Response of mangroves in South Africa to anthropogenic and natural impacts

Hoppe-Speer, Sabine Clara-Lisa

Title: Response of mangroves in South Africa to anthropogenic and natural impacts
Creator: Hoppe-Speer, Sabine Clara-Lisa
Subject: Mangrove forests
Subject: Climatic changes
Subject: Forest resilience
Date Issued: 2012
Date: 2012
Type: Thesis
Type: Doctoral
Type: PhD
Identifier: vital:10630
Identifier: http://hdl.handle.net/10948/d1012123
Identifier: Mangrove forests
Identifier: Climatic changes
Identifier: Forest resilience
Description: The total mangrove area cover in South Africa is 1631.7 ha, with the largest area cover in a few estuaries in the KwaZulu-Natal Province (1391.1 ha) and the remainder recorded in the Eastern Cape Province with 240.6 ha. This represents 0.05 percent of Africa‟s mangrove area cover and although small adds irreplaceable value to the biodiversity of South Africa. Mangroves are threatened by over-utilization through harvesting for firewood and building materials as well as excessive browsing and trampling by livestock. The objective of this study was to investigate the response of mangroves to different stressors from natural change as well as anthropogenic pressures. This was done by identifying pressures, measuring area cover, population structure and environmental parameters such as sediment characteristics. Mangroves in 17 estuaries along the east coast were investigated. Population structure and the area covered by mangroves in 2011/2012 were compared with data from the same area for 1999. Detailed studies were conducted in St. Lucia Estuary to investigate the response of mangroves to reduced tidal flooding; mangrove expansion at a latitudinal limit in a protected area at Nahoon Estuary was studied and the effect of cattle browsing on mangroves was measured at Nxaxo Estuary. The St. Lucia Estuary (28°S; 32°E) represented a unique study site as the mouth has been closed to the sea since 2002 and the mangrove habitats have been non-tidal. St. Lucia Estuary is both a Ramsar and World Heritage site and therefore understanding the response of mangroves to changes in the environment is important. In 2010 sediment characteristics and mangrove population structure were measured at four sites which were chosen to represent different salinity and water level conditions. The site fringing the main channel had the highest density of mangrove seedlings and saplings. The dry site had a lower density of mangroves with mostly only tall adult trees and few saplings. Mangrove tree height and density increased at sites with high sediment moisture and low surface sediment salinity. Few seedlings and saplings were found at sites with dry surface sediment and high salinity. Long term data are needed to assess the influence of mouth closure on recruitment and survival of the mangrove forest at St. Lucia Estuary; however this study has shown that sediment characteristics are unfavourable for mangrove growth at sites now characterized by a lack of tidal flooding. It is not known when exactly the mangroves were planted in Nahoon Estuary (32°S; 27° E), East London, but it is suspected that this was in the early 1970s. Avicennia marina (Forrsk.)Vierh. was planted first, followed a few years later by the planting of Bruguiera gymnorrhiza (L.) Lam. and Rhizophora mucronata (L.) among the larger A. marina trees. Surprisingly the mangrove population appears to be thriving and this study tested the hypothesis that mangroves have expanded and replaced salt marsh over a 33 year period. This study provides important information on mangroves growing at higher latitudes, where they were thought to not occur naturally due to lower annual average temperatures. It further provides insights on future scenarios of possible shifts in vegetation types due to climate change at one of the most southerly distribution sites worldwide. The expansion of mangroves was measured over a 33 year period (1978 - 2011) using past aerial photographs and Esri ArcGIS Desktop 10 software. In addition, field surveys were completed in 2011 to determine the population structure of the present mangrove forest and relate this to environmental conditions. The study showed that mangrove area cover increased linearly at a rate of 0.06 ha-1 expanding over a bare mudflat area, while the salt marsh area cover also increased (0.09 ha-1) but was found to be variable over time. The mangrove area is still small (< 2 ha) and at present no competition between mangroves and salt marsh can be deduced. Instead the area has the ability to maintain high biodiversity and biomass. Avicennia marina was the dominant mangrove species and had high recruitment (seedling density was 33 822 ± 16 364 ha-1) but only a few Bruguiera gymnorrhiza and Rhizophora mucronata individuals were found (< 10 adult trees). The site