Impacts of hydrological restoration on lowland river floodplain plant communities

Many lowland floodplain habitats have been disconnected from their rivers by flood defence banks. Removing or lowering these banks can reinstate regular flooding and thus restore these important wetland plant communities. In this study we analyse changes in wetland hydrology and plant community composition following the lowering of flood defence banks at a floodplain of the River Don in the United Kingdom (UK). The aim of the restoration project was to improve the quality of “floodplain grazing marsh” habitat, which is a group of wetland communities that are of conservation interest in the UK. We analyse changes in species richness and community composition over a period of 6 years, and compare the presence of indicator species from the target floodplain grazing marsh plant communities. The lowering of the flood banks increased the frequency of flood events, from an estimated average of 1.7 floods per year to 571 floods per year. The increased flooding significantly increased the proportion of time that the wetland was submerged, and the heterogeneity in hydrological conditions within the floodplain. There were significant differences in composition between the pre-restoration and restored plant communities. Plants with traits for moisture tolerance became more abundant, although the communities did not contain significantly more ‘target’ floodplain grazing marsh species at the end of the study period than prior to restoration. Colonisation by floodplain grazing marsh species may have been limited because environmental conditions were not yet suitable, or because of a shortage of colonising propagules. While the desired target plant community has not been achieved after 5 years, it is encouraging that the community has changed dynamically as a result of hydrological changes, and that moisture-tolerant species have increased in occurrence. Over the next few decades, the restored flood regime may cause further environmental change or colonisation events, thus helping increase the occurrence of desired floodplain grazing marsh indicator species.

     

Variant restoration trajectories for wetland plant communities on a channelized floodplain

Restoration efforts are typically based on an assumption that reestablishment of altered determinants of ecological structure and function will lead to a predictable reestablishment of populations and reassembly of communities. Dechannelization and reestablishment of natural hydrologic regimes provide the basis for the ongoing restoration of the Kissimmee River in Central Florida, United States. The expected reestablishment of historically dominant broadleaf marsh (BLM) and buttonbush shrub (BB) communities was evaluated over a 10‐year period following implementation of the first phase of the restoration project. Plant species composition and cover were assessed during dry (spring) and wet (summer) season sampling periods at five sites on the restored floodplain, and four “control” sites on the channelized floodplain. Mean daily stage data from nearby gauges indicated hydroperiods and depths on the reflooded floodplain were within the range of historic hydrologic conditions that selected for BLM and BB communities on the pre‐channelization floodplain. After reflooding, pasture grass and upland shrub communities rapidly transitioned to a fluid mix of obligate and facultative wetland species. Although signature BLM and BB species, Sagittaria lancifolia (bulltongue arrowhead), Pontederia cordata (pickerel weed), and Cephalanthus occidentalis (buttonbush), recolonized all study sites, the expected reestablishment of dominant cover of these species did not occur. Results indicate that restoration of BLM and BB communities has been impeded by deep flood pulse disturbances, establishment of invasive wetland grasses, and mineralized soil characteristics of the drained floodplain.     

Importance of low-flow and high-flow characteristics to restoration of riparian vegetation along rivers in arid south-western United States

