Helophyte Species Research Articles

Geochemical dynamics of phosphorus and metals in sediments of a shallow coastal lake (Xuño Lake, Galicia-NW Iberian Peninsula)

The shallow lakes are important freshwater ecosystems, since they support much of biodiversity and ecosystem services of life on land. Shallow lakes are highly dynamic ecological entities that can exist in several alternative stable states through regime shift caused by a natural or human disturbance that exceeds ecological thresholds for biological communities composition and structure equilibria. The sediment as a reservoir has a key role in the limnological regulation of wetlands linked to the fluxes of nutrients and elements in the biogeochemical interplay with the water and macrophytes. For this reason, the role of sediment in the limnology of the shallow coastal lake of Xuño (NW Iberian Peninsula) was explored by seasonally monitoring the chemical composition of water and sediments, also according to macrophyte species. The shallow depth determines the high availability of light in the bottom and a well-mixed water column maintain the surface of the water–sediment interface oxygenated. The oxic conditions of the bottom implies a top-down regulation of the water column in the Xuño shallow lake that limits the diffusion of phosphorus and trace metals (Fe, Mn, Cu, and Co) to the water, buffering eutrophication or contamination levels by immobilization in the sediments. In fact, the concentration of Hg in the lake water in spring, and also its bioavailability, are high due to its release from the sediment in suboxic conditions. The cover of helophyte species Phragmites Australis and Schoenoplectus Lacustris showed differences in the assimilation of organic monoester and diester phosphorus forms in the sediment. However, the water of the Xuño Lake shows an eutrophic status by the nutrient input associated with the birds populations as indicated by microbiological data.

Helophytes adapt to water and N-enrichment stresses by adjusting and coordinating stoichiometry characteristics in main organs

Exploring the adaptation strategies of plants under stressful environments from an ecological stoichiometry perspective is a critical but underexplored research topic, and multi-organ collaborative research for multi-species can provide a comprehensive understanding. In this study, helophytes were selected as the subjects, and water depth and water N-enrichment were set as the stressors. A simulation experiment including three water depths (drought stress, control and flooding stress) and four water N-enrichment levels (control, low, medium and high N-enrichment stresses) for six helophyte species was carried out. Overall, C concentrations in all plant organs remained stable under water (drought-flooding stress) and N-enrichment stress. N concentrations increased under both flooding and drought stresses, while P concentrations and the N:P ratio showed an increase and decrease under only flooding stress, respectively. N concentration and N:P ratio increased with water N-enrichment level. The interaction only promoted the accumulation of N concentrations in aboveground organs. Especially, several species also changed organ C concentrations to adapt to water stress and adjusted root N concentrations for the combined stresses of flooding or drought and high N. Leaf and stem were strongly synergistic in N element, and leaf and root were mainly synergistic in P element. Water N-enrichment determined organ element concentrations more than water depth, and species identity dictated organ C:N:P ratios. Our results reveal that the allocation and synergy of nutrients among organs are important adaptive strategies for plants in stressful environments. Meanwhile, increasing water N-enrichment can be an unignored stressor, and species identity should be paid attention as a countermeasure.

Environmental determinants of lake macrophyte communities in Baikal Siberia

We investigated whether environmental filtering or dispersal-related factors mostly drive helophyte and hydrophyte species richness and community composition in 93 lakes situated in Baikal Siberia. Using partial linear regression and partial redundancy analysis, we studied (1) what are the relative roles of environmental variables, dispersal variables, spatial processes and region identity (i.e., river basins) in explaining variation in the species richness and species composition of helophytes and hydrophytes across 93 Siberian lakes, and (2) what are the differences in the most important explanatory variables driving community variation in helophytes versus hydrophytes? We found that, for both species richness and species composition, environmental variables clearly explained most variation for both plant groups, followed by region identity and dispersal-related variables. Spatial variables were significant only for the species composition of hydrophytes. Nutrient-salinity index, a proxy for habitat trophic-salinity status, was by far the most significant environmental determinant of helophytes and hydrophytes. Our results indicate that environmental factors explained the most variation in both species richness and species composition of helophytes and hydrophytes. Nevertheless, dispersal-related variables (i.e. spatial and dispersal) were also influential but less important than environmental factors. Furthermore, the dispersal-related variables were more important for hydrophytes than for helophytes. Most brackish permanent lakes were mostly located in the steppe biomes of southern Transbaikalia. This characteristic along with the oldest age, the largest distances to both river and settlements and the lowest temperatures in the study region distinguished them from freshwater, drained and more nutrient-rich floodplain lakes.

