Monitoring Phragmites australis increases from 1937 to 1976 in the Siyai Lagoon (Natal, South Africa) by means of air photo interpretation

Abstract

The colonization o f the Siyai Lagoon on the north coast of Natal by Phragmites australis was studied by means of  air photo interpretation. It was possible to locate and estimate P. australis areas for 1957 (0,74 ha), 1965 (1,65 ha), 1969 (1,93 ha) and 1976 (2,94 ha). Phragmites australis first inhabited the shores of the middle section o f the lagoon followed by rapid expansion in the lower section. The upper section was colonized only at its lower end by expansion from the middle section. It is suggested that P. australis was unsuccessful in this section because of competition by the  Hibiscus tiliaceus—Barringtonia racemosa  Lagoon Fringe Forest. This same community is shading out  P. australis in some places. The notable increase in the rate of advance of land and littoral vegetation into the Siyai Lagoon was caused by sugar farming activities leading to erosion and sedimentation in the lagoon. A vegetation age gradient was observed from the upper section to the mouth region. The colonization of most of the Siyai Lagoon except the immediate mouth zone by  P. australis Reedswamp and  Hibiscus tiliaceus—Barringtonia racemosa Lagoon Fringe Forest, can be expected before the turn of the century. Dredging and mechanical control of vegetation will become necessary if major open water spaces are to be maintained.