Growth and physiological responses of two mangrove species (Bruguiera gymnorrhiza and Kandelia candel) to waterlogging

Effects of duration of waterlogging on growth and physiological responses of two mangrove species, Bruguiera gymnorrhiza and Kandelia candel, were investigated. The relative growth rate of B. gymnorrhiza decreased significantly with waterlogged time, with the highest value found for drained plants and the lowest in plants under 12 weeks waterlogging. On the contrary, no significant difference was found between waterlogged and drained K. candel plants. The shoot to root biomass ratio of K. candel increased when subjected to 8 or 12 weeks waterlogging but little change was recorded in B. gymnorrhiza, indicating a shift in biomass allocation from roots to shoots in K. candel under prolonged waterlogging but not in B. gymnorrhiza. These different growth responses between the two mangrove species supported the hypothesis that K. candel is more tolerant to waterlogging than B. gymnorrhiza. Under 12 weeks waterlogged treatment, root oxidase activity significantly decreased in B. gymnorrhiza but increased in K. candel. Chlorophyll contents of K. candel increased more rapidly in response to waterlogging than B. gymnorrhiza. Activities of both peroxidase and superoxide dismutase increased significantly in leaves of K. candel when the waterlogging period was longer than 8 weeks, while only the peroxidase activity of B. gymnorrhiza showed a significant increase, indicating that K. candel had stronger resistance to the oxidant damage resulting from waterlogging. These physiological indicators further supported the hypothesis that K. candel is more tolerant to waterlogging than B. gymnorrhiza.     

Salt and waterlogging stress impacts on seed germination and early seedling growth of selected endemic plant species from Western Australia

Six perennial species endemic to South West Western Australia (Acacia trulliformis, Austrostipa geoffreyi, Banksia oligantha, B. mucronulata, Hakea tuberculata and Orthrosanthus muelleri) were screened for salt tolerance and recovery during seed germination. Growth and survival of 6-month old seedlings of these six plus a further vegetatively propagated species (Myoporum turbinatum) were subsequently examined in response to salt and waterlogging application. Water uptake under elevated saline conditions (200 and 400 mM NaCl) was slow, but not restrictive to germination. Moreover, a large proportion of seeds that were unable to germinate under saline conditions recovered after being transferred to non-saline conditions. Germination, growth and survival varied with species and the salt concentration used. Increasing salt concentrations tended to increase time to germination. Germination of Acacia trulliformis seeds declined exponentially with increasing salinity, and seedlings suffered reduced growth under saline and non-saline waterlogging. Austrostipa geoffreyi seeds were sensitive to saline treatments but seedlings were highly tolerant of both saline and/or waterlogged conditions. Germination of the three proteaceous species declined significantly under highly saline conditions (400 mM NaCl) with seedlings of the two Banksia species not surviving any treatment with the exception of non-saline waterlogging. Seedlings of H. tuberculata were more resilient to treatment conditions. Orthrosanthus muelleri was sensitive to salt stress during germination but was highly resistant to waterlogging, both saline and non-saline. This study provides an insight into the response and resilience of components of the vegetation understorey of saline-affected regions of Western Australia not usually evaluated allowing for more informed restoration.     

Seedling survivorship,growth, and response to disturbance in Belizean mangal

Species zonation patterns across tidal gradients in mangrove forests are formed by successful seedling establishment and maintained by replacement of adults by conspecific seedlings. These two processes rarely have been examined experimentally in neotropical mangal. We studied survivorship and growth of seedlings of two species of mangrove, Rhizophora mangle L. and Avicennia germinans (L.) Steam, across a tidal gradient in Belize, Central America. Propagules of each species were planted in common gardens at tidal elevations corresponding to lowest low water (LLW), mean water (MW), and highest high water (HHW). Sixty-nine percent of Rhizophora seedlings planted at MW and 56% of those planted at LLW survived 1 year. Forty-seven percent of MW Avicennia seedlings also survived 1 year. No individuals of either species survived at HHW, and neither did any LLW Avicennia seedlings. Among the surviving Rhizophora seedlings, LLW seedlings grew more rapidly in terms of height, diameter, leaf production, and biomass than did MW seedlings. Insect herbivory was twice as high on MW seedlings as on LLW Rhizophora seedlings. We also examined the response of established Rhizophora seedlings to experimental removal of the adult Rhizophora canopy. Seedlings in canopy removal areas had higher survivorship, grew twice as fast, produced more leaves, and had less than half the herbivory of seedlings growing beneath an intact canopy. These results provide insights into underlying causes and maintenance of zonation in Caribbean mangrove forests.     

