Mangrove Forests of Northern KwaZulu-Natal: Sediment Conditions and Population Structure of the Largest Mangrove Forests in South Af

Abstract

The state of mangroves in the large forests of northern KwaZulu-Natal (KZN), South Africa, was assessed in terms of sediment characteristics and population structure in 2007. The tallest trees with the widest diameter at breast height were found at Mhlathuze (Avicennia marina) and Echwebeni (Bruguiera gymnorrhiza and Rhizophora mucronata) in Richards Bay. The height of trees at these estuaries increased with the age of the forest stands (2.4 m in the youngest to 14.3 m in the oldest stands). In South Africa, Ceriops tagal and Lumitzera racemosa occur under a narrow range of environmental conditions as they are only found at Kosi Bay. Avicennia marina and Bruguiera gymnorrhiza were found in all estuaries sampled in a wide range of conditions, from highly oxidised sediments (+125 to + 322.7 mV) to those that were very reduced (-360 to -35.8 mV), and from freshwater to saline conditions. Bruguiera occurred in drier habitats and tree density was negatively correlated with sediment moisture content. The density of Avicennia marina was not significantly correlated with any sediment parameter while the density of Rhizophora mucronata was positively correlated with pore water temperature. Although most forests were regenerating, indicated by their population structure (i.e. inverse, J-shaped curves), management plans are needed for each forest to ensure their long-term conservation as these mangroves have shown vulnerability to change in the past.

Keywords: Population structure, mangrove distribution, Avicennia marina, Bruguiera gymnorrhiza, Rhizophora mucronata.