Molecular phylogeny of mangroves V. Analysis of genome relationships in mangrove species using RAPD and RFLP markers

 DNA from pooled leaf samples of 11 true major mangrove, three true minor mangrove, two mangrove associate, two mangrove parasite, three terrestrial and one cultivated species were isolated for the present study. In total, 198 random amplified polymorphic DNAs (RAPDs) and 180 restriction fragment length polymorphism (RFLP) loci were scored by using ten primers and 14 enzyme-probe combinations respectively. The polymorphism observed for these markers revealed a high degree of genetic diversity in mangroves at both inter-specific or inter-generic levels. A dendrogram, constructed after pooling both RAPD and RFLP data, using a similarity index was analysed for genome relationships among these species. The dendrogram showed clustering of all the major mangroves, except for Nypa fruticans (Arecaceae), into one group. All species under the tribe Rhizophorae formed a sub-cluster, to which Xylocarpus granatum was found to be the most closesly related species. The clustering pattern implied that Excoecaria agallocha and Acanthus ilicifolius should be considered as true minor mangroves. The present study also provided molecular data favouring the separation of Avicennia spp. from the Verbenaceae to create a monotypic family the Avicenniaceae. The separation of Viscum orientale into the Viscaceae was also favoured. Received: 5 March 1998 / Accepted: 1 April 1998     

Frequency of occurrence of mangrove fungi from the east coast of India

This paper describes the frequency of occurrence and biodiversity of fungi from mangroves of the Godavari and Krishna deltas, on the east coast of India. Seventy three species were identified from Godavari and 67 from the Krishna mangroves. Fifty five species were common to both sites, 18 were found only at Godavari and 12 at Krishna mangroves. Verruculina enaliawas found to be very frequent at both sites with a higher frequency of occurrence at Godavari. Eutypa bathurstensis was very frequent at Godavari but only frequent at Krishna. Cirrenalia pygmea and Cryptosphaeria mangrovei were frequent at the Godavari mangrove, but were recorded occasionally at Krishna. Decaying samples of Rhizophora and Avicennia were studied in detail. Forty three species were common to both hosts, while 22 species were recorded only from Avicennia and 20 only from Rhizophora. Verruculina enalia was the only very frequent fungus recorded on both hosts with a lower percentage occurrence (14.8%) on R. apiculata as compared to Avicennia spp. (24.3%). Eutypa bathurstensis was the next most frequent fungus on Avicennia, while Rhizophila marina was next most frequent on Rhizophora. Dactylospora haliotrepha which was recorded frequently on Rhizophorawas infrequent on Avicennia.     

Holocene mangrove dynamics and sea-level changes in northern Brazil, inferences from the Taperebal core in northeastern Pará State

A 450 cm sediment core from Taperebal, in the mangrove region of northeastern Pará State in northern Brazil has been studied through pollen analysis in order to reconstruct mangrove development and dynamics and to infer relative sea-level (RSL) changes during the Holocene. Six AMS radiocarbon dates, which provide a somewhat limited age control with some uncertainties, suggest early and late Holocene deposits interrupted by a hiatus between them. A patchy vegetation of coastal Amazon rain forest, restinga, salt marsh and some mangrove, which was dominated by Avicennia, covered the study area during the early Holocene period. The occurrence of an early Avicennia dominated mangrove phase has not been reported so far from other sites in northern Brazil. During the mid Holocene mangroves mostly replaced the former coastal Amazon rain forest, restinga and some salt marsh vegetation, reflecting the rise in the RSL. Rhizophora trees expanded markedly and Avicennia became rare. In the sediment core there is apparently a gap between the depths of 115 and 85 cm (possibly starting between 5900 and 5750 b.p.). The deposits above 85 cm are of modern age and were probably deposited during the last decades. This gap can be explained by the lowering of the RSL as is shown for other northern Brazilian coastal sites. The deposition of sediments during the last decades suggests that the modern RSL is high compared to other periods in the Holocene. Pollen data from these deposits show that Rhizophora trees dominate the mangrove forests, also indicating a high RSL.     