The effect of temperature on salt uptake by tomato plants with diurnal and noctural waterlogging of salinized rootzones

Summary Tomato plants were grown at three constant temperatures (10, 20 and 28°C) with drained or waterlogged rootzones and were irrigated with saline solution (0.09M NaCl).Each increase in temperature resulted in an increase in leaf Na-ion and Cl-ion concentrations in plants grown with drained rootzones. However, with plants grown with waterlogged rootzones, maximum leaf concentrations of Na-ions and Cl-ions occurred at 20°C.At 10°C there were no differences between Na-ion and Cl-ion concentrations for drained or waterlogged treatments. At 20 and 28°C, waterlogging of the rootzone resulted in significantly higher concentrations of Na-ions and Cl-ions in leaf and stem tissues than occurred with drained rootzones.There were no differences in Na-ions and Cl-ions and Cl-ions in plant tops if plants were waterlogged with saline solution during the day or night.Transpiration increased significantly with each increase in temperature but showed no other treatment dependent responses.     

Root morphology is related to the phenotypic variation in waterlogging tolerance of two populations of Suaeda salsa under salinity

The effects of waterlogging and salinity on seedling emergence, seedling growth and ion accumulation in a euhalophyte Suaeda salsa in an intertidal zone and on saline inland soil were investigated. Seedlings of S. salsa from the intertidal zone emerged more rapidly than those of the inland population under both waterlogged and drained conditions. Waterlogging and salinity had no adverse effects on seedling emergence of S. salsa from the intertidal zone, but markedly inhibited this parameter in the inland population. Waterlogging did not affect the seedling survival, shoot dry mass, and shoot height in high salinity in S. salsa from the intertidal zone, while the opposite trend was shown in the inland population. The root dry mass was higher in S. salsa from the intertidal zone as compared to the inland population, in waterlogged treatments by 1.9, 1.3, and 1.5 times in 1, 200, and 600 mM NaCl, respectively, and in drained treatments by 1.8, 2.3, and 3.0 times in 1, 200, and 600 mM NaCl, respectively. Waterlogging increased Na⁺ and K⁺ concentrations in high salinity, but waterlogging had no effect on Cl^- concentration in shoots of S. salsa from the intertidal zone. In all NaCl treatments, waterlogging had no effect on concentrations of these ions in shoots of S. salsa from the saline inland site. In a field investigation, the fresh mass of shoots and roots were lower, whereas the root/shoot ratio was 1.5 times higher in S. salsa from the intertidal zone, compared with the inland population. These findings indicate that S. salsa population from the intertidal zone is more waterlogging tolerant than the inland population. S. salsa from the intertidal zone produced relatively more root biomass and this might help anchor plants against tidal action in the intertidal zone. The physiological and morphological characteristics may determine the natural distributions of the two S. salsa populations in their different saline environments.     

History and biogeography of the mangrove ecosystem,based on a critical reassessment of the paleontological record