The role of helophyte species on nitrogen and phosphorus retention from wastewater treatment plant effluents

In the Mediterranean region, water scarcity compromises stream water quality particularly downstream of wastewater treatment plants (WWTP). We tested the potential of four helophyte species to reduce dissolved inorganic nitrogen (N) and phosphorus (P) from WWTP effluents. We conducted an 11-month mesocosm experiment to assess differences in N and P content among plant compartments and among species. Moreover, we quantified the relative contribution of above and belowground parts of the plants to N and P retention. The experiment was conducted at the Urban River Laboratory (www.urbanriverlab.com) in artificial channels (12 m long x 0.6 m wide x 0.4 m deep) planted with monospecific stands of Iris pseudoacorus, Typha angustifolia, Phragmites australis and Scirpus lacustris. Channels (three replicates per species) received water from the WWTP effluent, which flowed at a constant rate of 5 L min−1 through the sub-surface. The helophytes were planted in November 2014 and biomass standing stocks of carbon (C), N and P were measured in October 2015 at the time of maximum plant biomass. Differences in the concentration of N and P were larger among plant compartments than among species. The highest N concentration was measured in leaves while rhizomes showed the highest P concentration. The total plant biomass varied greatly among species from 11.4 to 4.6 Kg DW m−2 for Iris and Scirpus, respectively. Iris accumulated the highest amount of N (256 g N m−2) and P (27 g P m−2) in biomass. Plants retained from 8% (Scirpus) to 19% (Iris) of total dissolved inorganic N inputs to the channels (10.4 kg N) during the experiment, and from 6% (Phragmites) to 14% (Iris) of total dissolved inorganic P inputs (1.3 kg P). This study provides quantitative evidence to water managers of the potential role of helophytes to improve water quality in freshwater ecosystems receiving water from WWTP effluents.

Emergent Macrophyte Root Architecture Controls Subsurface Solute Transport

AbstractEmergent macrophytes (helophytes) grow in the active channel of fluvial ecosystems. Subsurface flow beneath this area (i.e., hyporheic zone) is considered critical for ecological processes. However, little is known about the influence of helophyte roots on subsurface solute transport. We investigated the effect of three helophyte species with different root architecture (Iris pseudacorus L., Phragmites australis L., and Scirpus lacustris L.) on solute transport along subsurface flow paths. We considered both the physical and the biological roles of the roots, expecting that (1) roots will act as structures that create heterogeneities in the sediment (physical role); thus, root architecture will alter subsurface flow paths; (2) roots will remove water via evapotranspiration (biological role), leading to slower flow velocity; and (3) both scenarios will result in longer water residence times. We performed conservative tracer pulse additions in 12 flow‐through flumes subjected to four treatments: absence of helophytes (Control) and presence of helophytes (Iris, Scirpus, and Phragmites). Tracer breakthrough curves were used to compare solute transport patterns between the treatments by fitting a mobile‐immobile model and by applying temporal moment analysis. Results showed that helophyte roots increase subsurface water residence time by creating heterogeneities in the substrate and by removing water. Furthermore, hydraulic retention increased with the percent volume of fine roots but decreased in the presence of thicker roots. Based on these results we suggest that the root architecture of helophytes and their capacity to remove water via evapotranspiration should be considered when planning stream restoration activities aimed to improve water quality.

