Scrub Mangroves Research Articles

Climatic and Structural Factors Influencing Epiphytic Bromeliad Community Assemblage along a Gradient of Water-Limited Environments in the Yucatan Peninsula, Mexico

There is a lack of knowledge on the factors driving epiphytic community assemblage along water-limited environments. Epiphytic bromeliad and host communities were analyzed in a range of vegetation types, following a precipitation gradient from 500 to 1500 mm y−1. Plots were set up in scrub mangrove, coastal sand dune scrub, deciduous, semi-deciduous and sub-perennial forests within the Yucatan Peninsula in Mexico. Identity and abundance of hosts and epiphytes, as well as host height and seasonal microenvironmental variables, were recorded at each vegetation type. The study found epiphytic bromeliads to be mainly located in the lower canopy strata of all but the wettest site (sub-peren nial forest). Total epiphyte richness (estimated using species accumulation curves) increased with annual precipitation. Bromeliad species density decreased with vapor pressure deficit and increased with host species density. Semi-variograms and kriging analysis showed a high spatial correlation in bromeliad and host species density. The species composition, however, was unrelated to space, according to a Mantel test, but related to host species composition. The current study shows that diversity and structural integrity of the canopy may be as important as climate in the conservation of epiphytic composition in water-limited environments, where epiphytes are found in sheltered, lower canopy strata.

Data fusion and classifier ensemble techniques for vegetation mapping in the coastal Everglades

This study examined the applicability of data fusion and classifier ensemble techniques for vegetation mapping in the coastal Everglades. A framework was designed to combine these two techniques. In the framework, 20-m hyperspectral imagery collected from Airborne Visible/Infrared Imaging Spectrometer was first merged with 1-m Digital Orthophoto Quarter Quads using a proposed pixel/feature-level fusion strategy. The fused data set was then classified with an ensemble approach based on two contemporary machine learning algorithms: Random Forest and Support Vector Machine. The framework was applied to classify nine vegetation types in a portion of the coastal Everglades. An object-based vegetation map was produced with an overall accuracy of 90% and Kappa value of 0.86. Per-class classification accuracy varied from 61% for identifying buttonwood forest to 100% for identifying red mangrove scrub. The result shows that the framework is promising for automated vegetation mapping in the Everglades.

Dietary variation in the crab Aratus pisonii (H. Milne Edwards, 1837) (Decapoda, Sesarmidae) in a mangrove gradient in northwestern Venezuela

In order to investigate if the variety of resources consumed by Aratus pisonii (H. Milne Edwards, 1837) changes along a structural mangrove gradient, the natural diet of this crab species was studied in five mangrove forests. Mangrove forests with different degrees of structural development (arboreal, arbustive, scrub) and located in different environments (estuarine, marine, hypersaline) on the northwest coast of Venezuela were considered, during the rainy and dry seasons. Rhizophora mangle Linnaeus, 1753 was the predominant mangrove species. The gut contents of 313 individuals were analysed, yielding a total of 12 prey categories. The diet of A. pisonii was based on mangrove fragments (leaves and bark). The index of relative importance (IRI) of mangrove fragments varied between 58% and 96%. Other items supplemented the diet: seagrass, algae, insects and crustaceans. The variety of resources consumed by A. pisonii was related to the mangrove structure and varied with the season. During the rainy period food diversity in the gut content increased, as mangrove heterogeneity and complexity increased, but during the dry season the trend was reversed. Aratus pisonii appears to enhance an opportunistic feeding behaviour when leaf quality decreases, which occurs during the dry season mainly in the hypersaline mangroves with less structural development (arbustive and scrub). During the drought the IRI of the items of animal origin increased in all the localities; this could be a response to the nutritional needs of the crabs during this season. In this work, we report the first documented egg consumption of its conspecifics, which supports the idea of cannibalistic behaviour associated mainly with crabs that live in the hypersaline and scrub mangrove with the lowest structural development and leaf quality.