Regeneration and colonisation of mangrove on clay-filled reclaimed land in Singapore

A mature mangrove community was preserved at Pasir Ris during the reclamation of its foreshore by maintaining a channel (river) so that the plot could be subjected to inundation frequency of 40 to 50 times per month. By subjecting the embankments of the river and another plot of vacant reclaimed land adjacent to the mature community to a similar frequency of inundation, new mangrove communities comprising Avicennia spp and Sonneratia alba were regenerated. When the depth of the mangrove soil increased proportionately with time, succession by another seral community (Rhizophora spp) occurred. Factors affecting the development of the mangrove soil and its nutrient accumulation over time, and the regeneration of new mangrove communities through colonisation by different plant and animal species are discussed.     

Fungal diversity and plant disease in mangrove forests: salt excretion as a possible defense mechanism

Keywords. Salt excretion in leaves of some mangrove species may serve as an important defense against fungal attack, reducing the vulnerability of typically high-density, monospecific forest stands to severe disease pressure. In field surveys of a Caribbean mangrove forest in Panama, Avicennia germinans suffered much less damage from foliar diseases than did Laguncularia racemosa or Rhizophora mangle. Similarly, Avicennia leaves supported the least superficial fungal growth, endophytic colonization, and diversity, followed by Laguncularia and Rhizophora. Host specificity of leaf-colonizing fungi was greater than expected at random. We hypothesize that the different salt tolerance mechanisms in the three mangrove species may differentially regulate fungal colonization. The mangroves differ in their salt tolerance mechanisms such that Avicennia (which excretes salt through leaf glands) has the highest salinity of residual rain water on leaves, Laguncularia (which accumulates salt in the leaves) has the greatest bulk salt concentration, and Rhizophora (which excludes salt at the roots) has little salt associated with leaves. The high salt concentrations associated with leaves of Avicennia and Laguncularia, but not the low salinity of Rhizophora, were sufficient to inhibit the germination of many fungi associated with mangrove forests.     

Plant-mediated methane emission from an Indian mangrove

Mangroves have been considered for a long time to be a minor methane source, but recent reports have shown that polluted mangroves may emit substantial amounts of methane. In an unpolluted Indian mangrove, we measured annual methane emission rates of 10 g CH₄ yr^?1 from the stands of Avicennia marina. This rate is of the same order of magnitude as rates from Northern wetlands. Methane emission from a freshwater‐influenced area was higher, but was lower from a stunted mangrove growing on a hypersaline soil. Methane emission was mediated by the pneumatophores of Avicennia. This was consistent with the methane concentration in the aerenchyma, which decreased on average from 350 ppm_v in the cable roots to 10 ppm_v in the emergent part of the pneumatophores. However, the number of pneumatophores varied seasonally. The minimum number occurred during the monsoon season, which reduced methane emissions largely. Ebullition from unvegetated areas may also be important, at least during monsoon season when measured bubble fluxes were occasionally about five times as high as pneumatophore‐mediated emissions.     

Quantitative distribution of meiobenthos and the structure of the free-living nematode community of the mangrove intertidal zone in Nha Trang bay (Vietnam) in the South China Sea

Meiobenthic studies were performed in an intertidal area in the Be River estuary (Nha Trang Bay, Vietnam). The study area is an area of riverine-type mangroves that have been heavily damaged by human impacts, including timber cutting and waste. Three biotopes are situated in the middle intertidal zone: a fringe of Rhizophora stylosa, a bush area composed of Avicennia aff. alba behind it, and muddy sand with fiddler crabs (Uca spp.), which is free of mangrove plants. Three replicate samples of meiobenthos were collected in each biotope and each sample was subdivided into two layers: 0–1 and 1–4 cm. The abundance of metazoan meiobenthos varied from 735 specimens/10 cm² in the Uca spp. biotope to 244 specimens/10 cm² beneath the Rhizophora trees. Six taxonomic groups of high rank were found among the meiofauna: Nematoda, Copepoda (Harpacticoida), Oligochaeta, Turbellaria, Kinorhyncha, and Foraminifera (Allogromiida). The spatial variability of meiobenthos and its key taxa was estimated and the spatial distribution patterns of free-living nematode species were described. About 90% of the total meiobenthos inhabited the upper 0–1 cm of the sediments. Nematodes constituted 90–95% of all meiobenthic organisms in the samples. A total of 48 species of free-living nematodes were found in the investigated mangrove intertidal area. In terms of species composition and set of dominants, the nematode community is comprised of three local assemblages: one of them inhabits the uppermost centimeter in the Uca and Avicennia biocenoses; the second assemblage occupies the upper sediment layer in the Rhizophora stand; a less abundant but specific assemblage of several nematode species occurs in the subsurface sediments at all three sites.     