Interactions between environmental factors and vertical extension of helophyte zones in lakes in Finland

Sheltered shores in boreal lakes are typically dominated by emergent helophytes in which the perennating organ lies in soil below the water level, but the aerial shoots protrude above the water. Changes in natural water-level regimes due to hydropower development may significantly affect the longitudinal extent of the helophyte zone. We explored the vertical distribution of helophyte zones under water-level regulation to develop tools for estimating hydropower impacts. We recorded the vertical extension of the three most common helophyte species (Carex L. spp., Equisetum fluviatile L., and Phragmites australis (Cav.) Trin. ex Steud.) along 238 macrophyte transects in four regulated lakes and one nonregulated lake in Finland. We used random forest (RF) models to quantify the degree to which water-level fluctuation and site-specific environmental variables uniquely explained the variation in helophyte zonation. The vertical extension of Carex spp. was most strongly associated with the water-level fluctuation of the open-water period (OWP), followed by Phragmites and Equisetum. Overall, the RF models explained 4–41% of the variation observed in the helophyte zones. The models indicated that OWP fluctuation, slope, openness, and cover of other macrophyte groups were key factors explaining the extent of the helophyte zones. OWP fluctuation was the most important variable for zonation of Carex spp. and the upper limit of Phragmites. Slope was the most significant variable for the lower limit of Equisetum and Phragmites. Our study demonstrates the importance of taking into account natural habitat-level variability to increase confidence in quantifying the environmental impacts of hydropower production. This approach could be used in future impact assessments of new water-level regulation practices.

Phyto-dehydration of confined sludge: a sustainable approach for the management of polluted ponds

Leaf transpiration drives many of the processes involved in phyto-technologies, and it can represent a useful mechanism to remove water from different kind of storage basins presenting inorganic, organic or microbiological contamination (phyto-dehydration), with the aim to reduce the risk of environmental contamination. In this framework, a mesocosm-scale trial was carried out to test the capacity of different helophyte species to reduce the excess of water in an artificial pond filled with oversaturated sludge. The sludge derives from the digestion of pig slurries, presents high levels of zinc and copper and for most of the year is covered by a water layer of about 20 cm due to rainfalls. This layout (water layer over the sludge) was reproduced inside the mesocosms, where four helophyte species (Phragmites australis and a mix of Carex acutiformis, Iris pseudacorus and Juncus effusus) were planted on floating frames. Plant growth and functionality were monitored for one year, along with their water consumption capacity; the vegetation impact on sludge chemistry, sludge microbial community and sludge greenhouse gases emission/uptake were also evaluated. The sensitivity of the phyto-dehydration system to the reduction of water level occurring during summer in the pond was investigated reducing the water input to the mesocosms. P. australis and C. acutiformis successfully established in the mesocosms, while a significant mortality was recorded for I. psudacorus and J. effusus. Once established, plants were able to grow in the mesocosms, and no metal toxicity effect was observed on photosynthesis rates that were comparable with values reported for natural stands of the species. Plants significantly increased (from 24 to 63%, depending on the species) the amount of water lost by the mesocosms and counteracted the reduction of sludge organic carbon that could lead to a mobilization of the heavy metals bound to organic matter: furthermore, plants decreased the rates of mesocosm greenhouse gas emission and reduced the sludge pathogen (Enterobacteriaceae) occurrence. Water limitations reversibly reduced the water consumption and CO2 uptake capacity of the mesocosms. The results of this study demonstrated that the water balance of a sludge/water system can be effectively modified through the phyto-dehydration approach, increasing significantly the amount of water lost. Although the low tolerance of two species to the sludge/water environment after plantation needs to be further investigated, this phyto-technology can represent a promising approach to manage the excess of water in polluted ponds.

Helophyte impacts on the response of hyporheic invertebrate communities to inundation events in intermittent streams

AbstractThe effects of experimental inundation on invertebrate communities in artificial flumes fed with treated wastewater were investigated. Flumes, designed to simulate intermittent river conditions, were planted with 3 species of helophytes widely used in river restoration around the water‐stressed regions of Europe. Different species of vegetation had different capabilities to reduce the invertebrate's negative reaction to inundation, related mostly to rhizosphere density. Of the 3 helophyte species tested, only 1, Lysimachia vulgaris, showed significant capabilities to reduce invertebrate negative reaction on inundation. Species richness of invertebrates before and after the inundation did not change in any of the flumes, whereas species density significantly declined in all flumes except one planted with L. vulgaris. This helophyte species was associated with high densities of the Culicidae larvae (common mosquitos), which has severe implications for river restoration and vector management in the region. This study indicates that the selection and establishment of different helophyte species will impact invertebrate communities in restored streams, especially streams experiencing variable inundation conditions.

