Destruction of the Phoenix / Hibiscus and Barringtonia racemosa Communities at Richards Bay , Natal , South Africa

The destruction of the Phoenix!Hibiscus and Barringtonia racemosa Communities described by Venter in 1972 on the southern shores of Richards Bay is reported. The cause was the artificial openingof a new mouth about 5,5 km south of the original mouth, which increased tidal range and salinity. These swamp communities occupied a narrow band about 6 ha in area behind the Bruguiera gymnorrhiza Community. An estimated 95 % of the communities was affected and only on the landward border were some isolated remnants of species such as Acrostichum aureum, Hibiscus tiliaceus and Phoenix reclinata detected .Young stands of Phragmites australis, seedlings of Bruguiera gymnorrhiza and Avicennia marina and epipelic algae are recoIonizing the affected area.


INTRODUCTION
The construction o f Richards Bay H a rbo u r resulted in the Richards Bay Lagoon (28°49' S and 32°05' E ) being divided in to a harbour zone and a natural sanctuary by the construction o f a berm w all and the opening o f a new m outh fo r the nature reserve during 1975.Begg (1978) reports on the many abiotic and bio tic changes occurring after the h a b ilitio n o f the Mgobezeleni Lake (near Sodwana Bay) fo llow in g the construction o f a bridge that impeded tid a l flo w and caused the swamps to be flooded.D uring studies on N atal estuaries (W ard, M S) and on the conservation p rio ritie s between Richards Bay and M lalazi M outh (Weisser, M S) the vegetation o f the southern shores was rem apped, sampled w ith eight releves and compared w ith findings o f V en ter (1972), harbour.This w ork focuses on one o f these changes affecting vegetation.
Tree m o rta lity along the Zululand Coast has been described previously.Breen & H ill (1969)   Open parts o f the destroyed zone were colonized by epipelic ( = growing on m ud) algae.
V en ter (1972) described the Phoenix!Hibiscus C om m unity p rio r to its destruction as occupying a small zone im m ediately behind the Bruguiera gymnorrhiza C om m unity at the south-eastern shore o f Richards Bay.Landw ard it bordered on the Barringtonia racemosa C om m unity (see Fig. 3  M any Phoenix reclinata trees were cut by Z u lu people living nearby to tap the sugar-rich sap.The damaged plants coppice freely fro m the base to form dense thickets. The Barringtonia racemosa C om m unity was quantitatively studied by V en te r (1972), who took samples at three places nearby: at M zingazi L a k e , west It was not possible to establish to what degree this description tru ly represents the vegetation o f the area destroyed, as some samples fo r V e n te r's analysis were taken outside the study area (e.g.M zingazi Lake).H owever, Voacanga thouarsii and Rauvolfia caffra w ould seem to have played a more dom inant role in the study area.Nephrolepis biserrata was observed as an epiphyte low down on stems o f dead trees.
The death o f the Phoenix/Hibiscus and Barringto nia racemosa Communities in the Richards Bay Sanctuary owing to the opening o f the new m outh was an unforeseen penalty that was paid fo r the opening up o f the Richards Bay H arbour and is an example o f how infra-structural developments may affect even areas that were specifically set aside as natural sanctuaries.

Fig
Fig. 1.-Remnants of Phoenix!Hibiscus and Barringtonia ra cemosa Communities on southern shore of Richards Bay destroyed by increased tidal range and salinity following opening of new mouth at Richards Bay.In background landward side of Bruguiera gymnorrhiza Zone.
reported on a mass m o rta lity o f mangroves o f the Kosi Estuary (N atal) in 1965 follow ing natural closure o f the m outh fo r five months.B ruton & A pp le to n (1975, in Begg, 1978) described the death o f a mangrove swamp at * Botanical Research Institute, Department of Agriculture and Fisheries, Private Bag X101, Pretoria, 0001."University of Durban Westville, Department of Botany, Private Bag X54001, Durban, 4000.who studied the vegetation before the harbour development.RESULTS D u ring mapping, the almost complete destruction o f 6 ha o f the Phoenix!Hibiscus and Barringtonia racemosa Swamp Com m unities (sensu V enter, 1972) in the Richards Bay Sanctuary A rea was observed.Dead trees o f up to 12 m ta ll were found and most o f the trunks o f the bigger trees were s till upright (Fig. 1).A t high spring tide water com pletely flooded the dead forest.The opening o f the new m outh in 1975 about 100 m

TABLE 1 .-Eight relevds of the Bruguiera gymnorrhiza and Phoenix!Hibiscus Communities in the study area. The values correspond to Braun-Blanquet cover-abundance estimates (Braun-Blanquet 1964). The plots 25 m2, were approximately level, with muddy substrate and usually with stagnant water (dated 1980.11.20-24)
Phoenix reclinata and Acrostichum aureum were found to occur near the southern corner o f the bay, on slightly higher parts farthest fro m the sea (Table1, releve 445).A plant that has lately increased in the affected area is Phragmites australis w hich, in places, form s dense stands between the dead trunks and debris (Fig.2and Table1,Releves 441,442,443,446).
Bruguiera gymnorrhiza seedlings and Avicennia marina saplings were observed to be colonizing the area (Table1, Releves 441 & 445).This suggests a landward extension o f the M angrove C om m unity.

Rapanea melanophloeos, Ficus trichopoda, F. capensis, Syzygium cor datum, Bruguiera gymnorrhi za and Avicennia marina.
in V enter 1972) and on prim ary or secondary Dune Forests.It was a dense th icket form ed m ainly by the palm Phoenix reclinata, Hibiscus tiliaceus and the fern Acrostichum aureum.O ther species present were The last tw o species were in poor condition and were probably remains o f the neighbouring Bruguiera gymnorrhiza C om m unity which, in the course o f tim e, was apparently displaced by the Phoenix!Hibiscus C om m unity.Clim bers were Mikania

cordata, Ipomoea cairica, I. congesta, Dioscorea sylvatica and Rhus nebulosa, the
firs t tw o species being pa rticu larly common.Field layer species recorded were Acrostichum