Water use patterns of estuarine vegetation in a tidal creek system

Water availability is a key determinant of the zonation patterns in estuarine vegetation, but water availability and the use of different water sources over space and time are not well understood. We have determined the seasonal water use patterns of riparian vegetation over an estuarine ecotone. Our aim was to investigate how the water use patterns of estuarine vegetation respond to variations in the availability of tidal creek water and rain-derived freshwater. The levels of natural stable isotopes of oxygen and hydrogen were assessed in the stem of the mangrove Avicennia marina (tall and scrub growth forms), Casuarina glauca and Melaleuca quinquenervia that were distributed along transects from river/creek-front towards inland habitats. The isotopic composition of plant tissues and the potential water sources were assessed in both the wet season, when freshwater from rainfall is present, and the dry season, when mangrove trees are expected to be more dependent on tidal water, and when Casuarina and Melaleuca are expected to be dependent on groundwater. Our results indicate that rainwater during the wet season contributes significantly to estuarine vegetation, even to creek-side mangroves which are inundated by tidal creek water daily, and that estuarine vegetation depends primarily on freshwater throughout the year. In contrast, high intertidal scrub mangroves were found to use the greatest proportion of tidal creek water, supplemented by groundwater in the dry season. Contrary to prediction, inland trees C. glauca and M. quinquenervia were found also to rely predominantly on rainwater--even in the dry season. The results of this study reveal a high level of complexity in vegetation water use in estuarine settings.

Litterfall dynamics in carbonate and deltaic mangrove ecosystems in the Gulf of Mexico

From 1996 to 2002, we measured litterfall, standing litter crop, and litter turnover rates in scrub, basin, fringe and riverine forests in two contrasting mangrove ecosystems: a carbonate-dominated system in the Southeastern Everglades and a terrigenous-dominated system in Laguna de Terminos (LT), Mexico. We hypothesized that litter dynamics is driven by latitude, geomorphology, hydrology, soil fertility and soil salinity stress. There were significant temporal patterns in LT with litterfall rates higher during the rainy season (2.4 g m−2 day−1) than during the dry season (1.8 g m−2 day−1). Total annual litterfall was significantly higher in the riverine forest (12.8 Mg ha−2 year−1) than in the fringe and basin forests (9.7 and 5.2 Mg ha−2 year−1, respectively). In Southeastern Everglades, total annual litterfall was also significantly higher during the rainy season than during the dry season. Spatially, the scrub forest had the lowest annual litterfall (2.5 Mg ha−2 year−1), while the fringe and basin had the highest (9.1 and 6.5 Mg ha−2 year−1, respectively). In LT, annual standing litter crop was 3.3 Mg ha−1 in the fringe and 2.2 Mg ha−1 in the basin. Litter turnover rates were significantly higher in the fringe mangrove forest (4.1 year−1) relative to the basin forests (2.2 year−1). At Southeastern Everglades there were significant differences in annual standing litter crop: 1.9, 3.3 and 4.5 Mg ha−1 at scrub, basin and fringe mangrove sites, respectively. Furthermore, turnover rates were similar at both basin and fringe mangrove types (2.1 and 2.0 year−1, respectively) but significantly higher than scrub mangrove forest (1.3 year−1). These findings suggest that litter export is important in regulating litter turnover rates in frequently flooded riverine and fringe forests, while in infrequently flooded basin forests, in situ litter decomposition controls litter turnover rates.