Factors determining the occupation of Marsh Harrier Circus aeruginosus breeding grounds in a Mediterranean environment

Capsule Wetland occupation by breeding Marsh Harriers is influenced by wetland and vegetation area, distance to other wetlands where conspecifics are present and also by the characteristics and conditions of the vegetation such as predominant species and its height at the start of the breeding period.Aims To determine factors influencing the probability of wetland occupation by Marsh Harrier in relation to wetland dimensions, vegetation and hydric conditions, and to determine the effect of the predominant helophyte species in the wetlands and its physical characteristics.Methods Three hundred and thirty-two wetlands were monitored during a breeding season in NW Spain, a Mediterranean area that hosts 10% of the total number of breeding pairs of Marsh Harriers in Spain. The probability of wetland occupation for breeding in terms of wetland size and proximity, vegetation characteristics, human disturbance and also hydrological variables, was analysed using logistic regression.Results One hundred and sixty-five pairs were located in 131 wetlands. Variables affecting occupation include vegetation composition and characteristics, wetland dimensions and distance to other occupied wetlands. Wetlands with the highest probability of being occupied were larger, with greater areas of marsh vegetation, taller vegetation, rush and reed communities as the predominant species and which were closer to another wetland occupied by Marsh Harriers.Conclusions The dimensions and predominant species of helophyte vegetation, and size and location determine the occupation of wetlands as breeding grounds by Marsh Harrier. Changes in the natural supply of water and nutrients due to the implementation of intensive irrigation farming are likely modifying flora in wetlands and affecting the distribution and population size of Marsh Harriers in this region of Spain.

Impact of organic pollutants on metal and As uptake by helophyte species and consequences for constructed wetlands design and management

Various industrial processes and anthropogenic activities in urban areas induce a release of metals, metalloids and organic pollutants. Phytoremediation of co-contaminated waters in constructed wetlands is a promising solution for reducing the impact on natural environments. In order to improve the design and management of constructed wetlands, more knowledge is needed concerning the effect of organic pollutants on plant metal and metalloid uptake. In this study, the effects of a mixture of organic pollutants commonly found in industrial effluents (hydrocarbons, polycyclic aromatic hydrocarbons, anionic detergent) on the uptake of ten metals and metalloids (MM), i.e. Al, As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn, by five helophytes having a wide European distribution were studied. Main effects of plant species and pollutant conditions on metal uptake and interactions between factors were determined by a statistical treatment of a microcosm experiment. Overall, the order of element uptake in plants was Fe > Al > Mn > Cr, Ni, Zn, > Cu > As, Cd, Pb, which was consistent with relative concentrations in the rhizosphere environment of microcosms. Larger amounts of metals were retained in belowground biomass of plants than in aboveground parts. Statistical analysis showed that organic pollutants enhanced the accumulation of Mn in whole plants and the retention of Fe in belowground parts, while they reduced the accumulation of Cd, Ni, and Zn in whole plants and the retention of Cu in belowground parts. For the other MM (Al, As, Cr, Pb), effects were variable, depending on the plant species. Among the five plants tested, Carex cuprina generally removed the highest quantities of MM, which was the result of both a high metal accumulation capacity and high biomass production. Nevertheless, no significant proportion of the MM total loading could be removed in plants' aboveground parts.