Stand structure influences nekton community composition and provides protection from natural disturbance in Micronesian mangroves

Structurally complex mangrove roots are thought to provide foraging habitat, predation refugia, and typhoon protection for resident fish, shrimp, and crabs. The spatially compact nature of Micronesian mangroves results in model ecosystems to test these ideas. Tidal creek nekton assemblages were compared among mangrove forests impacted by Typhoon Sudal and differing in stand structure. Structurally complex Rhizophora spp. stands were predicted to support higher densities and different communities of nekton and to provide greater protection from typhoons compared to less complex Sonneratia alba/Bruguiera gymnorrhiza stands. Lift net data revealed that structural complexity did not support greater nekton densities, but did support significantly different nekton assemblages. The cardinalfish Apogon ceramensis and goby Oxyurichthys lonchotus had significantly higher densities in S. alba/B. gymnorrhiza mangrove creeks, whereas the silverside Atherinomorus lacunosus and diogenid crabs had significantly higher densities in Rhizophora spp. creeks. Similar nekton densities 17 and 4 months after the typhoon in Rhizophora spp. creeks provided indirect evidence that structural complexity increased protection for resident nekton from disturbances. Findings indicate that studies of structural complexity and nekton densities may be better served when individual species are compared and that diverse mangrove tree assemblages will support diverse nekton assemblages that may be more resilient to disturbance.     

Analysing ethnobotanical and fishery-related importance of mangroves of the East-Godavari Delta (Andhra Pradesh, India) for conservation and management purposes

Mangrove forests, though essentially common and wide-spread, are highly threatened. Local societies along with their knowledge about the mangrove also are endangered, while they are still underrepresented as scientific research topics. With the present study we document local utilization patterns, and perception of ecosystem change. We illustrate how information generated by ethnobiological research can be used to strengthen the management of the ecosystem. This study was conducted in the Godavari mangrove forest located in the East-Godavari District of the state Andhra Pradesh in India, where mangroves have been degrading due to over-exploitation, extensive development of aquaculture, and pollution from rural and urbanized areas (Kakinada). One hundred interviews were carried out among the fisherfolk population present in two mangrove zones in the study area, a wildlife sanctuary with strong conservation status and an adjacent zone. Results from the interviews indicated that Avicennia marina (Forsk.) Vierh., a dominant species in the Godavari mangroves, is used most frequently as firewood and for construction. Multiple products of the mangrove included the bark of Ceriops decandra (Griff.) Ding Hou to dye the fishing nets and improve their durability, the bark of Aegiceras corniculatum (L.) Blanco to poison and catch fish, and the leaves of Avicennia spp. and Excoecaria agallocha L. as fodder for cattle. No medicinal uses of true mangrove species were reported, but there were a few traditional uses for mangrove associates. Utilization patterns varied in the two zones that we investigated, most likely due to differences in their ecology and legal status. The findings are discussed in relation with the demographic and socio-economic traits of the fisherfolk communities of the Godavari mangroves and indicate a clear dependency of their livelihood on the mangrove forest. Reported changes in the Godavari mangrove cover also differed in the two zones, with significantly less perceptions of a decrease in the protected area, as compared to the adjacent non-protected area. A posteriori comparisons between sequential satellite imagery (retrospective till 1977) and respondents that were at least 15 years back then, revealed a mangrove decrease which was however perceived to different extents depending on the area with which the fishermen were familiar. While local needs had not been incorporated in the existing policy, we created a framework on how data on ethnobotanical traditions, fishery-related activities and local people's perceptions of change can be incorporated into management strategies.     

Effects of waterlogging and salinity on plant-water relations and on the accumulation of solutes in three mangrove species

