Historic and recent (2006) state of mangroves in small estuaries from Mlalazi to Mtamvuna in KwaZulu-Natal, South Africa

Abstract

The new forest type classification considers mangroves to be the rarest and most threatened of forest types in South Africa. The aim of this study was to determine the change in distribution and the current population and community structure of mangrove forests along the  KwaZulu-Natal coastline. Potential threats to mangroves in South Africa include wood harvesting, altered water-flow patterns coupled with salinity  changes, and the potential for permanently open-mouth conditions to become prolonged closed-mouth conditions. A literature review and field sampling were undertaken to gather information on historic and current mangrove forest status. Population structure was assessed using height and diameter at breast height in three 25 m² quadrats per site in four estuaries with mangroves. Mangroves were completely lost from 11 estuaries between 1982 and 1999. These losses can be related to changes in open-mouth conditions and intertidal habitat, which are important for mangrove survival. The mouth condition of these estuaries was observed over a nine-year period from 1996 to 2005. Mangroves only occurred in those estuaries where the mouth was open for more than 56% of the time. Rhizophora mucronata has been lost from the Mkomazi Estuary and almost completely from the Mlalazi Estuary; the distribution of this mangrove species is very limited and density is low at 0.5 ± 0.4 m^-2. Most mangrove forests were regenerating as they had inverse J-shaped curves as well as high adult:seedling ratios (1:2 for Bruguiera gymnorrhiza, 1:19 to 1:7 for Avicennia marina, and 1.8 to 3:1 for R. mucronata). None of the forests showed signs of harvesting for poles, and the greatest threat seems to be altered water-flow patterns due to freshwater use in the catchments and the change of land use from wetland to sugar-cane plantations. Effective management of these mangroves begins with determining the freshwater requirements of the estuaries to maintain the mouth dynamics and biotic communities. Further management is needed to ensure that mangroves are cleared of pollutants (plastic and industrial), and any further developments near them should be minimised. Management plans for each mangrove forest should be drafted to ensure long-term conservation of the mangroves in KwaZulu-Natal.

Keywords: Africa; Avicennia marina; Bruguiera gymnorrhiza; population dynamics; Rhizophora mucronata

Southern Forests 2009, 71(4): 287–296