Ludwigia grandiflora and L. peploides Onagraceae – Water primroses

<i>Ludwigia grandiflora</i> and <i>L. peploides</i> Onagraceae – Water primroses

Quantitative evaluation of water-level effects on “regeneration safe-sites” for lakeshore plants in Lake Kasumigaura, Japan

Nishihiro, J. and I. Washitani. 2009. Quantitative evaluation of water-level effects on “regeneration safe-sites” for lakeshore plants in Lake Kasumigaura, Japan. Lake Reserv. Manage. 25:217–223. We evaluated the effects of water-level regime on the area of “regeneration safe-sites” for seed germination and initial establishment of seedlings of helophytes occurring in the emergent vegetation zone of Lake Kasumigaura, Japan. Based on the results of previous studies and an original experiment, potential regeneration safe-sites (PRS) for helophytes were defined as sites in which the ground surface was inundated for less than 3 consecutive days from 1 April to 15 May. We estimated and compared the areas of PRS both before and after the implementation of an artificial water-level regime in the mid-1970s. Comparisons were made using data on daily water levels during 1960–1969 and 1996–2005, topographic profiles of the lakeshore in 1967 and 2005, and areas of the emergent vegetation in 1972 and 1997. We estimated that the current area of PRS in the lake has been reduced to 24% of past levels. This reduction was accompanied by both a complete loss of a spring drawdown and a decrease in the area of emergent vegetation. The areas of PRS under various water management regimes indicated that a relatively slight lowering of the spring water level could cause a marked increase in the area of PRS (e.g., a 10-cm decrease in the managed target water level could result in a doubling of the area of PRS). Our results suggest a water-level management policy that includes a spring drawdown can be effective in the restoration of helophyte species diversity through the recovery of PRS.

Can vertical-flow wetland systems treat high concentrated sludge from a food industry? A mesocosm experiment testing three plant species

We performed a mesocosms experiment using a vertical-flow wetland system to treat liquid sludge in the Mediterranean region. Three common helophyte species, common reed ( Phragmites australis Cav.), broadleaf cattail ( Typha latifolia L.), and yellow flag ( Iris pseudacorus L.), were planted as monoculture and irrigated with a liquid sewage sludge from a food industry, characterised by very high organic concentrations (COD > 8000 mg/L). We studied the benefits of plants by comparing unplanted to planted mesocosms. Results showed the high performance of such vertical-flow wetland systems. Removal efficiency was more than 98% for total suspended solids (TSS) and chemical oxygen demand (COD), and more than 87% for total Kjeldahl nitrogen (TKN). The main removal process was physical filtration by the substrate due to the high proportion of particulate elements in the sludge. Planted mesocosms were more efficient than those unplanted, confirming the positive role of the plants. Mesocosms planted with Phragmites or Typha showed better performances in TKN removal than those planted with Iris. Only in mesocosms planted with Phragmites was there no outflow in summer due to high evapotranspiration.

Effect of Cellulose Wastes Upon the Growth of Phragmites Australis

Growth responses of Phragmites australis (Cav.) Trin. Ex Steud, (reed grass), a helophyte species, were examined under in vitro and greenhouse conditions in the presence of various residues from a Kraft pulp mill. Plant tolerance to solid residues (ashes, dregs, flyashes, grits, primary sludge, and brown stock rejects) was tested in vitro. Solid residues were added separately up to 30% (w/v), as well a liquid residue up to 30% (v/v), to a Murashige and Skoog (1962) sucrose-free nutrient media with (5 mg l−1) 6-benzylaminopurine. After 2 mo in vitro, plantlets developed well in the presence of up to 10% solid or liquid wastes, but higher concentrations of either limited growth. This effect was mainly attributed to the plant's uptake and accumulation of various elements such as sodium, iron, copper, manganese, and boron, which are common to these waste types, thus showing an efficient phytoremediation potential. When added to MS media, the concentration of these elements generally decreased in the residual media after 2 mo of culture: the initial sodium, iron, and copper content in the growth media was reduced ca. 10-fold detected; a 5-fold reduction occurred for manganese and boron. In experiments under greenhouse conditions with in vitro propagated plantlets potted in mixtures of a commercial organic soil and residues, significant differences in plant development (plant size and fresh weight increase) were observed in the presence of ashes mixed at levels of 20% and 30%, compared to the control in organic soil. For other solid wastes, plant growth was inhibited as the concentration of each waste increased, causing chlorosis and/or plant necrosis.