provides opportunities for studies on mangrove / salt marsh interactions in response to a changing climate at the most southern limit of mangrove distribution in Africa. This research has provided the baseline data, permanent quadrats and tagged trees to be used in future long-term monitoring of population growth and sediment characteristics. At Nxaxo Estuary (32°S; 28°E) the response of mangrove trees (Avicennia marina) to cattle browsing and trampling was investigated by using cattle exclusion plots. Exclusion plots were established by fencing in five 25 m2 quadrats and adjacent to each experimental quadrat a control quadrat (not fenced in, 25 m2) was set-up. Trees were tagged and measured annually from 2010 to 2012. Sediment salinity, pH, moisture, organic content, compaction as well as sediment particle size was also measured in each quadrat. Sediment characteristics did not vary between control and experimental plots but did show changes between the years. The mangrove trees in the cattle exclusion plots grew exponentially over a period of two years. There was a significant increase in mean plant height (5.41 ± 0.53 cm), crown volume (0.54 ± 0.01 m3) and crown diameter (7.09 ± 0.60 cm) from 2010 to 2012. Trees in the control plots had significantly lower growth (p < 0.05). There was a decrease in plant height (-0.07 ± 0.67cm1) and only small increases in crown volume (0.14 ± 0.1 m3) and crown diameter (2.03 ± 2.61 cm). The research showed that browsing on mangroves by cattle stunts growth and causes a shrubby appearance as a result of coppicing. The browsed trees were dwarfed with horizontal spreading of branches and intact foliage close to the ground while the plants in the cattle exclusion plots showed an increase in vertical growth and expansion. In the cattle exclusion plots there was a significantly higher percentage of flowering (67 percent) and fruiting (39 percent) trees in 2012 compared to the control sites where 34 percent of the plants were flowering and 5.4 percent of the plants carried immature propagules. Observations in the field also indicated that cattle had trampled a number of seedlings thus influencing mangrove survival. The study concluded that browsing changes the morphological structure of mangrove trees and reduces growth and seedling establishment. This is an additional stress that the mangroves are exposed to in rural areas where cattle are allowed to roam free. Seventeen permanently open estuaries provide habitat for mangrove forests along the former Transkei coast. This part of the Eastern Cape is mostly undeveloped and difficult to access. Mangrove area cover, species distribution, population structure and health of the mangrove habitat were compared with results from previous studies in 1982 and 1999. The mangrove Bruguiera gymnorrhiza had the densest stands and was widely distributed as it was present in 13 of the 17 estuaries. Avicennia marina was dominant in those estuaries which had the largest area cover of mangroves and was present in 10 estuaries, while Rhizophora mucronata was rare and only present in five estuaries. Anthropogenic and natural impacts were noted within the mangrove habitats in each of these estuaries. Harvesting of mangrove wood, livestock browsing and trampling and footpaths occurred in most of the estuaries (> 70 percent). It was observed that browsing on trees resulted in a clear browse-line and browsing on propagules mainly by goats resulted in reduced seedling establishment in most of the estuaries except those in protected areas. Mangroves had re-established in estuaries where they had been previously lost but mouth closure due to drought and sea storms resulted in the mass die back of mangroves in the Kobonqaba Estuary. There was a total loss of 31.5 ha in mangrove area cover in the last 30 years and this was a total reduction of 10.5 ha (11 percent) for every decade. This is high considering that the present total mangrove area cover is only 240.6 ha for all the Transkei estuaries. In this study it was concluded that the anthropogenic impacts such as livestock browsing and trampling as well as harvesting in these estuaries contributed most to the mangrove degradation as these are continuous pressures occurring over long periods and are expected to increase in future with increasing human population. Natural changes such as sea storms occur less frequently but could result in large scale destruction over shorter periods. Examples of these are mouth closure that result in mangrove mass mortality as well as strong floods which destroy forest by scouring of the banks.
Format: xiii, 174 leaves
Format: pdf
Publisher: Nelson Mandela Metropolitan University
Publisher: Faculty of Science
Language: English
Rights: Nelson Mandela Metropolitan University

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