Dispersal units of Avicennia marina (Forsk.) Vierh., Rhizophora mucronata Lam. and Bruguiera gymnorrhiza (L.) Lam. were cultivated in mangrove sediment under greenhouse conditions. After 7 months plants were subjected to the following waterlogging and salinity treatments for 60 days: drained non-saline, drained saline, waterlogged non-saline and waterlogged saline. Measurements were made of stomatal resistance (r_s), tissue water potentials, relative water content, and the concentration of Na, K, Ca and Mg in plant organs. Measurements were made 4 days before the termination of waterlogging and continued for 12 days thereafter. Generally, the highest stomatal resistance occurred in the waterlogged saline treatments and the least stomatal resistance occurred in the drained non-saline treatments. Water potentials were usually lower in the saline treatments than in the non-saline treatments. Depression in water potential was greatest in Avicennia and least in Bruguiera. In all 3 species, the concentration of cations was high in the roots. Salinity had greater effects on the concentration of ions in Avicennia than in Rhizophora or Bruguiera. The low stomatal resistance, low tissue water potentials, high relative water content and high tissue cation concentration in Avicennia suggest that it is ideally suited as the pioneer in the mangrove association. Generally, Rhizophora was more tolerant of salinity than Bruguiera. The responses of the plants reveal the remarkable adaptability of mangroves to the saline environment. The adaptive features of each species determine its characteristic habitat in intertidal areas.     

A review of mangrove rehabilitation in the Philippines: successes,failures and future prospects

From half a million hectares at the turn of the century, Philippine mangroves have declined to only 120,000 ha while fish/shrimp culture ponds have increased to 232,000 ha. Mangrove replanting programs have thus been popular, from community initiatives (1930s–1950s) to government-sponsored projects (1970s) to large-scale international development assistance programs (1980s to present). Planting costs escalated from less than US$100 to over $500/ha, with half of the latter amount allocated to administration, supervision and project management. Despite heavy funds for massive rehabilitation of mangrove forests over the last two decades, the long-term survival rates of mangroves are generally low at 10–20%. Poor survival can be mainly traced to two factors: inappropriate species and site selection. The favored but unsuitable Rhizophora are planted in sandy substrates of exposed coastlines instead of the natural colonizers Avicennia and Sonneratia. More significantly, planting sites are generally in the lower intertidal to subtidal zones where mangroves do not thrive rather than the optimal middle to upper intertidal levels, for a simple reason. Such ideal sites have long been converted to brackishwater fishponds whereas the former are open access areas with no ownership problems. The issue of pond ownership may be complex and difficult, but such should not outweigh ecological requirements: mangroves should be planted where fishponds are, not on seagrass beds and tidal flats where they never existed. This paper reviews eight mangrove initiatives in the Philippines and evaluates the biophysical and institutional factors behind success or failure. The authors recommend specific protocols (among them pushing for a 4:1 mangrove to pond ratio recommended for a healthy ecosystem) and wider policy directions to make mangrove rehabilitation in the country more effective.     

Barnacle fouling impedes the gaseous exchange and food production of the mangroves <Emphasis Type="Italic">Kandelia obovata</Emphasis>, a dominant mangrove species in Hong Kong and Taiwan

Barnacle fouling has shown to impede gas exchange ability of mangroves. Fouled mangrove plants may therefore obtain less carbon dioxide and water for photosynthesis, resulting in reduced food and chloroplasts production, but such hypothesis remains untested. The objective of the present study compared the stomata density (essential for obtaining carbon dioxide and water molecules) and leaf chlorophyll concentration (essential for photosynthesis) of fouled and non-fouled (control) of seedlings, juveniles and adults of the mangroves Kandelia obovata, in Hong Kong and Taiwan. The seedlings and juveniles of the dominant mangrove plant species, Kandelia obovata, in Hong Kong and Taiwan had a higher density of stomata but a lower chlorophyll concentration in the leaves, when the trunks and twigs were fouled by the barnacle Fistulobalanus albicostatus. Fouled K. obovata appears to develop more stomata in the leaves to compensate the blocking effect of the lenticel from barnacle fouling. As fouling impacts the gaseous exchange ability of mangroves, fouled plants could obtain less carbon dioxide and water for photosynthesis, resulting in reduced food and chloroplasts production. Fouled adult plants, however, had variable responses in leaf chlorophyll concentrations among the study sites, suggesting adults were more tolerant of barnacle fouling. The present study reveals seedling and juvenile mangrove plants are very susceptible to barnacle fouling, which impedes the gaseous exchange mechanism and food production, which can subsequently result in reduced growth, fitness and survival. Handling editor: P. Viaroli     

Detrital traits affect substitutability of a range‐expanding foundation species across latitude

