Morphological and biomechanical responses of floodplain willows to tidal flooding and salinity

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River connectivity and climate behind the long‐term evolution of tropical American floodplain lakes

This study presents the long‐term evolution of two floodplains lakes (San Juana and Barbacoas) of the Magdalena River in Colombia with varying degree of connectivity to the River and with different responses to climate events (i.e., extreme floods and droughts). Historical limnological changes were identified through a multiproxy‐based reconstruction including diatoms, sedimentation, and sediment geochemistry, while historical climatic changes were derived from the application of the Standardised Precipitation‐Evapotranspiration Index. The main gradients in climatic and limnological change were assessed via multivariate analysis and generalized additive models. The reconstruction of the more isolated San Juana Lake spanned the last c. 500 years. Between c. 1,620 and 1,750 CE, riverine‐flooded conditions prevailed as indicated by high detrital input, reductive conditions, and dominance of planktonic diatoms. Since the early 1800s, the riverine meander became disconnected, conveying into a marsh‐like environment rich in aerophil diatoms and organic matter. The current lake was then formed around the mid‐1960s with a diverse lake diatom flora including benthic and planktonic diatoms, and more oxygenated waters under a gradual increase in sedimentation and nutrients. The reconstruction for Barbacoas Lake, a waterbody directly connected to the Magdalena River, spanned the last 60 years and showed alternating riverine–wetland–lake conditions in response to varying ENSO conditions. Wet periods were dominated by planktonic and benthic diatoms, while aerophil diatom species prevailed during dry periods; during the two intense ENSO periods of 1987 and 1992, the lake almost desiccated and sedimentation rates spiked. A gradual increase in sedimentation rates post‐2000 suggests that other factors rather than climate are also influencing sediment deposition in the lake. We propose that hydrological connectivity to the Magdalena River is a main factor controlling lake long‐term responses to human pressures, where highly connected lakes respond more acutely to ENSO events while isolated lakes are more sensitive to local land‐use changes.     

Shoot dynamics of the giant grass Gynerium sagittatum in Peruvian Amazon floodplains,a clonal plant that does show self-thinning

The giant rhizomatous grass Gynerium sagittatum is an early successional species that forms dense monocultures in Peruvian Amazon floodplains. We studied the shoot population structures by recording shoot densities and shoot heights. Leaf areas and stem volumes were allometrically estimated. Stands of two varieties of G. sagittatum were examined that differ in height and in the degree of shoot branching. In stands of increasing age, marked decreases in shoot densities were accompanied with an increase in mean shoot size. Self-thinning was indicated by the negative correlation between log stem volume per unit ground area and log shoot density, significant at least for one of the two varieties. The difference in thinning slope between the varieties could be largely accounted for by their different shoot geometry, as was revealed by calculations based on the allometric model of Weller (1987b). The relationship between log leaf area per shoot and log shoot density was significantly negative with slopes close to –1. Shoot size inequalities decreased with increasing mean stem volume per shoot, probably as a result of density-dependent mortality of the smaller shoots. All of these results accord with expectations for shoot self-thining. Gynerium sagittatum is the first clear example of a clonal plant species that exhibits self-thining in natural monospecific stands. It is argued that self-thinning occurs in this giant tropical grass because its shoots are perennial and do not experience seasonal die-back (periodic density-independent mortality), in contrast to many of the clonal plant species that have been studies so far.     

Relationship between vegetation and environmental conditions in the northern Pantanal of Mato Grosso, Brazil

This paper analyses the relationship between the distribution of tropical floodplain vegetation and environmental factors such as the soil types, their physical‐chemical properties and inundation dynamics in the northern Pantanal of Mato Grosso, Brazil. Fifty‐four soil profiles of the four principal geomorphologic units of the lowland were classified into twenty‐one units of the FAO‐UNESCO system and related to eight vegetation formations. Under the deciduous and semideciduous woodland and forest formations on flood free riverbanks and relict dunes Arenosols, Solonetz and Alisols dominate. Inundation forests grow on a large variety of soil groupings such as Alisols, Solonetz, Fluvisols, Gleysols and Cambisols, mainly on fluvial deposits. Under the grassland formations of periodically flooded spreadings and depressions Planosols, Acrisols, Alisols, Fluvisols, Arenosols and Cambisols were cored, while Gleysols are common on swamps bearing Thalia geniculata or Cyperus giganteus. Canonical Correspondence Analysis was applied to study the influence of monitored environmental factors on spatial distribution of vegetation formations. Water regime and soil texture were found to define the grassland‐forest boundaries as well as the transition between different grassland formations and between seasonal as well as evergreen forests.     

