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Factors affecting mangrove cover in northern Vietnam

Nguyen, Huong Thuy Thi (2022) Factors affecting mangrove cover in northern Vietnam. PhD thesis, Murdoch University.

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Vietnam, with 3,260 km of coastline, is one of the most vulnerable countries to global warming. The mangrove area in Vietnam has declined markedly in the past 60 years. Many afforestation and reforestation projects have been undertaken in Vietnam but these efforts have not always been successful. This PhD study was undertaken in Thai Binh, Thanh Hoa and Nghe An Provinces, in north-central Vietnam, where mangrove forests play a vital role in reducing the impacts of natural disasters from typhoons and storms. From an understanding of historic change in mangrove cover, the thesis explored mangrove growth, health and survival in field experiments.

Chapter 2, through a review of the literature, provides background to the research undertaken in the thesis with particular reference to (i) mangrove health and tree decline, (ii) mangrove restoration projects; and (iii) the sustainable management of mangroves. Chapter 3 used satellite imagery from 1973 to 2020 for two provinces to determine the spatial extent of mangroves, and identified factors responsible for the temporal change in mangrove cover. The drivers of change over this period were in the order of aquaculture > other land use > natural factors > afforestation. By comparing the data in project documents and government audits, it was estimated that the average rate of success of mangrove restoration programs is about 30%.
Afforestation trials (Chapter 4) were established in Thanh Hoa and Thai Binh provinces to explore two species (Sonneratia apetala and S. caseolaris), two seedling ages at planting (12-month and 18-month), shore position (upper shore and lower shore), and planting configuration (monoculture, mixed culture) on mangrove survival and growth. After 18 months, the survival rate of both species was >90%. In the upper shore site, 12-month-old seedlings grew faster than 18-month-old seedlings, but the opposite occurred in the lower shore site. For the same age of seedlings, trees in the lower shore site grew faster than in the upper shore site. Also, tree growth was influenced by planting configurations in both the upper and lower shore site.

The potential contribution of fungal pathogens to canopy decline in the afforestation trials was assessed in Chapter 5. Fungi were isolated from the main symptoms observed in the field, namely shoot dieback, and pink and black leaf spots. Using morphological and molecular methods, it was found that Neopestalotiopsis sp.1 and Pestalotiopsis sp.3 were associated with shoot dieback; Pestalotiopsis aff. humus, Pestalotiopsis aff. neolitseae and Pestalotiopsis sp.1 were associated with pink leaf spots; whilst Curvularia aff. tsudae, Pestalotiopsis sp.2 and Pestalotiopsis sp.4 were associated with black leaf spots. Pathogenicity of the main isolates was confirmed by inoculating Sonneratia apetala and S. caseolaris seedlings in a nursery.

Data on the spatial and temporal extent of mangrove cover in Thanh Hoa and Nghe An provinces should assist local authorities to better manage coastal mangrove forests in the future. The early results from the afforestation trials showed that where care is taken in site selection and planting that it is possible to achieve good outcomes in afforestation projects in Vietnam. The data will increase our knowledge of best practices for design and species selection in mangrove afforestation. Although tree survival was high, the presence of fungal pathogens may be a threat for the long-term health and survival of planted mangroves. The field trials provide a base for future long-term studies on mangrove performance in northern Vietnam.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Agricultural Sciences
Supervisor(s): Dell, Bernard, Hardy, Giles and Que, Ngo Dinh
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