The geological record of mangrove plants is based on comparablemorphological characteristics of pollen, fruits and wood, of fossil andmodern species. But this record relies on the assumption that the ecologicaland habitat preferences of ancestral taxa have remained similar throughages. A reexamination of fossil evidence of Avicennia, Pelliciera,Sonneratia, Rhizophora, Bruguiera, Ceriops, etc.reveals that the modern mangrove flora was pantropic by the Eocene, andappears to have originated during Paleocene times. Earlier Paleozoic andMesozoic candidates for a mangrove ecology lack conclusive evidence oftheir exclusive association with tidal environments. It is therefore clear thatcontinental drift had a limited role in the dispersal and development ofmodern mangrove floras. The Eocene/Oligocene boundary crisis appears toherald a beginning of the biogeographic split between the current-dayeastern and western provinces of mangrove plants. But, while the climaticorigins of this major disjunction is not clearly understood, our reassessmentof Tertiary paleoclimates suggests that the major cooling events of themiddle Paleocene, the end of the Eocene and the middle Pliocene were themost likely influences on the evolution of mangrove floras. The associatedinvertebrates, especially molluscs, further support our assertion that amodern mangrove ecosystem was established only during the earliestEocene times. We summarize our interpretation in a set of 9 palinspasticmaps of fossil mangrove genera through their evolution ending with thecurrent, bipartite distribution of present day taxa.     

Mangrove sesarmid crab feeding experiments in Peninsular Malaysia

The feeding ecology of mangrove sesarmid crabs in Peninsular Malaysia was investigated by field and laboratory experiments using four mangrove leaf species (Avicennia officinalis, Bruguiera gymnorrhiza, B. parviflora and Rhizophora apiculata) and leaves of different condition (fresh and senescent). Leaves tethered on strings at high (Bruguiera zone) and low (Rhizophora zone) intertidal positions, both upstream (Sungai Pasir) and downstream (Lower Merbok) showed significant amounts of leaf litter removal in 24 h (mean 79±3% initial dry mass). Significantly more B. gymnorrhiza was consumed in Bruguiera zones and significantly less senescent A. officinalis in the upstream Rhizophora zone. In Bruguiera zones, significant numbers of leaves were taken down burrows but there were no preferences for leaf species or condition of leaf taken down burrows at all sites. In 24 h, under laboratory conditions, the sesarmid crabs Sesarma (Perisesarma) eumolpe and S. (Perisesarma) onychophorum were offered with a mangrove species choice of either fresh or senescent leaves. There was no difference in mangrove species taken when the leaves were senescent for both crab species, but when the leaves were fresh, significantly more A. officinalis leaves were consumed by both sesarmid crab species. S. onychophorum ate significantly more B. parviflora than did S. eumolpe. The crab distribution in the field was related to the preferred tree species dominance, indicating that tree species may be important for crab species distribution, or vice versa. The mean rate of leaf consumption was not significantly different between the crab species; S. eumolpe was 29.9±5.9 and S. onychophorum was 35.3±7.2 mg dry mass per wet mass gram of crab in 24 h. Rhizophora spp. were the least preferred species in all feeding experiments, a finding which may have implications for ecosystem functioning in monoculture rehabilitation projects.     

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.     

The effect of combined salinity and waterlogging on the halophyte Suaeda maritima: The role of antioxidants

Suaeda maritima is a halophytic plant and its habitat is salt marsh. In order to adapt to saline or waterlogged conditions, plants have evolved mechanisms that include antioxidant protection. However, the combined effect of salinity and waterlogging on antioxidants in S. maritima is unknown. The aim of this study was therefore to investigate the effect of saline-flooding on levels of glutathione and phenolic compounds (antioxidants) and the correlation between their concentration and activity in S. maritima shoots grown in their natural habitat and in a glasshouse.Shoots were collected from two different elevations (and so different degrees of flooding) of a salt marsh while other plants were grown in half strength seawater in the controlled conditions of a glasshouse for 8 weeks (drained and flooded). Shoot samples were used to measure dry weight, glutathione and its reduction state, malonyldialdehyde content (MDA), polyphenol content, superoxide anion and 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) scavenging activity in the shoots of S. maritima.Growth of S. maritima was greater in plants growing on the high marsh than at a lower elevation and in drained medium as opposed to waterlogged conditions in the glasshouse. Waterlogging caused an increase in glutathione and its reduction state. The glutathione half-cell redox potential (E_GSSG/2GSH) was more negative in plant shoots grown under waterlogged conditions than in plants grown under normal conditions. Higher DPPH and superoxide anion scavenging activity was associated with high antioxidant concentrations (glutathione and polyphenols).Conclusions. Under saline-flooded conditions in the field and in the glasshouse, plants produced a higher concentration of antioxidants than under drained conditions. These result indicated that antioxidant molecules play an important role in S. maritima plants under combined salinity and waterlogging stress.     