PENGARUH KADAR ABU GOSOK SELAMA PEREBUSAN DAN LAMA PERENDAMAN AIR TERHADAP KADAR TANIN BUAH DAN TEPUNG MANGROVE (Avicennia marina)

Mangrove fruit has the potential to be developed as a potential food source, but the presence of toxins in the fruit are considered harmful if consumed in the long run. Therefore, research needs to be done to eliminate or reduce the levels of toxins in the mangrove fruit, The poison in one of which is the fruit of mangrove tannins. One alternative to reduce the levels of toxins in the fruit of the mangrove scrub with ash and water immersion. This research used Completely Randomized Design, factorial pattern consisting of two factors with three replications, factor I was rubbing ash powder content (5%, 10%, and 15%), and factor II is long soaking water for 6 hours (with 4, 8, 12 times the turnover of water). The results showed that treatment with boiling some rubbing ash content (5%, 10%, and 15%) and long soaking water to give a significant influence (P <0.05) toward decreased levels of mangrove Avicennia marina fruit tannins. Treatment of boiling ash content of 15% gave the lowest reduction is 28.92% and the long soaking water for 6 hours with a change of water 12 times giving the low tannin levels decrease is 28.80%, as well as mangrove fruit flouring will A.marina lower levels of tannin by 28.44%.

Influence of patch, habitat, and landscape characteristics on patterns of Lower Keys marsh rabbit occurrence following Hurricane Wilma

Degradation of coastal systems has led to increased impacts from hurricanes and storm surges and is of concern for coastal endemics species. Understanding the influence of disturbance on coastal populations like the endangered Lower Keys marsh rabbit (Sylvilagus palustris hefneri) is important to understanding long-term dynamics and for recovery planning. We evaluated the effect of disturbance on the rabbits by determining which patch, habitat, and landscape characteristics influenced habitat use following Hurricane Wilma. We determined patch-level occurrence 6–9 months prior to Hurricane Wilma, within 6 months following the hurricane, and 2 years after the storm to quantify rates of patch abandonment and recurrence. We observed high patch abandonment (37.5% of used patches) 6 months after Hurricane Wilma and low rates of recurrence (38.1% of abandoned patches) 2 years after the storm, an indication that this storm further threatened marsh rabbit viability. We found the proportion of salt-tolerant (e.g., mangroves and scrub mangroves) and salt-intolerant (e.g., freshwater wetlands) vegetation within LKMR patches were negatively and positively correlated with probability of patch abandonment, respectively. We found patch size and the number of used patches surrounding abandoned patches were positively correlated with probability of recurrence. We suggest habitat use following this hurricane was driven by the differential response of non-primary habitats to saline overwash and habitat loss from past development that reduced the size and number of local populations. Our findings demonstrate habitat use studies should be conducted following disturbance and should incorporate on-going effects of development and climate change.

Study on the utility of IRS 1D LISS-III data and the classification techniques for mapping of Sunderban mangroves

Mangrove conservation and management is a stupendous task chiefly due to the inaccessibility and the hostile substrate conditions. Remote sensing technology serves as an important tool in providing fast, accurate and up-to-date baseline information on the status of mangroves. It is almost impossible to carry out conventional field surveys in these swampy areas. The present study aims at the classification and mapping of the mangroves in Sunderban Biosphere Reserve (SBR) in the West Bengal province of India using IRS 1D LISS-III satellite data. Different classification approaches, viz., on-screen visual interpretation, supervised and unsupervised classifications were tried. The study showed that four mangroves classes, viz., Avicennia, Phoenix, mixed mangroves, and mangrove scrub and eight non-mangrove classes could be delineated using all the three approaches. All the mangrove and non-mangrove classes were field verified and the overall accuracy as well as user’s and producer’s accuracies for each category were determined. It was observed that among the three approaches, on-screen visual interpretation yielded higher classification accuracy (91.67%) compared to supervised (79.90%) and unsupervised classifications (71.08%). The results obtained through on-screen visual interpretation showed that all mangrove categories together cover 23.21% of the total geographical area of SBR, of which the mixed mangrove category covers maximum area (18.31%). Among the non-mangrove classes, the waterbody occupies largest area (35.36%) followed by agriculture (34.51%).

