Peter Saenger

Mangrove ecology, silviculture and conservation

Preface. 1. Introduction: The Mangrove Environment. 2. The Mangrove Flora. 3. Adapting to the Mangrove Environment. 4. Physico-chemical Factors and Mangrove Performance. 5. Biotic Interactions and Mangrove Performance. 6. Mangrove Structure and Classification. 7. The Value of Mangroves. 8. Mangrove Silviculture and Restoration. 9. Conservation and Management of Mangroves. 10. References. Index.

Redox stratification and heavy metal partitioning in Avicennia-dominated mangrove sediments: a geochemical model

Abstract Mangrove forest sediments can provide a sink for trace metals because the mangroves create a baffle that promotes the accumulation of fine-grained organic matter-rich sediment, which is usually sulphidic due to the presence of sulphate-reducing bacteria. Direct adsorption, complexing with organic matter, and the formation of insoluble sulphides all contribute to the trapping of metals. The concentration and chemical speciation of the metals are influenced by the distribution of geochemically distinct horizons within the sediment. In horizons with a pH>7 and an Eh +100 mV (oxidation horizons), most metals are present as exchangeable or oxide-bound species. In most cores, two oxidation and two reduction horizons can be recognised, but dark mottles of low Eh ( +100 mV) sediment can be found around mangrove roots and burrows in the reduction horizons. The depth to each horizon, differs between cores and can change in response to seasonal shifts in the position of the water table. A model is presented that accounts for the development of the oxidation and reduction horizons within the Avicennia -dominated mangrove forest sediment and describes the major controls on metal cycling within the sediment.

Comparative analysis of genetic diversity in the mangrove species Avicennia marina (Forsk.) Vierh. (Avicenniaceae) detected by AFLPs and SSRs

Abstract  Avicennia marina is an important mangrove species with a wide geographical and climatic distribution which suggests that large amounts of genetic diversity are available for conservation and breeding programs. In this study we compare the informativeness of AFLPs and SSRs for assessing genetic diversity within and among individuals, populations and subspecies of A. marina in Australia. Our comparison utilized three SSR loci and three AFLP primer sets that were known to be polymorphic, and could be run in a single analysis on a capillary electrophoresis system, using different- colored fluorescent dyes. A total of 120 individuals representing six populations and three subspecies were sampled. At the locus level, SSRs were considerably more variable than AFLPs, with a total of 52 alleles and an average heterozygosity of 0.78. Average heterozygosity for AFLPs was 0.193, but all of the 918 bands scored were polymorphic. Thus, AFLPs were considerably more efficient at revealing polymorphic loci than SSRs despite lower average heterozygosities. SSRs detected more genetic differentiation between populations (19 vs 9%) and subspecies (35 vs 11%) than AFLPs. Principal co-ordinate analysis revealed congruent patterns of genetic relationships at the individual, population and subspecific levels for both data sets. Mantel testing confirmed congruence between AFLP and SSR genetic distances among, but not within, population comparisons, indicating that the markers were segregating independently but that evolutionary groups (populations and subspecies) were similar. Three genetic criteria of importance for defining priorities for ex situ collections or in situ conservation programs (number of alleles, number of locally common alleles and number of private alleles) were correlated between the AFLP and SSR data sets. The congruence between AFLP and SSR data sets suggest that either method, or a combination, is applicable to expanded genetic studies of mangroves. The codominant nature of SSRs makes them ideal for further population-based investigations, such as mating-system analyses, for which the dominant AFLP markers are less well suited. AFLPs may be particularly useful for monitoring propagation programs and identifying duplicates within collections, since a single PCR assay can reveal many loci at once.