1. Riparian vegetation in dry regions is influenced by low‐flow and high‐flow components of the surface and groundwater flow regimes. The duration of no‐flow periods in the surface stream controls vegetation structure along the low‐flow channel, while depth, magnitude and rate of groundwater decline influence phreatophytic vegetation in the floodplain. Flood flows influence vegetation along channels and floodplains by increasing water availability and by creating ecosystem disturbance. 2. On reference rivers in Arizona's Sonoran Desert region, the combination of perennial stream flows, shallow groundwater in the riparian (stream) aquifer, and frequent flooding results in high plant species diversity and landscape heterogeneity and an abundance of pioneer wetland plant species in the floodplain. Vegetation changes on hydrologically altered river reaches are varied, given the great extent of flow regime changes ranging from stream and aquifer dewatering on reaches affected by stream diversion and groundwater pumping to altered timing, frequency, and magnitude of flood flows on reaches downstream of flow‐regulating dams. 3. As stream flows become more intermittent, diversity and cover of herbaceous species along the low‐flow channel decline. As groundwater deepens, diversity of riparian plant species (particularly perennial species) and landscape patches are reduced and species composition in the floodplain shifts from wetland pioneer trees (Populus, Salix) to more drought‐tolerant shrub species including Tamarix (introduced) and Bebbia. 4. On impounded rivers, changes in flood timing can simplify landscape patch structure and shift species composition from mixed forests composed of Populus and Salix, which have narrow regeneration windows, to the more reproductively opportunistic Tamarix. If flows are not diverted, suppression of flooding can result in increased density of riparian vegetation, leading in some cases to very high abundance of Tamarix patches. Coarsening of sediments in river reaches below dams, associated with sediment retention in reservoirs, contributes to reduced cover and richness of herbaceous vegetation by reducing water and nutrient‐holding capacity of soils. 5. These changes have implications for river restoration. They suggest that patch diversity, riparian plant species diversity, and abundance of flood‐dependent wetland tree species such as Populus and Salix can be increased by restoring fluvial dynamics on flood‐suppressed rivers and by increasing water availability in rivers subject to water diversion or withdrawal. On impounded rivers, restoration of plant species diversity also may hinge on restoration of sediment transport. 6. Determining the causes of vegetation change is critical for determining riparian restoration strategies. Of the many riparian restoration efforts underway in south‐western United States, some focus on re‐establishing hydrogeomorphic processes by restoring appropriate flows of surface water, groundwater and sediment, while many others focus on manipulating vegetation structure by planting trees (e.g. Populus) or removing trees (e.g. Tamarix). The latter approaches, in and of themselves, may not yield desired restoration outcomes if the tree species are indicators, rather than prime causes, of underlying changes in the physical environment.     

Effects of flooding regime on wetland plant growth and species dominance in a mesocosm experiment

Flooding regimes are a primary influence on the wetland plant community. Human-induced disturbance often changes the duration and frequency of flooding in wetlands, and has a marked influence on wetland plant composition and viability. Comprehensive studies of the environmental thresholds of wetland plants are required for the development of proper practices for wetland management and restoration after hydrological disturbance. This study provides a quantitative assessment of the establishment, growth, and community shifts in dominance of three emergent plant species (Scirpus tabernaemontani, Typha orientalis, and Zizania latifolia) typical of South Korean wetlands, under five hydrological regimes (waterlogged, low-level standing water, high-level standing water, intensive periodic flooding, and intermittent flooding) over four growing seasons. A mesocosm experiment was conducted in the campus of Seoul National University, South Korea. The number and biomass of shoots of Z. latifolia responded positively to increased water level and flooding frequency, while that of the other plants did not. Zizania latifolia outcompeted S. tabernaemontani and T. orientalis irrespective of hydrological regime. This study suggests that Z. latifolia can outcompete the other two macrophytes in the field. This study will improve our ability to predict the dynamics of wetland vegetation and so facilitate the formulation of wetland management and restoration strategies.     

Wetland plant growth under contrasting water regimes associated with river regulation and drought: implications for environmental water management

An important characteristic of many wetland plants in semi-arid regions is their capacity to withstand fluctuations between extended dry phases and floods. However, anthropogenic river regulation can reduce natural flow variability in riverine wetlands, causing a decline in the frequency and duration of deep flooding as well as extended droughts, and an increase in shallow flooding and soil saturation. Our aim in this paper was to use an experimental approach to examine whether reductions in flooding and drought disadvantage species adapted to both these extremes, and favours those with water requirements that match the new regime of frequent low-level flooding. We compared the growth characteristics and biomass allocation of three native Australian aquatic macrophytes (Pseudoraphis spinescens, Juncus ingens and Typha domingensis), which co-occur at Barmah Forest, south-eastern Australia, under three water treatments: drought, soil saturation and deep flooding. The responses of species to the treatments largely reflected changes in their relative abundance at Barmah Forest since river regulation. Typha domingensis, which has remained uncommon, performed relatively poorly in all treatments, while J. ingens, which has increased its range, exhibited more vigorous growth under soil saturation. Pseudoraphis spinescens, which was once widespread but has declined markedly in its distribution, grew strongly under all water treatments. These findings suggest that a return to more natural, variable river flow regimes can potentially be an important conservation and restoration strategy in ecosystems characterised by species that have adaptations to extreme hydrological growing conditions.     

