlled by an `allogenic' factor imposed upon them.