Climate‐driven range shifts of foundation species could alter ecosystem processes and community composition by providing different resources than resident foundation species. Along the US Atlantic coast, the northward expanding foundation species, black mangrove Avicennia germinans, is replacing the dominant salt marsh foundation species, marsh cordgrass Spartina alterniflora. These species have distinct detrital attributes that ostensibly provide different resources to epifauna. We experimentally examined how detritus of these species affects decomposition and community composition in different habitat contexts at regional and local scales. First, we manipulated detritus identity (Avicennia, Spartina) at 13 sites across a 5° latitudinal gradient spanning mangrove, mixed marsh‐mangrove and salt marsh habitats. Across latitude, we found that Avicennia detritus decomposed 2–4 times faster than Spartina detritus, suggesting that detrital turnover will increase with mangrove expansion. Epifaunal abundance and richness increased 2–7 times from south to north (mangrove to salt marsh) and were equivalent between Avicennia and Spartina detritus except for crabs, a dominant taxonomic group that preferred Spartina detritus. Second, to examine the whether changing habitat context affected regional patterns, we manipulated detritus identity and surrounding habitat type (mangrove, salt marsh) at a single mixed site, also including inert mimics to separate structural and nutritional roles of detritus. Epifaunal richness was similar between the two detrital types, but crabs were 2–7 times more abundant in Spartina detritus due to its structural attributes. Surrounding habitat type did not influence decomposition rate or community patterns, which suggests that latitudinal influences, not surrounding habitat, drove the regional community patterns in the first experiment. Overall, mangrove expansion could alter epifaunal communities due to the lower structural value and faster turnover of mangrove detritus. As species shift with changing climate, understanding foundation species substitutability is critical to predict community change, but we must account for concomitant environmental changes that also modify communities.     

An 800 year record of mangrove dynamics and human activities in the upper Gulf of Thailand

A multiproxy record comprising pollen, charcoal, loss on ignition and particle size analyses from two radiocarbon dated sediment cores from Klong Kone subdistrict on the western coast of the Gulf of Thailand provides insights on mangrove dynamics, environmental changes and human activities during the last 800 years. The mangroves were dominated by Rhizophora which indicates that the area has been influenced by the sea level from at least 820 cal bp until 720 cal bp. An intertidal area may have formed that supported mangrove development as part of an old shoreline during 820–720 cal bp. After 720 cal bp, mangroves decreased and were replaced by grasses, suggesting that a lower sea level caused the mangroves to grow closer to the sea until around 140 cal bp. Cereal pollen increased from 720 cal bp suggesting probable use of the shoreline for intensive cultivation. The mangroves were characterised by Avicennia, which increased toward the top of the 2 cores, suggesting that the mangroves then grew further inland, probably due to recent sea-level rise. Intensive human activity is recorded during the 20th century, as indicated by increased particle size, charcoal and carbonate content. At present, human activity in the area includes dams and construction as well as aquaculture.     

Characterisation of the effect of a simulated hydrocarbon spill on diazotrophs in mangrove sediment mesocosm

An analysis of the effect of an oil spill on mangrove sediments was carried out by contamination of mesocosms derived from two different mangroves, one with a history of contamination and one pristine. The association between N₂ fixers and hydrocarbon degradation was assessed using quantitative PCR (qPCR) for the genes rrs and nifH, nifH clone library sequencing and total petroleum hydrocarbon (TPH) quantification using gas chromatography. TPH showed that the microbial communities of both mangroves were able to degrade the hydrocarbons added; however, whereas the majority of oil added to the mesocosm derived from the polluted mangrove was degraded in the 75 days of the experiment, there was only partially degradation in the mesocosm derived from the pristine mangrove. qPCR showed that the addition of oil led to an increase in rrs gene copy numbers in both mesocosms, having almost no effect on the nifH copy numbers in the pristine mangrove. Sequencing of nifH clones indicated that the changes promoted by the oil in the polluted mangrove were greater than those observed in the pristine mesocosm. The main effect observed in the polluted mesocosm was the selection of a single phylotype which is probably adapted to the presence of petroleum. These results, together with previous reports, give hints about the relationship between N₂ fixation and hydrocarbon degradation in natural ecosystems.     