Genetic structure at range edge: low diversity and high inbreeding in Southeast Asian mangrove ( Avicennia marina ) populations

Understanding the genetic composition and mating systems of edge populations provides important insights into the environmental and demographic factors shaping species’ distribution ranges. We analysed samples of the mangrove Avicennia marina from Vietnam, northern Philippines and Australia, with microsatellite markers. We compared genetic diversity and structure in edge (Southeast Asia, and Southern Australia) and core (North and Eastern Australia) populations, and also compared our results with previously published data from core and southern edge populations. Comparisons highlighted significantly reduced gene diversity and higher genetic structure in both margins compared to core populations, which can be attributed to very low effective population size, pollinator scarcity and high environmental pressure at distribution margins. The estimated level of inbreeding was significantly higher in northeastern populations compared to core and southern populations. This suggests that despite the high genetic load usually associated with inbreeding, inbreeding or even selfing may be advantageous in margin habitats due to the possible advantages of reproductive assurance, or local adaptation. The very high level of genetic structure and inbreeding show that populations of A. marina are functioning as independent evolutionary units more than as components of a metapopulation system connected by gene flow. The combinations of those characteristics make these peripheral populations likely to develop local adaptations and therefore to be of particular interest for conservation strategies as well as for adaptation to possible future environmental changes.

Microsatellite analysis of genetic structure in the mangrove species Avicennia marina (Forsk.) Vierh. (Avicenniaceae)

The level of genetic variation throughout the entire worldwide range of the mangrove species Avicennia marina (Forsk.) Vierh. was examined using microsatellite markers. Three microsatellite loci detected high levels of allelic diversity (70 alleles in total), essential for an accurate estimation of population genetic parameters. The informativeness of the microsatellite loci tended to increase with increasing average number of repeats. The levels of heterozygosity detected for each population, over all loci, ranged from 0.0 to 0.8, with an average of 0.407, indicating that some populations had little or no genetic variation, whereas others had a large amount. Populations at the extremes of the distribution range showed reduced levels of heterozygosity, and significant levels of inbreeding. This is not unexpected as these populations may be subject to founder effects and environmental constraints. The presence of genetic structure was tested in A. marina populations using three models: (i) a single panmictic model; (ii) the discrete subpopulation model; and (iii) the isolation by distance model. The discrete subpopulations model was supported by the overall measures of population differentiation based on the infinite alleles model (F‐statistics), and the stepwise mutation model (R statistics). In addition, an analysis of molecular variance (amova), using both theoretical models, found that most of the variation was between populations (41–71%), and within individuals in the total population (31–49%). There was little variation among individuals within populations (0–10%). There was no significant isolation by distance. The high levels of genetic differentiation observed among populations of A. marina may be due to environmental and ecological factors, particularly past sea level and climatic changes.

Characterisation and analysis of microsatellite loci in a mangrove species, Avicennia marina (Forsk.) Vierh. (Avicenniaceae)

Abstract An enriched microsatellite library of the mangrove species Avicennia marina was constructed, in which 85.8% of the clones contained microsatellite sequences. Of the microsatellite repeat sequences isolated, 55.0% were di-nucleotides, 34.2% were tri-nucleotides, 50.0% were perfect, 24.2% were imperfect, and 15.0% were compound. Four different di-nucleotide repeats were isolated with repeat lengths ranging from 5 to 33; ten different tri-nucleotide repeats were isolated with repeat lengths ranging from 3 to 25. The most common di-nucleotide was the AC/TG repeat; the most common tri-nucleotide was the CCG/GGC repeat. Sixteen microsatellite sequences were selected for primer design, and 6 primers were selected to investigate the polymorphism detected among 15 individuals of A. marina from three natural populations in Australia. A total of 40 alleles were detected at 6 microsatellite loci. The number of alleles per microsatellite locus ranged from 5 to 13. On average, 7 alleles were detected per locus. All microsatellite loci showed high levels of gene diversity (heterozygosity), with values ranging from 0.53 to 0.88; the mean value of gene diversity was 0.70. Microsatellite loci were also tested for conservation across Avicennia species. There was a decline in amplification success with increasing divergence between Avicennia species. The results indicate that microsatellites are abundant in the Avicennia genome and can be valuable genetic markers for assessing the effects of deforestation and forest fragmentation in mangrove communities, which is an important issue for mangrove conservation and afforestation schemes.

Mangrove zonation in Mobbs Bay - Australia

Zonation of mangrove vegetation at Mobbs Bay NSW was studied in relation to the physicochemical characteristics of the substrate and canopy cover at the season of seedling dispersal and establishment. The zonation pattern of mangrove species across such a low salinity site (4 to 14) was found to be a function of three environmental gradients, namely sulphide concentration in the sediments, canopy cover and height above the watertable. It is unlikely to be explained as a result of a single or particular gradient. It is suggested that the specific segregation of species is the outcome of the cumulative interaction between different environmental gradients on one hand and tolerance boundaries of each species to each particular gradient on the other.