Geospatial modelling of biological richness in Sunderbans

The present study attempts to assess the biological richness in Sunderban Biosphere Reserve (SBR) using a three-pronged approach i.e. satellite image (IRS 1D LISS-III) for vegetation/land use stratification, landscape analysis for disturbance regimes assessment and the disturbance regimes together with the ecosystem uniqueness, species richness and importance value for biological richness modelling. The study showed that four mangrove categories, viz., Avicennia, Phoenix, mixed mangroves and mangrove scrub, cover 23.21 per cent of the total geographical area of SBR. The largest area is occupied by mixed mangroves (18.31%). The overall accuracy of the vegetation/land use map worked out to be 91.67 per cent. The disturbance analysis revealed that the vegetation types were not much disturbed. Shannon-Weaver’s index of diversity was highest in case of mixed mangrove. The results revealed that 75 per cent forest area has high biological richness.

Hydrological drivers of wetland vegetation community distribution within Everglades National Park, Florida

The influence of hydrological dynamics on vegetation distribution and the structuring of wetland environments is of growing interest as wetlands are modified by human action and the increasing threat from climate change. Hydrological properties have long been considered a driving force in structuring wetland communities. We link hydrological dynamics with vegetation distribution across Everglades National Park (ENP) using two publicly available datasets to study the probability structure of the frequency, duration, and depth of inundation events along with their relationship to vegetation distribution. This study is among the first to show hydrologic structuring of vegetation communities at wide spatial and temporal scales, as results indicate that the percentage of time a location is inundated and its mean depth are the principal structuring variables to which individual communities respond. For example, sawgrass, the most abundant vegetation type within the ENP, is found across a wide range of time inundated percentages and mean depths. Meanwhile, other communities like pine savanna or red mangrove scrub are more restricted in their distribution and found disproportionately at particular depths and inundations. These results, along with the probabilistic structure of hydropatterns, potentially allow for the evaluation of climate change impacts on wetland vegetation community structure and distribution.

Sedimentation within and among mangrove forests along a gradient of geomorphological settings

Coastal wetlands provide important ecological services to the coastal zone, one of which is sediment retention. In this study we investigated sediment retention across a range of geomorphological settings and across vegetation zones comprising coastal wetlands. We selected six coastal wetlands dominated by mangroves over a gradient from riverine to tidal settings in Southeast Queensland, Australia. Each site was comprised of three distinct vegetation communities distributed as parallel zones to the coast line: seaward fringe mangroves, landward scrub mangroves and saltmarsh/ cyanobacteria mat of the high intertidal zone. We measured suspended sediment retention and sedimentation rates. Additionally, in order to assess the origin of sediment transported and deposited in the mangroves, glomalin, a novel terrestrial soil carbon tracer, was used. Our results show a mean average sedimentation of 0.64 ± 0.01 mg cm −2 spring tide −1, which was variable within sites, regardless of geomorphological setting. However, geomorphological setting influenced spatial patterns of sediment deposition. Riverine mangroves had a more homogeneous distribution of sediments across the intertidal zone than tidal mangroves, where most sedimentation occurred in the fringe zone. Overall, the fringe zone retained the majority of sediment entering the coastal wetland during a tidal cycle with 0.90 ± 0.22 mg cm −2 spring tide −1, accounting for 52.5 ± 12.5% of the total sedimentation. The presence of glomalin in suspended sediments, and thus the relative importance of terrigenous sediment, was strongly influenced by geomorphological setting, with riverine mangroves receiving more glomalin in suspended solids than tidal mangroves. Glomalin was also differentially deposited within the vegetation zones at different geomorphological settings: primarily at the fringe zone of tidal mangroves and within the scrub zone of riverine mangroves. The differences we observed in the spatial distribution of sedimentation and the difference in the origin of the sediment deposited in riverine and tidal mangroves are likely to have an impact on ecological processes.

Photosynthetic performance and resource utilization of two mangrove species coexisting in a hypersaline scrub forest.