Prolonged inundation and ecological changes in an Avicennia mangrove: implications for conservation and management

The mangrove around Sungei Pemburongunan, just west of Tanjong Batu and Istana Darul Aman in the Brunei-Muara District, is unique in that it contains the only pure stand of Avicennia marina in Brunei Darussalam. In mid-October, 1990 the mouth of Sungei Pemburongunan was closed due to a natural build-up of a sandbar across it. The mangrove quickly became flooded and adverse effects on the flora and fauna were observed during the following weeks. In mid-December a channel was dug across the sand bar to reduce the flooding and normalise the ecology. Some of the dying Avicennia and Lumnitzera have since recovered but others such as Acrostichum and Casuarina have not. Avicennia showed some interesting responses to prolonged inundation. Many of the Avicennia that had flowered during the flood produced seedlings which are now well rooted and 40–80 cm in height. The survival of large numbers of these seedlings is attributed to the floor-related decrease in the number of grapsid and sesarmid crabs which predate on them. The crab and mollusc populations have largely recovered. Unlike periodic short-term flooding which does not seem to have any apparent adverse affect on the ecosystem, prolonged inundation can result in the loss of the Avicennia and its associated flora and fauna. Thus, fragile mangrove ecosystems such as this need to be monitored and managed closely.This work demonstrates that contrary to conventional wisdom, human intervention and management can be beneficial to more fragile ecosystems, which could otherwise succumb to natural processes. Furthermore, it highlights the importance of the dynamic nature of the environment which should be considered in management and conservation programmes.     

Lipid composition of mangrove and its relevance to salt tolerance

Lipid compositions of mangrove trees were studied in relation to the salt-tolerance mechanism. Leaves and roots were obtained from seven mature mangrove trees on Iriomote Island, Okinawa: Bruguiera gymnorrhiza, Rhizophora stylosa, Kandelia candel, Lumnitzera racemosa, Avicennia marina, Pemphis acidula and Sonneratia alba. Lipids of mangrove leaves mainly consisted of 11 lipid classes: polar lipids, unknown (UK) 1–6, sterols, triacyl glycerols, wax ester and sterol ester (UK 3 and 4 were found to be tri-terpenoid alcohol in this study). Of these lipid classes, sterol ester was the main lipid in all species comprising 17.6–33.7% of total lipids. Analysis of the chemical structure found that the sterol esters mainly consisted of fatty acid esters of tri-terpenoid alcohols. One major tri-terpenoid alcohol was identified to be lupeol by interpretation of infrared resonance, nuclear magnetic resonance and mass spectrometry. Because of the unique anatomy of the mangrove root, lipid analyses were made separately for epidermis, cortex and innermost stele, respectively. The concentration of free tri-terpenoid alcohols showed a higher tendency in the outside part than in the inside portion of the roots, suggesting their protective roles. Relevance of lipid composition to salt tolerance was studied with propagules of K. candel and B. gymnorrhiza planted with varied salt concentrations. The proportions of free tri-terpenoids increased with salinity in both leaves and roots of K. candel, and only in roots of B. gymnorrhiza. No salt-dependent changes were noted in the phospholipid and fatty acid compositions in both species. These findings suggested that salt stress specifically modulated the terpenoid concentrations in mangroves. Electronic Publication     

Modern pollen rain in mangroves from San Andres Island, Colombian Caribbean

The precise characterisation of present-day mangrove ecosystems from modern pollen rain facilitates the accurate use of fossil pollen data for late Quaternary sea level and environmental reconstructions. Here, we investigate whether the analysis of pollen rain data corroborates existing floristic and structural characterisation of different mangrove types at the Caribbean island of San Andrés, Colombia. At 82 plots along 20 transects of four distinct mangrove types, samples were obtained of (i) surface sediments for pollen analysis, and (ii) a range of environmental parameters (including inundation levels, salinity and pH). This information was compared to previously sampled mangrove composition and tree basal area. In surface sediment samples 82 pollen taxa were found, from which 19 were present in the vegetation plots. However, because pollen may be transported by wind and/or watercourses, the overall floristic composition of the different forest types may not necessarily be reflected by the pollen spectra. Local vegetation (i.e. mangroves and beach) represented > 90% of the pollen spectra, while the regional one (i.e. hinterland forests) represented < 5% of it. Unlike the four mangrove types that were previously described in the vegetation, the analysis of pollen samples suggested only three distinct types of forest.The groups were characterised based on (i) the dominance of at least one of the true mangrove species from pollen data ordination and the presence of associated species, and (ii) their relationship with environmental parameters. Rhizophora was present in all plot samples, but did not contribute to forest type separation. In fact, just three true mangrove species proved reliable indicators of (i) high salinity and fringe mangroves (i.e. Avicennia), (ii) high pH levels and landward mangroves (i.e. Conocarpus), and (iii) natural or anthropogenic caused disturbance of forest stands (Laguncularia and associated Acrostichum fern). Hence our study confirms that mangrove pollen spectra can be accurately used to describe different mangrove environments for fossil based palaeoecological reconstructions.     