Predictability of flood pulse driven assembly rules for restoration of a floodplain plant community

Community assembly rules were formulated to evaluate the restoration of wet prairie along the periphery of the floodplain of the Kissimmee River in central Florida. Restoration of this plant community is expected to be driven by the reestablishment of flood pulse hydrology following the ongoing dechannelization of the river. Assembly rules were assessed with plant species composition and cover data from 15 permanent plots on the restored floodplain and 6 control plots on the channelized floodplain. These sites were sampled biannually from 1998 to 2010. Mean annual hydroperiods and depths confirmed that the frequency, duration and amplitude of post-restoration flood pulses at study sites were similar to historic reference locations. Elimination of pasture grasses (primarily Paspalum notatum Flüggé) following restoration of the flood pulse validated the hypothesized deletion rule for initial transformation of the wet prairie zone. Predicted increased dominance of obligate and facultative wetland species, a “community addition rule”, also was confirmed. An index of weighted averages of wetland indicator taxa showed significant short-term responses to antecedent hydroperiods and depths, and a restoration trajectory for wetland plant species. As predicted, recruitment of wet prairie indicator species from the extant seed bank correlated with reestablishment of the flood pulse, but was greatest when inundation extended from the wet season into the dry season. Restoration of a wetland plant community did not result in the predicted increase in species richness and diversity. Colonization and expansion of the exotic grass, Hemarthria altissima (Poir.) Stapf & C.E. Hubb., disrupted community reassembly processes. By summer 2007, mean cover of this species and several other exotic grasses increased to 24%, and necessitated herbicide treatments. Assembly rules provided useful predictions for the initial restoration of wet prairie vegetation, but were eventually confounded by the spread of an exotic species that was new to the regional flora.     

Managed flooding can augment the benefits of natural flooding for native wetland vegetation

Managed flooding is increasingly being used to maintain and restore the ecological values of floodplain wetlands. However, evidence for its effectiveness is sometimes inconsistent and water available for environmental purposes often limited. We experimentally inundated a floodplain wetland (or “billabong”) in late spring by pumping water from its adjacent creek, aiming to promote the native wetland flora and suppress terrestrial exotics. Vegetation was surveyed before (spring) and after (late summer) the managed flood in the experimental billabong and in three control billabongs. Floodplain water levels were continuously monitored. Wet conditions caused two of the control billabongs to also flood, but to a lesser extent than the experimental billabong. We therefore assessed vegetation changes relative to flooding duration. With increasing flooding duration, the cover of wetland vegetation (amphibious and aquatic species) increased and the cover of terrestrial and exotic vegetation decreased, with these effects largest in the deliberately flooded billabong. Flooding durations greater than 20 days generally resulted in increased cover of wetland plants and restricted the growth of terrestrial plants. Reinstatement of more appropriate flooding regimes can thus promote native wetland plants, while suppressing terrestrial exotic species. Our study also provides evidence for the use of modest water allocations to augment the benefits of natural flooding in the maintenance and restoration of native wetland plant communities.     

Flood pulses drive the temporal dynamics of assemblages of aquatic insects (Heteroptera and Coleoptera) in a temperate floodplain

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Response of Plant Height,Species Richness and Aboveground Biomass to Flooding Gradient along Vegetation Zones in Floodplain Wetlands,Northeast China

Flooding regime changes resulting from natural and human activity have been projected to affect wetland plant community structures and functions. It is therefore important to conduct investigations across a range of flooding gradients to assess the impact of flooding depth on wetland vegetation. We conducted this study to identify the pattern of plant height, species richness and aboveground biomass variation along the flooding gradient in floodplain wetlands located in Northeast China. We found that the response of dominant species height to the flooding gradient depends on specific species, i.e., a quadratic response for Carex lasiocarpa, a negative correlation for Calamagrostis angustifolia, and no response for Carex appendiculata. Species richness showed an intermediate effect along the vegetation zone from marsh to wet meadow while aboveground biomass increased. When the communities were analysed separately, only the water table depth had significant impact on species richness for two Carex communities and no variable for C. angustifolia community, while height of dominant species influenced aboveground biomass. When the three above-mentioned communities were grouped together, variations in species richness were mainly determined by community type, water table depth and community mean height, while variations in aboveground biomass were driven by community type and the height of dominant species. These findings indicate that if habitat drying of these herbaceous wetlands in this region continues, then two Carex marshes would be replaced gradually by C. angustifolia wet meadow in the near future. This will lead to a reduction in biodiversity and an increase in productivity and carbon budget. Meanwhile, functional traits must be considered, and should be a focus of attention in future studies on the species diversity and ecosystem function in this region.     