Variation in plant species richness in and adjacent to 64 ponds in SE Norwegian agricultural landscapes

All vascular plants, classified by life and growth form into six groups, four groups of hydrophytes (lemnids, nymphaeids, elodeids and isoetids), helophytes, and terrestrial species of pond margins, were inventoried in 64 SE Norwegian agricultural landscape ponds and their adjacent margins. The study sites varied considerably with respect to species richness; 0–4 for each hydrophyte group, 0–9 for helophytes, and 13–77 for terrestrial species. A total of 56 explanatory variables were recorded for each pond and adjacent margin to explain the observed richness variability. Species richness in each plant group was modelled separately by generalised linear models (GLM), using the recorded explanatory variables as predictors. Pond area, water depth and water chemical variables were the most important predictors of hydrophyte and helophyte species richness, followed by fluctuation and human impact variables such as liming and presence of ducks and fish. Pond margin area and geographical variables were significant predictors of richness of terrestrial species of pond margins. The contribution of different structuring processes to explanation of species richness patterns is discussed. The generally weak explanatory power of the selected variables indicates high importance of apparent randomness in this ecosystem, and, notably, that the ponds and their adjacent margins represent islands in the agricultural landscape that accumulate species more or less individualistically.

Phytoremediation of selenium by two helophyte species in subsurface flow constructed wetland

The phytoremediation of selenium by two different wetland species was investigated. Selenium (20.4 μg/l) was supplied continuously to subsurface flow constructed wetlands, one vegetated with Typha latifolia L. and the other with Phragmites australis (Cav.) Trin. ex Steud. The beds of both species had same hydraulic loading rate (0.079 m 3/m 2/d) and water retention time (24 h). However, the mass loading rate was 1.27 mg Se/m 2/d for Phragmites and 1.35 mg Se/m 2/d for Typha. In the Typha bed Se migrated faster than in the Phragmites bed. After 25 d of Se supplementation in the Typha bed about 54% of the Se inlet concentration remained in the outlet water. In the Phragmites bed Se was removed completely from the water after passing through 3/4 of the bed length. After 65 d of Se supplementation the highest amount of Se (2.8 μg/g dry matter) was determined in the organic material of the Typha bed. Roots and rhizomes accumulated 2.2 and 1.8 μg/g dry matter respectively. Phragmites accumulated Se in the leaves and stems, but not in the rhizomes. The accumulation in the leaves (1.8 μg Se/g dry matter) was three times higher than in the stems (0.6 μg Se/g dry matter).

Abilities of Helophyte Species to Release Oxygen into Rhizospheres with Varying Redox Conditions in Laboratory-Scale Hydroponic Systems

ABSTRACT Plantlets of the wetland species cattail (Typha latifolia), reed (Phragmites australis), rush (Juncus effusus), and yellow flag (Iris pseudacorus) grown from seedlings or cuttings were investigated in laboratory-scale hydroponic systems in order to determine the intensity of oxygen release into the rhizosphere under various redox conditions. The initial redox conditions of the rhizosphere were modified by adding different amounts of titanium (III) citrate. All the plantlets investigated released oxygen into the rhizosphere (0.01 mg/h * plant to 1.41 mg/h * plant), depending on the actual conditions of each experiment. The intensity of release was found to be controlled by the external oxygen demand in the rhizosphere for the whole range of conditions from extremely reduced to oxidized. The capacities of the plantlets to release oxygen were found to be species-specific under reduced conditions in the rhizosphere (–400 mV to +200 mV). Under conditions of higher redox potential (>+200 mV) the oxygen release depends on the physiological status of the individual plant. Oxygen release rates are highest in the range–250 mV < Eh < −150 mV. For the species investigated, the highest rates were observed for Typha latifolia (1.41 mg/h * plant) followed by Phragmites australis (1 mg/h * plant), Juncus effusus (0.69 mg/h * plant), and Iris pseudacorus (0.34 mg/h * plant). In general, the plantlets released oxygen with different rates into a nonbuffered rhizosphere until highly oxidized conditions in the root-near environment prevailed. The presented oxygen-releasing behavior is a process dominating natural conditions within the rhizosphere and is relevant to wetland systems and for conceptual approaches in phytoremediation.