Spatial and temporal variations in densities of small fishes across different temporary floodplain types of the lower Okavango Delta,Botswana

Small-sized and juvenile fishes as well as physicochemical water parameters in various primary, secondary and rarely flooded temporary floodplains of the Okavango Delta were assessed during the different hydrological phases of the 2009–2010 flooding season. Small fishes were sampled in the marginal zone of the floodplains using a throw-trap net and a suite of physicochemical properties were measured. Both physicochemical water quality parameters and densities of small fishes showed spatiotemporal variations across the temporary floodplain types. Turbidity, dissolved oxygen, conductivity, woody debris and chlorophyll a were all highest in rarely flooded floodplains compared to both primary and secondary floodplains (ANOVA, p < 0.05). Fish from 38 fish taxa belonging to 11 families were identified and classified during the study. The fish families Cichlidae (dominated by tilapias), Poeciliidae (Aplocheilichthys spp.) and Cyprinidae (Barbus spp.), were the most abundant across the study floodplains. The poeciliids were most abundant in frequently flooded primary and secondary floodplains, whereas juvenile cichlids dominated in rarely flooded floodplains. During high floods the rarely flooded portions of the delta function as important nursery habitats for juvenile cichlids, implying that a significant reduction in flooding may have negative effects on the delta's tilapia stocks due to reduced recruitment success.     

On ecology,life history and survival strategies of a wing‐dimorphic ground beetle (col.: Carabidae: Odacanthini: Colliuris) inhabiting central Amazonian inundation forests

The discovery of a lowland Amazonian wing‐dimorphic carabid species, Colliuris batesi Chaudoir, prompted a complete study of its biology and behaviour over a period of more than 4 years. Studies were made on development, fecundity, gonad development, life span and habitat shifts. Survival strategies and possible adaptations are examined with respect to seasonal flooding of their habitat. It is proposed that evolution of brachyptery in this species is rather recent in origin because macropterous specimens are widespread across the upper Amazon Basin, and because the only body change associated with short wings has been narrowing of the humeri.     

Fish species diversity in sand bank habitats of a neotropical river

The fish fauna of the Cinaruco River, an intermediate sized floodplain river that forms the southern boundary of the newly established Santos Luzardo National Park in the llanos of Venezuela, was sampled in 1993-1994. Due to highly seasonal rainfall, the nutrient-poor Cinaruco undergoes dramatic changes in water level, creating a variety of seasonally available habitats for fishes. Sand bank habitats are conspicuous features in both main-channel and lentic backwater areas, and support fish assemblages that are different from adjacent rocky shore and shrubby shore habitats. Seine samples from sand bank habitats revealed high alpha diversity, dominated numerically by a few species of small Characiformes. Comparisons among and between lagoon, side-channel, and main-channel sand bank habitats showed little assemblage similarity. Overall, sand bank habitats were used by 8 orders, 21 families, and 105 species of fish, mostly of size classes less than 100 mm. Several species are currently undescribed. Elements which may contribute to high fish diversity include proximity to a diverse Amazonian fauna, seasonally dynamic habitat availability, the influence of keystone predators, and episodes of intermediate disturbance, such as seasonal release from intense biotic interactions.     

Physiological responses to flooding and light in two tree species native to the Amazonian floodplains

In Amazonian floodplains, plant survival is determined by adaptations and growth strategies to effectively capture sunlight and endure extended periods of waterlogging. By measuring gas exchange, quantum efficiency of photosystem 2 (PSII), and growth parameters, we investigated the combined effects of flooding gradients and light on two common evergreen floodplain tree species, the light-tolerant Cecropia latiloba and the shade-tolerant Pouteria glomerata. Individual plants were subjected to different combinations of light and flooding intensity in short-term and long-term experiments. Plants of C. latiloba lost all their leaves under total submersion treatments (plants flooded to apex and with reduced irradiance) and showed highest maximum assimilation rates (A_max) in not flooded, high light treatments (6.1 μmol CO₂ m⁻² s⁻¹). Individuals of P. glomerata showed similar patterns, with A_max increasing from 1.9 μmol CO₂ m⁻² s⁻¹ under total flooding to 7.1 μmol CO₂ m⁻² s⁻¹ in not flooded, high light treatments. During the long-term flooding experiment, quantum efficiency of PSII (F_v/F_m) of C. latiloba was not affected by partial flooding. In contrast, in P. glomerata F_v/F_m decreased to values below 0.73 after 120 days of total flooding. Moreover, total submergence led P. glomerata to reduce significantly light saturation point (LSP), as compared to C. latiloba. For both species morphological adjustments to long-term flooding, such as the production of adventitious roots, resulted in reduced total biomass, relative growth rate (RGR) and leaf mass ratio (LMR). Growth increase in C. latiloba seemed to be more limited by low-light than by flooding. Therefore, the predominant occurrence of this species is in open areas with high light intensities and high levels of inundation. In P. glomerata flooding induced high reductions of growth and photosynthesis, whereas light was not limiting. This species is more abundant in positions where irradiance is reduced and periods of submergence are slightly modest. We could show that the physiological requirements are directly responsible for the flooding (C. latiloba) and shade (P. glomerata) tolerance of the two species, which explains their local distribution in Amazonian floodplain forests.     