In a hypersaline mangrove scrub forest in northern Florida, coexisting trees of Laguncularia racemosa and Avicennia germinans were either fertilized with nitrogen or phosphorus, or not fertilized (controls). We aimed to test whether nutrient additions differentially altered photosynthetic performance and resource utilization in these two species. In control trees, photosynthetic rates were higher in L. racemosa than A. germinans. However, leaf nitrogen concentrations were higher in A. germinans than L. racemosa. Avicennia germinans responded to fertilization with nitrogen by increasing leaf nitrogen concentrations and rates of photosynthesis such that they were equivalent to photosynthesis in L. racemosa. Laguncularia racemosa did not show a response to nitrogen additions. Neither species showed strong responses to phosphorus fertilization. Avicennia germinans had high photosynthetic water-use efficiency (photosynthesis/transpiration), but low photosynthetic nitrogen-use efficiency (photosynthesis/leaf nitrogen). In contrast, L. racemosa had comparatively low photosynthetic water use efficiency and high photosynthetic nitrogen use efficiency. Leaf level characteristics lead us to hypothesize that coexistence of A. germinans and L. racemosa should occur where nitrogen levels are low and salinity is moderate, or at least moderate for some period of the year.

Comparative study of water uptake and photosynthetic gas exchange between scrub and fringe red mangroves, Rhizophora mangle L.

The red mangrove (Rhizophora mangle L.) occurs frequently in both scrub and fringe mangrove forests. Our previous study demonstrated that individuals of this mangrove species growing in scrub and fringe forests differ significantly in both morphological and physiological characteristics. To further characterize physiological differences between scrub and fringe mangroves, we compared their differences in water uptake and photosynthetic gas exchange during different seasons. In the wet season (June-October, 1990), scrub mangroves showed lower δD and δ18O values of stem water than fringe mangroves, indicating more usage of rain-derived freshwater. In the dry season (Jan-April, 1991), however, scrub mangroves utilized the same water source as fringe mangroves, reflected by their similar δD and δ18O values of stem water. Consistently, there were significant differences in predawn water potentials between scrub and fringe mangroves in the wet season (October 1990) with higher values for scrub mangroves, but no significant differences in the dry season (January 1991). Higher elevation in the scrub forest seems to be the major factor responsible for the shift of water sources in scrub mangroves. On Apr. 27 and Aug. 8, 1990, scrub mangroves showed lower CO2 assimilation rate, stomatal conductance, and intercellular CO2 concentration than fringe mangroves. There were no differences in these gas exchange characteristics on the other two measuring dates: Oct. 17, 1990 and Jan. 11, 1991. Instantaneous water use efficiency was significantly higher for scrub mangroves than for fringe mangroves on three of the four sampling dates. Similarly, leaf carbon isotope discrimination of scrub mangroves was always significantly lower than that of fringe mangroves, indicating higher long-term water use efficiency. Higher water use efficiency in scrub mangroves is a result of stomatal limitation on photosynthesis, which may entail considerable carbon cost to the plants.

Differences in morphology, carbon isotope ratios, and photosynthesis between scrub and fringe mangroves in Florida, USA

All three mangrove species in Florida (USA), Rhizophora mangle L., Laguncularia racemosa Gaert. and Avicennia germinans (L.) L., are found frequently in scrub mangrove forests, in which individuals rarely exceed 1.5 m in height. In the present study, the differences in morphological characteristics, leaf carbon isotope ratios and photosynthetic gas exchange between individuals in scrub and fringe mangrove forests in south Florida were investigated quantitatively. Plants in the scrub forests had much lower canopy height, more main stems per tree and smaller leaves, relative to those in the fringe forests. There was a significant correlation between tree height and leaf δ 13C value, with higher δ 13C values (1–4% more positive) for plants in the scrub mangrove forests. Correspondingly, scrub mangroves showed significantly lower intercellular carbon dioxide (CO 2) concentration and higher intrinsic water use efficiency over long-term carbon assimilation, relative to fringe mangroves. Photosynthetic gas exchange measurements on R. mangle individuals showed a 15.5% lower CO 2 assimilation rate, 6.1% lower intercellular CO 2 concentration and 11.6% higher intrinsic water use efficiency in scrub mangroves, consistent with those estimated from leaf carbon isotope ratios. A higher slope for the linear correlation between CO 2 assimilation rate and stomatal conductance was observed for the individuals in the scrub mangrove forest, which is in agreement with other measurements indicating higher water use efficiency in scrub mangroves. Possible environmental factors responsible for these morphological and physiological differences between scrub and fringe mangroves are discussed.