Fine root production in three zones of secondary mangrove forest in eastern Thailand

Key message

High root productions, especially in the fine roots, estimated by ingrowth cores were confirmed in mangrove forests. The zonal variation in root production was caused by inundation regime and soil temperature.

Abstract

Mangrove forests have high net primary productivity (NPP), and it is well known that these trees allocate high amounts of biomass to their root systems. In particular, fine root production (FRP) comprises a large component of the NPP. However, information on root production remains scarce. We studied FRP in three zones (Avicennia, Rhizophora, and Xylocarpus) of a mangrove forest in eastern Thailand using ingrowth cores (0–30 cm of soil depth). The root biomass and necromass were periodically harvested from the cores and weighed during the one-year study. The FRP was determined by summation of the fine root biomass (FRB) and root necromass. The results showed that the FRB clearly increased in the wet and cool dry seasons. Magnitude of FRB in the Rhizophora and Xylocarpus zones was 1171.07 and 764.23 g/m²/30 cm, respectively. The lowest FRB (292.74 g/m²/30 cm) was recorded in the Avicennia zone locating on the river edge where there is a greater frequency of inundation than the other zones. Root necromass was high in the Rhizophora and Xylocarpus zones, and accumulated noticeably when soil temperatures rapidly declined during the middle of the wet season to cool dry season. However, root necromass in the Avicennia zone varied within a small range. We attributed the small accumulation of root necromass in the Avicennia zone to the relative high soil temperature that likely caused a high root decomposition rate. The average FRP (3.403–4.079 ton/ha/year) accounted for 74.4, 81.5, and 92.4 % of the total root production in the Avicennia, Rhizophora, and Xylocarpus zone, respectively. The root production and causative factors (i.e., soil temperature and inundation regime) are discussed in relation to the carbon cycle of a mangrove forest.

     

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.     

Molecular phylogeny of mangroves VIII: Analysis of mitochondrial DNA variation for species identification and relationships in Indian mangrove Rhizophoraceae

Ten species belonging to four genera of the mangrove tribe Rhizophoreaefound in the Indian subcontinent were analysed for species identification andgenetic relationship using nine mitochondrial gene probes. RFLP patternobservedwith 27 probe enzyme combinations the genera Rhizophora,Bruguiera, Ceriops andKandelia differentiated these species into three classesofmitotypes with further resolution within them. Clustering of these mitotypesindicated that Rhizophora was more closely related toCeriops-Kandelia than to theBruguiera. Though the component species of each genusclustered together, a high degree of heterogeneity was observed among fourspecies of the genus Rhizophora and three species of genusBruguiera. The variation between two species ofCeriops was minimal. Species-specific profiles wereobserved for all the species in some probe-enzyme combination. Though themonotypic genus Kandelia shared a number of loci withgenusCeriops, it remained distinct. The putative parents of thenaturally occuring interspecific hybrid in Pichavaram were reconfirmed to beR. apiculata and R. mucronata. Theresults are discussed with regard to the taxonomic and phylogeneticrelationships between different species and genera of the tribe Rhizophoreae.     

Review of wheat improvement for waterlogging tolerance in Australia and India: the importance of anaerobiosis and element toxicities associated with different soils

Background and Aims

The lack of knowledge about key traits in field environments is a major constraint to germplasm improvement and crop management because waterlogging-prone environments are highly diverse and complex, and the mechanisms of tolerance to waterlogging include a large range of traits. A model is proposed that waterlogging tolerance is a product of tolerance to anaerobiosis and high microelement concentrations. This is further evaluated with the aim of prioritizing traits required for waterlogging tolerance of wheat in the field.