Fish larvae, growth and biomass relationships in an Australian arid zone river: links between floodplains and waterholes

1. Floodplain inundation provides many benefits to fish assemblages of floodplain river systems, particularly those with a predictable annual flood pulse that drives yearly peaks in fish production. In arid‐zone rivers, hydrological patterns are highly variable and the influence of irregular floods on fish production and floodplain energy subsidies may be less clear‐cut. To investigate the importance of floodplain inundation to a dryland river fish assemblage, we sampled fish life stages on the floodplain of Cooper Creek, an Australian arid‐zone river. Sampling was focused around Windorah during a major flood in January 2004 and in isolated waterholes in March 2004 following flood drawdown. 2. Of the 12 native species known to occur in this region, 11 were present on the floodplain, and all were represented by at least two of three life‐stages – larvae, juveniles or adult fish. Late stage larvae of six fish species were found on the floodplain. There were site‐specific differences in larval species assemblages, individual species abundances and larval distribution patterns among floodplain sites. 3. Significant growth was evident on the floodplain, particularly by larval and juvenile fish, reflecting the combination of high water temperatures and shallow, food rich habitats provided by the relatively flat floodplain. 4. Low variation in biomass, species richness and presence/absence of juvenile and adult fish across four floodplain sites indicates consistently high fish productivity across an extensive area. 5. Similarities and differences in fish biomass between the floodplain and isolated post‐flood waterholes suggest high rates of biomass transfer (involving the most abundant species) into local waterholes and, potentially, biomass transfer by some species to other waterholes in the catchment during floodplain inundation and after floods recede. 6. The high concentration of fish on this shallow floodplain suggests it could be a key area of high fish production that drives a significant proportion of waterhole productivity in the vicinity. The Windorah floodplain provides favourable conditions necessary for the spawning of some species and juvenile recruitment of the majority of species. It is also appears to be a significant conduit for the movements of fish that underpin high genetic similarity, hence population mixing, of many species throughout the Cooper Creek catchment. The high floodplain fish production in turn provides a significant energy subsidy to waterholes after floodwaters recede. 7. The identification of key sites of high fish production, such as the Windorah floodplain, may be important from a conservation perspective. Key management principles should be: maintenance of the natural flooding regime; identification of the most productive floodplain areas; and maintenance of their connectivity to anastomosing river channels and the remnant aquatic habitats that ultimately sustain this fish assemblage through long‐term dry/drought and flood cycles.     

Adaptations to Flooding Stress: From Plant Community to Molecule

Abstract: This review highlights four major topics in plant flooding research: the processes underlying vegetation zonation in the floodplain, the challenges of using model species to reveal adaptive responses in shoots and roots, the role of micro-organisms in flooded soils in relation to plant growth, and the molecular regulation of the hormone ethylene which is heavily involved in the adaptation reaction of flood-resistant plants. Model species and vegetation strategies are used to unravel mechanisms of vegetation zonation in the river flood-plain. In the case of woodlands, hydrological conditions determine to a large extent their zonation patterns under natural conditions. For softwood species, such as Salicaceae, the interaction between water levels and timing of seed dispersal is the dominating process determining their establishment success on river banks. Their strategy is well adapted to irregular, high and prolonged floods. Hardwood species, Quercus, Fraxinus, UImus and Acer, are flood-sensitive and inhabit the higher sites. They mainly have heavy seeds and germinate under shaded conditions. The most shade-tolerant hardwood species are the least well adapted to flooding. Anthropogenically influenced parts of the floodplain are characterized by grasslands with elevation level and management practices determining the species composition. Low-lying grasslands have flood-tolerant species; elevated zones are seldom flooded and have flooding-sensitive species. Following Grime (1998^[59]), plant species of major vegetation types within the floodplain zone can be divided into three categories–dominants, subordinates and transients–illustrating the diversity in plant species in relation to environmental properties. Model species that are indicative of the different conditions in the various zones are chosen to help in the understanding of morphological and physiological adaptations at the plant level. The formation of aerenchymatous roots and the capacity to elongate shoot parts upon submergence are among the main responses of surviving plants. The role of hormones in the adaptation reaction is emphasized. Owing to high porosities in roots of flood-tolerant plants, radial oxygen loss greatly influences nitrification and denitrification processes in the flooded soil. Nutrient cycles are restored by root-derived oxygen and the oxygenated rhizosphere is detoxified. A new development in flooding ecology is the unravelling of the molecular regulation of hormonally controlled processes. The expression of an ethylene receptor gene in Rumex palustris is highlighted. This paper ends with some suggestions for future flooding research.