Effect of Growth Form, Salinity, Nutrient and Sulfide on Photosynthesis, Carbon Isotope Discrimination and Growth of Red Mangrove (Rhizophora mangle L.)

The red mangrove (Rhizophora mangle L.), a dominant mangrove species in Florida, frequently occurs in two distinct growth forms, scrub and tall trees. These two growth forms show significant differences in physiology in the field, with lower CO2 assimilation rate, stomatal conductance, and carbon isotope discrimination or higher transpiration efficiency for the scrub form. To elucidate the possible factors responsible for these physiological differences, we studied the physiological and growth responses of scrub and tall red mangrove seedlings grown hydroponically in the greenhouse under 12 different growth conditions combining three salinities (100, 250, 500 mM NaCl), two nutrient levels (10, 100% strength of full nutrient solution), and two sulfide concentrations (0, 2.0 mM Na2S). The two growth forms showed similar physiological and growth responses to these treatments, suggesting no genetic control of physiological and growth differences between the growth forms of this species. High salinity, low nutrient level, and high sulfide concentration all significantly decreased CO2 assimilation, stomatal conductance, and plant growth, but only salinity significantly decreased intercellular CO2 concentration and leaf carbon isotope discrimination, suggesting that the lower carbon isotope discrimination, or higher transpiration efficiency, observed for scrub mangroves in the field is caused only by high salinity during the dry season. Hypersalinity thus seems to be one of the stressful environmental conditions common to all scrub red mangrove forests studied in southern Florida.

The inland mangroves of Inagua

Summary Mangroves located 50 km inland in the Lake Windsor basin in Inagua are described and compared with the island coastal mangroves. Measurements of soil and surface water salinity yielded high values typical of arid climates. Avicennia germinans and Laguncularia racemosa were found growing in the higher salinity soils while Rhizophora mangle was found fringing inland ocean holes with low salinities. Conocarpus erecta grew in elevated ridges. Scrub mangroves grow in the most extreme conditions of high salinity or over rocky soils. It is postulated that fresh sea water flows from ocean holes into the large Lake Windsor basin where high evaporation concentrates salts. Storms leach salts and maintain stable salinity conditions. Thus, the inland mangroves are not isolated from the sea but critically dependent on it as a source of fresh sea water. A critical examination of the data presented for the island of Barbuda by Stoddart, Bryan and Smith (1973) supports the idea that inland mangroves are connected ...