High Molecular Weight Organic Compounds in the Xylem Sap of Mangroves: Implications for Long-Distance Water Transport

The rise of sap in mangroves has puzzled plant physiologists for many decades. The current consensus is that negative pressures in the xylem exist which are sufficiently high to exceed the osmotic pressure of seawater (2.5 MPa). This implies that the radial reflection coefficients of the mangrove roots are equal to unity. However, direct pressure probe measurements in xylem vessels of the roots and stems of mangrove (Rhizophora mangle) grown in the laboratory or in the field yielded below-atmospheric, positive (absolute) pressure values. Slightly negative pressure values were recorded only occasionally. Xylem pressure did not change significantly when the plants were transferred from tap water to solutions containing up to 1700 mOsmol kg^?1 NaCl. This indicates that the radial reflection coefficient of the roots for salt, and therefore the effective osmotic pressure of the external solution, was essentially zero as already reported for other halophytes. The low values of xylem tension measured with the xylem pressure probe were consistent with previously published data obtained using the vacuum/leafy twig technique. Values of xylem tension determined with these two methods were nearly two orders of magnitude smaller than those estimated for mangrove using the pressure chamber technique (?3 to ?6MPa). Xylem pressure probe measurements and staining experiments with alcian blue and other dyes gave strong evidence that the xylem vessels contained viscous, mucilage- and/or protein-related compounds. Production of these compounds resulting from wound or other artifactual reactions was excluded. The very low sap flow rates of about 20–50 cm h^?1 measured in these mangrove plants were consistent with the presence of high molecular weight polymeric substances in the xylem sap. The presence of viscous substances in the xylem sap of mangroves has the following implications for traditional xylem pressure measurement techniques, development of xylem tension, and longdistance water transport: (1) high external balancing pressures in the pressure chamber are needed to force xylem sap to the cut surface of the twig; (2) stable tensions much larger than 0.1 MPa can be developed only occasionally because viscous solutions provide nucleation sites for gas bubble formation; (3) the frequent presence of small gas bubbles in viscous solutions allows water transport by interfacial, gravity-independent streaming at gas/water interfaces and (4) the increased density of viscous solutions creates (gravity-dependent) convectional flows. Density-driven convectional flows and interfacial streaming, but also the very low radial reflection coefficient of the roots to NaCl are apparently the means by which R. mangle maintains water transport to its leaves despite the high salinity of the environment.     

Recovery Deficiency Following Tree Mortality in Mangroves of Two Caribbean Islands: Field Survey and Statistical Classification

Mangrove species are well adapted to the harsh ecological conditions of their environment throughout the tropics. However, in the islands of Guadeloupe and Martinique (Lesser Antilles), deficient forest recovery was evidenced in 43 mangrove sites (>1000 m²) affected by apparently natural tree mortality. Such sites were recorded from four chronological sets of aerial photographs between 1950 and 1995, and field-investigated in terms of environment and vegetation characteristics. Given the speculative relationship between the slow-regenerating vegetation and its present environment within non-steady state, disturbed sites, statistical analyses arbitrarily matching physicochemical and biological data were primarily avoided. On one hand, principal component analysis (PCA), combined with an agglomerative hierarchical classification, was performed on environmental, rank-ordered data; on the other hand, multidimensional scaling (MDS) was implemented on vegetation data. Discriminant analyses (DA) further characterized the environment/vegetation interrelationships for each site type. Ultimately, three main types of mortality sites have been distinguished among the study areas. One type clustered the sites showing the lowest salinity values and the highest surge vulnerability whose dominant mangrove species is Rhizophora. Another type presented highly saline sites having clayey soils with pure, stunted, Avicennia stands. The last type consisted of peculiar forest gaps on deep, compact, peat soils. During the rainy season, these sites turned into shallow ponds scattered with living, young Rhizophora and large, standing, dead Avicennia. The authors suggest that this classification may serve as a comprehensive framework to test subsequent hypotheses (hurricanes, droughts…) on the origins of natural massive tree mortality and the causes of recovery deficiency in mangroves of the Caribbean.