Methods

Waterlogging tolerance mechanisms of wheat are evaluated in a range of diverse environments through a review of past research in Australia and India; this includes selected soils and plant data, including plant growth under waterlogged and drained conditions in different environments. Measurements focus on changes in redox potential and concentrations of diverse elements in soils and plants during waterlogging.

Key Results

(a) Waterlogging tolerance of wheat in one location often does not relate to another, and (b) element toxicities are often a major constraint in waterlogged environments. Important element toxicities in different soils during waterlogging include Mn, Fe, Na, Al and B. This is the first time that Al and B toxicities have been indicated for wheat in waterlogged soils in India. These results support and extend the well-known interactions of salinity/Na and waterlogging/hypoxia tolerance.

Conclusions

Diverse element toxicities (or deficiencies) that are exacerbated during waterlogging are proposed as a major reason why waterlogging tolerance at one site is often not replicated at another. Recommendations for germplasm improvement for waterlogging tolerance include use of inductively coupled plasma analyses of soils and plants.Key words: Waterlogging, microelements, toxicity, redox potential, wheat, anaerobiosis

‘No grain is ever produced without water, but too much water tends to spoil the grain and inundation is as injurious to growth as dearth of water.’ Narada Smriti XI, 19; circa 3000 bc.‘Waterlogging’ is defined as a condition of the soil where excess water limits gas diffusion; while ‘waterlogging tolerance’ is defined as survival or the maintenance of high growth rates, biomass accumulation or grain yield under waterlogging relative to non waterlogged (usually drained soil) conditions (Setter and Waters, 2003).

     

The abundance and growth of Avicennia marina and Rhizophora stylosa in the low shore zone of Darwin Harbour,Northern Territory

Abstract The growth and distribution of the two mangrove seedling species, Avicennia marina (Forsk.) Vierh. and Rhizophora stylosa Griff., were studied in the low-shore zone at three locations along two estuaries of Darwin Harbour to determine how growth varied along the estuary and within the habitats of the low shore zone. The low shore environment was heterogenous for factors such as salinity and light, with substantial variation within and between locations. Seedlings of Avicennia were in higher densities in light gaps than in the adjacent forest, and density in both habitats decreased towards the mouth of the estuary. Growth of Avicennia seedlings, as measured through in situ growth trials and plant harvests, increased towards the mouth of the estuary. Results for Rhizophora seedlings were more complex although some similar trends were evident. This may have been due to the greater early survival of Rhizophora seedlings compared to Avicennia seedlings. The importance of propagule origin was investigated, for Rhizophora, by transplanting propagules between locations along one estuary. Increased seedling survival was observed at the propagule source site; however, this effect was subtle and probably not the most important factor affecting seedling survival at a site.     

Zonal distribution of coarse woody debris and its contribution to net primary production in a secondary mangrove forest

Coarse woody debris (CWD) plays an important role in long-term carbon storage in forest ecosystems. However, few studies have examined CWD in mangrove forests. A secondary mangrove forest on an estuary of the Trat River showed different structures along vegetation zones ranging from the river’s edge to inland parts of the forest (the Sonneratia–Avicennia, Avicennia, Rhizophora, and Xylocarpus zones, respectively). The mass distribution of CWD stock in downed wood and standing dead trees along these vegetation zones was evaluated. Most of the CWD stock in the Sonneratia–Avicennia and Avicennia zones was found in downed wood, while it mainly accumulated in standing dead trees in the Rhizophora and Xylocarpus zones. The total mass of CWD stock that accumulated in each zone ranged from 1.56–8.39 t ha^?1, depending on the forest structure and inundation regimes. The annual woody debris flux in each zone was calculated by summing the necromass (excluding foliage) of dead trees and coarse litter from 2010 to 2013. The average woody debris flux was 5.4 t ha^?1 year^?1, and its zonal variation principally depended on the necromass production that resulted from forest succession, high tree-density, and lightning. Over all the zones, the above- and below-ground net primary production (ANPP and BNPP, respectively) was estimated at 18.0 and 3.6 t ha^?1 year^?1, respectively. The magnitude of BNPP and its contribution to the NPP was markedly increased when fine root production was taken into consideration. The contribution of the woody debris flux without root necromass to the ANPP ranged from 12 to 28%.     