Half a century of changes in the riverine landscape of Limay River: the origin of a riparian neoecosystem in Patagonia (Argentina)

Our study aims to determine the dynamic that led to the spread of exotic Salicaceae on the Limay River floodplain and its implications in shaping the current neoecosystem. We used images obtained by the HRG sensor on board the SPOT-5 satellite. We selected two images dates allowed a comparison of the floodplain under different flooding regimes programmed by the Interjuridiccional Basin Authority, at a rate of 1290 m³/s in Spring and a flow rate of less than 400 m³/s in Summer. To characterize the vegetation cover, the Normalized Difference Vegetation Index (NDVI) was used to compare images of September and December, also the Normalized Difference Water Index (NDWI) was applied to delineate the riverbed and floodplain in September. To evaluate the influence of flooding regime on the detected patches a Principal Component Analysis (PCA), was performed; and biotic and abiotic factors on the composition of the dominant tree species in each patch by Multiple Factor Analysis (MFA) were analized. A 58.4 % coverage of forest patches had developed on the Limay River floodplain. Patches of older trees grew on surfaces at 1.5 m above the water level. However, the surfaces not reached by floods >1.5 m have a very low rocky coverage (1 %). The analysis of the age of the trees downstream of the dam system showed that the vegetation, often exposed to high floods before the dams were built (1971), was composed of the native willow (Salix humboldtiana) and the exotic Salix alba. As the river regime was attenuated and extraordinary floods disappeared with the operation of the dam Arroyito in 1980, the first patches of Populus nigra spread.     

The Significance of Meander Restoration for the Hydrogeomorphology and Recovery of Wetland Organisms in the Kushiro River,a Lowland River in Japan

The meanders and floodplains of the Kushiro River were restored in March 2011. A 1.6‐km stretch of the straightened main channel was remeandered by reconnecting the cutoff former channel and backfilling the straightened reach, and a 2.4‐km meander channel was restored. Additionally, flood levees were removed to promote river–floodplain interactions. There were four objectives of this restoration project: to restore the in‐stream habitat for native fish and invertebrates; to restore floodplain vegetation by increasing flooding frequency and raising the groundwater table; to reduce sediment and nutrient loads in the core wetland areas; to restore a river–floodplain landscape typical to naturally meandering rivers. In this project, not only the natural landscape of a meandering river but also its function was successfully restored. The monitoring results indicated that these goals were likely achieved in the short term after the restoration. The abundance and species richness of fish and invertebrate species increased, most likely because the lentic species that formerly inhabited the cutoff channel remained in the backwater and deep pools created in the restored reach. In addition, lotic species immigrated from neighboring reaches. The removal of flood levees and backfilling of the formerly straightened reach were very effective in increasing the frequency of flooding over the floodplains and raising the water table. The wetland vegetation recovered rapidly 1 year after the completion of the meander restoration. Sediment‐laden floodwater spread over the floodplain, and approximately 80–90% of the fine sediment carried by the water was filtered out by the wetland vegetation.     

Unrealized Expectations for Restoration of a Floodplain Plant Community

The ongoing restoration of the channelized Kissimmee River is expected to promote reestablishment of the prolonged, deep inundation regimes that sustained broadleaf marsh as the dominant wetland plant community on the historical floodplain. The success of the restoration was evaluated at locations on the remnant floodplain where broadleaf marsh had been replaced by a mesophytic shrub community, and on the lower portion of the reconstructed floodplain, which was recreated by backfilling of a flood control canal and degradation of associated spoil mounds. During the 8‐year post‐restoration period (2001–2008) mean annual hydroperiods and depths on the restored floodplain were not significantly different from pre‐channelization hydrologic conditions at historical reference sites. Increased hydroperiods and depths eliminated the mesophytic shrub (primarily Myrica cerifera) and associated fern cover, and led to colonization of floating and mat‐forming species, but did not result in the reestablishment of a broadleaf marsh community. Signature broadleaf marsh species, Sagittaria lancifolia and Pontederia cordata, were found in all remnant floodplain plots and colonized 8 of the 10 reconstructed floodplain plots, but had mean cover ranging from only 0.9 to 6.1%. Several factors may have contributed to unsuccessful reestablishment of broadleaf marsh, including unfavorable edaphic conditions, brief drawdown (low stage) periods for establishment of seedlings, flood induced mortality, and an invasion of the exotic shrub, Ludwigia peruviana, which had post‐restoration mean cover of 17–19%. Study results indicate hydrologic restoration of floodplain plant communities can be influenced by more discrete aspects of the river flood pulse than average hydroperiods and depths.     

