Mangrove Forest Landcover Changes in Coastal Vietnam: A Case Study from 1973 to 2020 in Thanh Hoa and Nghe An Provinces

Genetic diversity and population structure of natural provenances of Sonneratia caseolaris in Vietnam

BACKGROUND

Sommeratia caseolaris is considered the most important mangrove species for reforestation and conservation programs. Therefore, the knowledge of genetic diversity and the population structure of the species has important implications both for the conservation of existing genetic resources and development programs. In the present study, the genetic diversity and structure population of eight populations of S. caseolaris from the Northern to the Southern Coast of Vietnam were determined using nine ISSR molecular markers.

RESULTS

Eight populations of the mangrove species Sonneratia caseolaris were sampled across the natural range in Vietnam to evaluate the genetic diversity of the species. Nine ISSR markers were used to analyse 30 individuals from each population. There were moderate to high levels of genetic diversity (I = 0.447; h = 0.300). PCoA analysis gave very similar results to UPGMA dendrogram construction with the eight populations clustered into three genetic groups which mostly aligned with geographical distances among them. AMOVA analysis results indicated that most (81 %) of the genetic variation was within populations.

CONCLUSION

The current study also indicates the high level of genetic variation existing among and within the natural population of S. caseolaris in Vietnam. These results open new perspectives towards the conservation of the species' genetic resources and their future use in conservation and reforestation programs.

New and Emerging Insect Pest and Disease Threats to Forest Plantations in Vietnam

The planted forest area in Vietnam increased from 3.0 to 4.4 million hectares in the period 2010–2020, but the loss of productivity from pests and diseases continues to be a problem. During this period, frequent and systematic plantation forest health surveys were conducted on 12 native and 4 exotic genera of trees as well as bamboo across eight forest geographic regions of Vietnam. Damage caused by insects and pathogens was quantified in the field and laboratory in Hanoi. The threats of greatest concern were from folivores (Antheraea frithi, Arthroschista hilaralis, Atteva fabriciella, Hieroglyphus tonkinensis, Lycaria westermanni,Krananda semihyalina, and Moduza procris), wood borers (Batocera lineolata, Euwallacea fornicatus, Tapinolachnus lacordairei, Xyleborus perforans, and Xystrocera festiva), sap-sucking insects (Aulacaspis tubercularis and Helopeltis theivora) and pathogens (Ceratocystis manginecans, Fusarium solani, and Phytophthora acaciivora). The number of new and emerging pests and pathogens increased over time from 2 in 2011 to 17 in 2020, as the damage became more widespread. To manage these pests and diseases, it is necessary to further invest in the selection and breeding of resistant genotypes, improve nursery hygiene and silvicultural operations, and adopt integrated pest management schemes. Consideration should be given to developing forest health monitoring protocols for forest reserves and other special-purpose forests.

Mangrove Dieback and Leaf Disease in Sonneratia apetala and Sonneratia caseolaris in Vietnam

Even though survival rates for mangrove restoration in Vietnam have often been low, there is no information on fungal pathogens associated with mangrove decline in Vietnam. Therefore, this research was undertaken to assess the overall health of mangrove afforestation in Thanh Hoa Province and fungal pathogens associated with tree decline. From a survey of 4800 Sonneratia trees, the incidence of disorders was in the order of pink leaf spot > shoot dieback > black leaf spot for S. caseolaris and black leaf spot > shoot dieback > pink leaf spot for S. apetala. Approximately 12% of S. caseolaris trees had both pink leaf spot and shoot dieback, while only 2% of S. apetala trees had black leaf spot and shoot dieback. Stem and leaf samples were taken from symptomatic trees and fungi were cultured in vitro. From ITS4 and ITS5 analysis, four main fungal genera causing leaf spots and shoot dieback on the two Sonneratia species were identified. The most frequently isolated fungal taxa were Curvularia aff. tsudae (from black leaf spot),Neopestalotiopsis sp.1 (from stem dieback), Pestalotiopsis sp.1 (from pink leaf spot), and Pestalotiopsis sp.4a (from black leaf spot). The pathogenicity of the four isolates was assessed by under-bark inoculation of S. apetala and S. caseolaris seedlings in a nursery in Thai Binh Province. All isolates caused stem lesions, and Neopestalotiopsis sp.1 was the most pathogenic. Thus, investigation of fungal pathogens and their impact on mangrove health should be extended to other afforestation projects in the region, and options for disease management need to be developed for mangrove nurseries.