Structure of Mangrove Forests in Florida, Puerto Rico, Mexico, and Costa Rica

Structural parameters of mangrove forests were measured at seven geographical locations in Florida (U.S.A.), Puerto Rico, Mexico, and Costa Rica. One-tenth hectare plots were subdivided into twenty 5 x 10 m plots where all individuals greater than 2.5 cm DBH were measured and recorded. Height of the tallest tree in each plot was measured. The complexity index developed by Holdridge (1967) as an integrative measure that combines floral characteristics (number of species) (s), stand density (d), basal area (b), and height (h) was computed as follows: (s) (d) (b) (h) 10-3. The riverine and basin mangrove forests of the southwestern coast of Florida had considerably taller trees (6-9 m) and larger basal areas (20.3-38.5 m2/ha) than did scrub mangrove growing on the southeastern Coast of Florida where a low canopy (1.0 m), a low basal area (6.0 m2/ha), and a correspondingly low complexity index (1.5) were measured. The riverine forests of the Marismas Nacionales located on the Pacific coast of Mexico had high complexity indices (49.7-73.2) due to the large basal areas (57.8-60.8 m2/ha), tall canopies (16-17 m) and a large number of trees greater than 10 cm diameter (103-145/ha). The entire estuarine system of the Marismas Nacionales acts as a receiving basin for large quantities of freshwater runoff and nutrient-rich sediments from surrounding upland watersheds. Except for local sport fishing, the commercial fishing industry of Florida, Puerto Rico, and Costa Rican mangrove waters is not nearly as active as that observed near Teacap'an, Mexico. The structural characteristics of the north- and south-coast mangroves of Puerto Rico are probably best described by the direct and indirect influences of the climate at each coastline. The basin mangrove forest on the humid north coast (annual rainfall of 1631 mm) has a basal area of 17.8 m2/ha, canopy height of 13.6 m, and a complexity index of 16.7. The driest site (rainfall of 860 mm/year) on the south coast of Puerto Rico was a red mangrove fringe forest which had a complexity index of 0.9, low canopy height (7 m), and low basal area (6.9 m2/ha). Here upland runoff, draining into the mangroves through porous limestone outcrops, only occurs 1-2 months annually. In Costa Rica, the riverine mangroves near Puerto Limon on the Caribbean coast (annual rainfall- of 3306 mm) are more structurally developed than either the riverine or fringe mangroves of the drier (1800 mm annual rainfall) Pacific coast sites. A basal area of 96.4 m2/ha (primarily Pterocarpus officinalis Jacq.) and a tall canopy (16 m) gave this forest the largest complexity index (84.5) of all forests inventoried. The Pacific coast mangroves of Costa Rica are exposed to seasonal rainfall (six months dry season), and this drier environment was reflected in a shorter canopy (9.510.0 m) and a lower basal area (23.2-32.9 m2/ha).

Reconstructing Triassic vegetation of eastern Australasia: a new approach for the biostratigraphy of Gondwanaland

As is widely recognised, fossil florules are difficult to correlate because they appear to have been more controlled by environmental and ecological factors than uniform changes which reflect the passage of geological time. Here I present a worked example of a generally applicable approach to this problem in dating and mapping fossil plant associations. Floral associations are best described and named by the classical methods of phytosociology. The three dimensional shape of an association in rocks and its palaeoecology can then be critically assessed. For example, during Middle Triassic time the following plant associations would have been encountered on a traverse from the coast to several hundred kilometres inland from the Pacific margin of Gondwanaland: Pachydermophylletum (mangrove scrub), Linguifolietum (coastal swamp woodland), Dicroidietum odontopteroidium (floodplain forest), Phoenicopsetum (levee bank scrub), Dicroidietum odontopteroidium xylopterosum (xerophytic woodland) and Johnstonietum (mal...

Desert dunes and estuarine fill in the Fitzroy estuary (North-Western Australia)

Summary In the R. Fitzroy estuary, Western Australia, there is a remarkable interfingering of linear desert dunes and tidal flats. The dunes, fixed by Acacia shrub and at right angles to the resultant of present sand-shifting winds, belong to a former more arid climate. The extensive tidal flats form part of considerable Holocene estuarine sedimentation. The age of the dunes and associated aridity was the prime object of this investigation. Despite difficulties with insensitive sea level indicators and large tidal ranges, it is evident that sea level reached much closer to its present position by 7400–6000 BP than in Europe or N. America. This agrees with recent work elsewhere in Australia showing that it behaved differently in this respect. At the maximum of the Holocene transgression tidal action cut back the flanks and tips of the desert dunes and truncated others completely, leaving only concealed plinths. Sediment mixing through this erosion produced a shore facies, reaching to 1–2m above HW springs and subjected to pronounced pedogenesis. Whether there was a Mid-Holocene sea level of + 1–2m remains uncertain but various geomorphic traits fit with this hypothesis better than with self-exclusion of the tide by estuarine sedimentation. Differences between air photography in 1949 and 1967 witness rapid erosional loss and constructional gain of mangrove swamp and salt marsh, with a marked net loss. Much of the wide high-tidal land lacks vegetation because of dry season hypersalinity; other parts form shallow intertidal lagoons behind low estuarine levees. Low mangrove forest and mangrove scrub occupy restricted mid-tidal zones. Greater extent and vigour of mangrove growth, including tall mangrove forest, in the period 7400–6000 BP require a longer and rainier wet season than is found with the present tropical monsoonal semi-arid climate. The desert dunes are overlain by the sediments of the Holocene transgression, including mangrove clays dated at 6000 BP. Dune sands were proved to nearly 15m below HW springs, taking their minimum age back to about 8000 BP. Well preserved dune shape implies a maximum age within the Upper Pleistocene. Related evidence from the Timor Shelf points to a last pleniglacial age for the Fitzroy dunes, whose direction is best interpreted on the basis of the likely wind regime of a full glacial paleoclimate with more prolonged Trade Winds and reduced rainbearing monsoonal winds.