The effect of flooding and flood timing on leaf litter breakdown rates and nutrient dynamics in a river red gum (Eucalyptus camaldulensis) forest

Comparisons of litter standing-stocks in low-lying and higher areas of the floodplain and the effects of controlled flooding events on leaf litter decomposition and leaf litter nutrients were examined during autumn and winter in a southeastern Australian river red gum (Eucalyptus camaldulensis) floodplain forest. The mean mass of total litter and some litter components was significantly greater in autumn than in winter but there were few differences in litter mass between low-lying flood runners and higher sites (1.5 m) on the floodplain, regardless of season. Leaf decomposition was more rapid in flooded areas than in non-flooded areas and was significantly faster in autumn than in winter. In flooded leaves, concentrations of phosphorus and nitrogen dropped rapidly during the first 3 days of each experiment, increased to near original after 7–10 weeks and then decreased again. After 112 days of decomposition the C:N:P ratios of leaf litter increased, but this effect was most marked for flooded leaves. Simple models of leaf litter dynamics indicated that leaf litter standing-stocks in low-lying flood runners would be reduced by flooding, particularly during autumn. In contrast, models predicted a net gain in standing-stocks of leaf litter to be higher on the floodplain, particularly in autumn. Alteration to the seasonal timing of floods by river regulation has probably decreased litter standing-stocks and nutrients available in low-lying areas of the floodplain to support the production of macrophytes and biofilms during winter and spring floods.     

Summer floods shape meadow butterfly communities in a floodplain nature reserve in Central Europe

How flooding regimes shape temperate-zone butterfly communities has received little attention. At the river Danube in eastern Austria, a levee has largely interrupted natural river dynamics since the late nineteenth century. Only a fraction of the floodplain area still experiences annual summer inundations after snow-melt in the Alps. We surveyed meadow butterfly communities on either side of the levee in a year with an unusually strong flood (2013), and in a season with a weak flood typical for the region (2012). Altogether we observed 67 butterfly species. Butterfly abundance and species richness were lower on meadows with stronger flood impact, but differences were modest. In contrast, species composition differed prominently relative to flooding regime and nectar availability. Grass-feeding species tended to be rarer on flooded meadows, while Brassicaceae-feeding species were more prevalent on nutrient-rich flood-prone meadows. Highly dispersive butterflies made up a larger share on flooded meadows, whereas highly philopatric species were relatively more common at sites with little or no inundation. These results indicate that summer inundations at the river Danube act as filters for the local species composition of butterflies on floodplain meadows. Local resource availability and the differential potential of species to re-colonize meadows after catastrophic floods are likely drivers of these differences. Effects of inundations were not consistently stronger in a year of a catastrophic flood than in a normal season. Butterfly communities on non-flooded meadows had a higher regional conservation value.     

Cyanobacterial blooms in a temperate river-floodplain ecosystem: the importance of hydrological extremes

In the past decade, extreme hydrological events were expressed with extreme droughts and floods in temperate regions. The aim of this paper is to explain how such changes in hydrology can influence cyanobacterial populations in floodplain ecosystems. We therefore analyzed a 6-year (2003–2008) study of the phytoplankton in the Kopački Rit floodplain, one of the largest natural floodplains in the middle section of the Danube River (Europe). During the studied period, the shallow floodplain lake shifted between a state of turbid water, characterized by high phytoplankton biomass and regular appearance of cyanobacteria blooms, to a state of clear water with very low phytoplankton biomass and absence of cyanobacteria, and back to the turbid state. Apparently, the major forces driving the cyclic shift were closely related to extremely high and long-lasting flood events. Significant increase in water level, low hydraulic residence time of water, decrease in transparency and low-light climate, together with mass developed aquatic macrophyte vegetation in the whole inundated floodplain were unfavorable conditions for growth and proliferation of cyanobacteria. With the establishment of the flood regime characterized by long-lasting periods without flooding, in-lake processes prevailed leading to cyanobacterial bloom. The most successful were filamentous non-N-fixing cyanobacteria tolerant to mixed and low-light conditions (Planktothrix and Limnothrix) and invasive species Cylindrospermopsis raciborskii. Their massive development led to the establishment of a phytoplankton steady state. All our results demonstrate that the altered intensity and frequency of flood events will have pronounced effects on the appearance of cyanobacterial blooms and generally on alternative stable states in the floodplain. Relating to this, management objectives should be focused on qualifications of changes in hydrology and projecting those effects for potential floodplain restoration.     