The taxonomic relationships within the genus Excoecaria L. (Euphorbiaceae) based on leaf morphology and rDNA sequence data

The tropical Indo-Pacific genus Excoecaria L. (Euphorbiaceae) has several closely related species in Australia whose taxonomic relationships are unclear. The most widely reported species in Australia is the mangrove species Excoecaria agallocha L. (type species), whose taxonomic and geographic limits are difficult to define from its closely related species or sub-species. Two additional taxa have also been described but not clearly differentiated from the type species: Excoecaria dallachyana Baillon and Excoecaria ovalis Endl. This project aimed to determine the taxonomic relationships of the Australian Excoecaria species using both leaf morphological data and DNA sequence data from the internal transcribed spacer (ITS) region of ribosomal genes. The nucleotide differences in the examined ITS1 region show that E. agallocha from eastern Australia and E. ovalis from Western Australia respectively, are genetically uniform within species but differ from each other consistently, thus supporting species status. The leaf morphological data also support this view: single factor analysis of variance consistently separated E. ovalis from E. agallocha on the basis of leaf width, leaf length and length of petiole. In contrast, E. ovalis from the Gulf of Carpentaria differs only slightly from E. ovalis in Western Australia, but no evidence was found to suggest any leaf morphological differentiation within this species. The analysis also suggests that E. dallachyana is not closely related to either mangrove species E. agallocha or E. ovalis, despite superficial morphological similarities.

Mangroves: sustainable management in Bangladesh

Bangladesh faces two mangrove management challenges: the first is the management of the resources of the Sundarbans natural mangrove areas on a sustainable basis, while protecting the high levels of biodiversity. The second challenge is to manage the planted mangroves of the Bangladeshi shoreline under competing, and sometimes conflicting, management objectives and expectations. Management of the Sundarbans has been based on selective felling, with felling cycles and minimal DBHs adjusted for each of the main commercial species. The allowable annual cut is determined by ongoing forest inventories to ensure harvesting is equal or lower than the growth and reproduction rate. However, despite the adoption of sustainable yield, some degradation of the mangroves is occurring, the primary causes being human interference (e.g. illegal harvesting and pollution) and changed hydro-edaphic conditions (e.g. erosion and accretion, and increased soil salinity due to water abstraction). The protection from cyclone damage afforded by the Sundarbans mangrove forests, led the Forestry Department in 1966 to commence a programme of planting mangroves outside the protective coastal embankments in order to provide greater protection for the other coastal areas. Harvestable size is reached in 15–25 years, but prior to harvest, a new crop must be established, so that coastal lands are not unvegetated and liable to erosion. The mangrove greenbelt has brought many obvious benefits, but despite these benefits, there are problems which are not silvicultural, but result from population pressure; illicit felling of trees and unlawful grazing of coastal lands, threaten the mangroves and the encroachment in some areas by shrimp farms comprises a major concern. The protective benefits from the sustainable management of natural and planted mangroves in Bangladesh are beyond dispute – the minimal damage suffered by the coastal areas from the Indian Ocean tsunami on 26 December 2004 can be ascribed simply to the good condition of the natural and planted mangrove greenbelt of that country. When viewed together with the benefits that mangrove habitats bring to biodiversity conservation, it would seem to be obvious that mangrove greenbelts should be actively promoted. What has been shown in Bangladesh is that the silvicultural expertise needed has been developed.

Changes in the coastal zone of Abu Dhabi determined using satellite imagery (1972–2003)

The coastal and marine resources of Abu Dhabi have sustained the local inhabitants for thousands of years. However, rapid development following the discovery and exploitation of mineral oil reserves in the second half of the 20th Century changed the intensity and use of the natural resources. Changes in coastal geomorphology, landuse and vegetation were documented using remote sensed images covering the period from 1972 to 2003. The study indicated that there have been major geomorphologic and landuse changes within the coastal zone of Abu Dhabi as a result of urban and industrial development. Vegetation, both cultivated and intertidal, has increased within the study period. The data collected emphasises the urgency for developing sustainable coastal management strategies in Abu Dhabi and the region.