The effects of environmental variables on mortality and growth of mangroves at Mngazana Estuary, Eastern Cape, South Africa

Abstract

This study investigated the effects of environmental variables on mortality and growth of three mangrove species at Mngazana Estuary—Eastern Cape, South Africa, to determine the sustainability of harvesting at this estuary. Environmental variables changed with the sediment composition in the estuary, whereby moisture content and organic matter were positively correlated to silt content. A decrease in sediment pH increased the mortality of Avicennia marina seedlings (r = −0.71, p < 0.05) and decreased the growth of Rhizophoramucronata and Bruguiera gymnorrhiza seedlings (r = −0.8, r = 0.52, p < 0.05, respectively). The mortality of B. gymnorrhiza showed a positive correlation with sediment moisture content indicating that this species prefers drier conditions. The vertical growth rate of A. marina individuals increased from seedlings (0.31 ± 0.07 cm month−1) to adults (1.2 ± 0.2 cm month−1) whereas the growth of B. gymnorrhiza was not significantly different for trees of 150 cm height and taller (0.65 ± 0.1 cm month−1). The vertical growth of R. mucronata peaked at 0.72 ± 0.1 cm month−1 at a height of 151–250 cm. If the differences in vertical growth rate for each size class were taken into account, it will take approximately 13 years for R. mucronata seedlings, the preferred species for harvesting, to grow to a height of 390 cm. This is the preferred height at which trees are selected for harvesting at this estuary. Mangroves are harvested for building material (poles) and firewood. This is 2.6 times slower than those individuals growing in Gazi Bay situated in southern Kenya, along the east coast of Africa. Data from this study indicate that it is not sustainable to harvest mangroves from the forest due to the slow (vertical and horizontal) growth rate of individuals and the length of time required for new individuals to grow to the preferred height for harvesting.