Effects of mangrove soil ageing on the accumulation of hydrogen sulphide in roots ofAvicennia spp

The possibility for accumulation of hydrogen sulphide gas in roots of mangroves (Avicennia spp) which had colonized reclaimed coastal areas in Singapore was investigated using a simple potentiometric technique preceded by an extraction step. The study showed that detectable amounts of hydrogen sulphide gas were present in the underground cable roots of the mangroves and that the concentrations increased with the age of the plant. Furthermore concentrations of hydrogen sulphide in the roots were 30–40 times higher than the concentrations of the gas which were simultaneously generated by anaerobic processes in the surrounding, ageing mangrove sediment. The reasons for these patterns and their possible impact on the succession ofAvicennia spp by a different type of mangrove,Rhizophora spp, are discussed.Author to whom correspondence should be addressed     

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.     

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.     

Organic Matter Dynamics on the Forest Floor of a Micronesian Mangrove Forest: An Investigation of Species Composition Shifts1

Species composition shifts in mangrove forests may alter organic matter dynamics. The purpose of this study was to predict the effect of species replacements among mangrove trees on organic matter dynamics in a mangrove forest on the island of Kosrae, Federated States of Micronesia. We were particularly interested in elements of the carbon cycle that affect peat accumulation rates, organic matter exports to the estuary and coral reef systems, and soil microbiology. We compared organic matter production and decomposition rates among three mangrove species that commonly grow in similar hydrogeomorphic settings: Rhizophora apiculata BL, which is selectively harvested; Bruguiera gymnorrhiza, which may gradually replace Rhizophora; and Sonneratia alba, which is producing few mature fruits. Sonneratia had significantly higher rates of root production (estimated with ingrowth chambers) than Bruguiera or Rhizophora. Sonneratia foliage had significantly faster decomposition rates and significantly lower lignin:nitrogen ratios than Bruguiera foliage. Live root mass was positively correlated with ingrowth and soil carbon, although soil carbon and ingrowth were not significantly correlated with each other. Humic acid concentrations were significantly higher in Sonneratia rhizospheres than in either Bruguiera or Rhizophora rhizospheres and were positively correlated with root ingrowth. The species changes taking place on Kosrae are likely to result in lower rates of root production and foliage decomposition, and more refractory carbon pools in soil.     

Effects of light and intertidal position on seedling survival and growth in tropical tidal forests

Reciprocal transplant experiments were used to study the effects of tidal inundation and light level on growth and survival of four species of mangroves in Australia: Avicennia marina (Forsk.) Vierh., Bruguiera gymnorrhiza (L.) Lam., Ceriops tagal C.T. White, and Rhizophora stylosa Griff. Seedlings were planted in the high or low intertidal and in light gaps or under the shade of a closed canopy. Survival and growth of the seedlings were monitored for 30–36 months. Significant differences in survival were found among species, between intertidal zones and due to light level. Averaged across intertidal zones and light level, survival was greatest for Rhizophora and decreased in the order Ceriops, Avicennia, and Bruguiera. For all species survival was greater (P ⩽ 0.001) in the high than in the low intertidal treatment, regardless of light level. Within the high intertidal all species survived better in light gaps than under the canopy. Relative growth of Rhizophora stylosa, Avicennia marina, and Ceriops tagal were greater in the high versus low intertidal and in gaps versus under the canopy. For Bruguiera gymnorrhiza growth was not significantly different between gap and canopy or high and low intertidal. Although Avicennia marina, Bruguiera gymnorrhiza, and Rhizophora stylosa survived and/or grew best in the high intertidal they reach maximum abundance in the low intertidal. Ceriops tagal, however, performed best in the region where it is most abundant, yet even there, it was out-performed by the former three species. These results indicate that the species zonation patterns often observed across the intertidal cannot be explained by physiological adaptation alone. Factors such as propagule dispersal, competition and predation on propagules may also be important.