The Relation of Vegetation and Soil to Development of Mangrove Swamps for Rice Growing in Sierra Leone

Attempts to reclaim mangrove swamps for rice growing in Sierra Leone have shown the importance of a greater knowledge of the natural vegetation and its relation to soil and water conditions. Five species of mangrove commonly occur in West Africa, namely Rhizophora racemosa, R. mangle and R. harrisonii, together with Avicennia germinans and Laguncularia racemosa. Laguncularia is usually found in a stunted form in secondary mangrove scrub, but it has also been seen, apparently as a colonizer, on fresh silt. Its ecological status is therefore obscure, but it is of minor importance. The other species are all closely linked with the character of the soil and its mode of formation. In Sierra Leone Rhizophora racemosa is the primary colonizer on freshly deposited silts, where it forms pure stands along the edges of creeks. It rarely forms pure stands, or grows to its full height, in other situations. Avicennia does not normally occur along rivers and creeks, but rather on the sea coast. It appears to be associated with sandier soils and is sometimes a primary colonizer. Avicennia can occur in pure stands of mature trees, but this is not usual. Rhizophora harrisonii and R. mangle often occur as undershrubs in open A vicennia woodland, and they sometimes also form dense thickets, in association with scrub Rhizophora racemosa, on sites where primary R. racemosa has died out, leaving the remains of its fibrous rootlets. Herbaceous plants may be associated with this mixed woodland. The most typical of these are the salt-tolerant fern Achrostichum aureum and, in more open places, the halophytes Sesuvium portulacastrum and Philoxerus vermicularis. On sandier soils which have not borne Rhizophora racemosa and are not, therefore, fibrous, the palm Phoenix reclinata and Conocarpus erectus are typical of the fringing vegetation between the mangrove lands and higher ground, and may also occur mixed with the mangroves along their boundary. Annona glabra, Heteropteris leona and the climber Stigmaphyllon ovatum are also found in the sandier fringing areas, and they extend along the fringes of the fibrous mangrove soils where, presumably because they are acid, there is usually an area free of woody vegetation between the mangrove and the vegetation which fringes the higher ground. This area may be quite bare or may support a cover of the salt-tolerant grass Paspalun vaginatum. All this mangrove land is below the level of the high spring tides and all of it is consequently flooded to a greater or lesser extent by tide water. Along the coast, where the catchment areas of minor creeks are too small to accumulate fresh water, even during heavy rain, the tide water is always saline, but where major rivers bring large volumes of fresh water to the sea during the rainy season the mangrove land is flooded intermittently through a large part of the year by fresh water backed up by the tides. These fresh-flooding estuarine mangrove lands have been intensively cultivated for rice during the past 60 or 70 years, and they give good yields under natural flooding conditions. There is no trouble from salt; the small amount which comes on to the land in the dry season prevents the growth of weeds and is rapidly leached from the soil at the onset of the rains. In more recent years attention has turned to the development of the more saline mangrove lands near the coast, and the first attempts at reclamation were made about 20