A comparative study of the gas exchange potential between three wetland species using sulfur hexafluoride as a tracer

The gas-exchange potential of three wetland species (helophytes) was examined in an aquatic model vegetation facility (AMOVA) using sulfur hexafluoride (SF 6) as a tracer. Three beds containing gravel and vegetated with Phragmites australis, Typha latifolia and Schoenoplectus lacustris were compared to an unvegetated gravel bed as a reference. A mass balance of SF 6 emissions revealed a different seasonal emission pattern for the three species investigated. Helophytes capable of pressurized ventilation, e.g. Phragmites and Typha, showed the highest gas-exchange values. On an annual basis net SF 6 emission values of Phragmites were the highest in the autumn month of October (65.8%) and the lowest in the winter month of January (37.1%). P. australis represents a helophyte species with highly developed gas-exchange tissues facilitating diffusion and transport of the tracer gas SF 6 to the atmosphere via the intercellular lacunar system.

Channel instability as a control on silting dynamics and vegetation patterns within perifluvial aquatic zones

Many authors have shown that the sedimentology of former channels and subsequent vegetation changes are controlled by temporal (flood events and successional processes) and spatial (e.g. distance to the main channel) factors. River channel instability can disrupt these associations. The Ain River, France, has undergone a fluvial metamorphosis during the past 100 years, its braided pattern being replaced by a sinuous single-thread pattern. As a consequence, former channels have different geometrical characteristics and sediment trap efficiencies. Former meandering channels experience more frequent backflows and are more rapidly silted than the older former braided channels. The recently abandoned channels are characterized by the development of large-sized vegetation species with a relatively slow colonization rate, whereas the older channels are colonized predominantly by flood-tolerant aquatic plants. The locally derived discharge of former channels (from groundwater or from their own basin) may reduce or prevent sediment entry during flood events and thus may decrease the sedimentation rate. In such cases, the oligotrophic component of the water from the hillslope aquifer is high and the former channel is usually nutrient-poor, characterized by oligotrophic species. The main river channel also has experienced local incision, aggradation and horizontal displacement during recent decades, so that the dynamics of the former channels strongly depend on the dynamics of the reach in which they are located. In degraded reaches, former channels are often dry, and helophyte species have been replaced by mesophytes. The frequency and magnitude of flow connection between the river channel and the former channel can increase or decrease owing to the movement of the active river channel within the fluvial corridor, inducing varying modifications of former channel vegetation patterns. River channel instability at various time-scales is a key-factor controlling process diversity and thus biodiversity in the fluvial corridor. It can modify the geometry of abandoned channels, groundwater fluxes, the amount, mobilization and deposition of sediment within the corridor, and consequently the vegetation community patterns. This increases the complexity of successional patterns, because an old former channel may be characterized by pioneer species whereas a younger one can become quickly filled and colonized by terrestrial species. Copyright (C) 2000 John Wiley & Sons, Ltd.

Four helophyte species growing under salt stress: their salt of life?

We studied the capacity of pre-germinated seedlings of four helophyte species to develop under conditions of chronic and temporary salt stress in waterlogged sediments: Typha latifolia L., Phragmites australis (Cav.) Trin. ex Steud., Scirpus lacustris ssp. lacustris L., and Scirpus maritimus L. Salinity levels were 1.8‰ S and 18‰ S , applied constantly during four months, or for one or three weeks in otherwise freshwater conditions. The chronic salt stress treatment at 18‰ S led to a rapid die-off of both T. latifolia and S. lacustris ssp. lacustris. The other two species only barely survived, hardly forming new vertical shoots. At 1.8‰ S, the consequences were less, and limited to T. latifolia and S. lacustris ssp. lacustris. Both levels of short-term salinity stress were well-tolerated by P. australis and S. maritimus, while mortality of T. latifolia and S. lacustris ssp. lacustris was significantly increased after the three weeks, 18‰ S treatment. The surviving individuals of these two species after this treatment also showed a more than 50% reduction in their total biomass compared to the control plants. Remarkably, after the three weeks 18‰ S treatment, all species apart of T. latifolia showed a short period in which the growth rate of the number of new shoots was significantly increased. This increase was sufficient to restore the same development pattern as observed in the control plants.