Patterns of forest succession and impacts of flood in the Upper Mississippi River floodplain ecosystem

The widespread loss of oak-hickory forests and the impacts of flood have been major issues of ecological interest concerning forest succession in the Upper Mississippi River (UMR) floodplain. The data analysis from two comprehensive field surveys indicated that Quercus was one of the dominant genera in the UMR floodplain ecosystem prior to the 1993 flood and constituted 14% of the total number of trees and 28% of the total basal area. During the post-flood recovery period through 2006, Quercus demonstrated slower recovery rates in both the number of trees (4%) and basal area (17%). In the same period, Carya recovered greatly from the 1993 flood in terms of the number of trees (11%) and basal area (2%), compared to its minor status before the flood. Further analyses suggested that different species responded to the 1993 flood with varying tolerance and different succession strategies. In this study, the relation of flood-caused mortality rates and DBH, f_m(d), can be expressed in negative exponential functions for each species. The results of this research also indicate that the growth functions are different for each species and might also be different between pre- and post-flood time periods. These functions indicate different survival strategies and emergent properties in responding to flood impacts. This research enhances our understanding of forest succession patterns in space and time in the UPR floodplain. And such understanding might be used to predict long-term impacts of floods on UMR floodplain forest dynamics in support of management and restoration.     

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.     

The Rufiji River flood: plague or blessing?

The building of a large multipurpose dam is planned at Stiegler’s Gorge on the Rufiji River (Tanzania). Both national and local authorities have strongly emphasised the flood control aspect of the dam as they see the Rufiji floods as a major constraint to development. Though it is true that the Rufiji River has a high flow variability at various timescales, the flood perception in local communities differs from this view. The floods, essential for the sustenance of floodplain fertility, and therefore of the farming system, and vital to the productivity of most of the natural resources on which local communities depend, are perceived as a blessing, whilst droughts and the absence of regular flooding are perceived as the main threat. Historically, most of the food shortages in Rufiji District are associated with drought years and the myth of “the flood as a plague” emerged only in the late 1960s during the Ujamaa villagisation policy. The persistence of this myth is favoured by the inadequate assessment of the complexity of the local economies by the District technical staff. This difference in perception of the flood has major implications for development options. Under the current dam design, the alteration of the flooding pattern would have negative consequences for the downstream wetland and forest ecosystems and the flood-associated livelihoods of some 150,000 people. A cost-benefit analysis of flood control measures and a study of a dam design that would maintain the beneficial aspects of flooding should be accorded the highest priority.     

Physiological and morphological responses of red alder and sitka alder to flooding

Red alder (Alnus rubra Bong.) and sitka alder (A. viridis ssp. sinuata [Regel] Löve & Löve) are nitrogen-fixing woody species that grow sympatrically along the Pacific coast of North America. Red alder is found in poorly drained lowlands, as well as in soils of moist upland slopes, whereas sitka alder generally colonizes well-drained soils. To identify factors that contribute to flood tolerance, we conducted greenhouse experiments subjecting both species to a 20-day flood and 10-day recovery and red alder to a 50-day flood and 20-day recovery. We determined the effect of this stress on nitrogenase activity, root and nodule alcohol dehydrogenase (ADH) activity, lenticel and adventitious root development, relative growth rate (RGR), and leaf gas exchange. After 24 h of flooding, nitrogenase activity could not be detected in either species. Limited nitrogenase activity did return in red alder at the end of a 10-day recovery following the 20-day flood, but sitka alder showed no recovery of nitrogenase activity. After 50 days of continuous flooding, red alder nitrogenase activity returned to pretreatment levels. Red alder root and nodule ADH activity was more than twice that of sitka alder under flooded conditions. Sitka alder showed extensive root mortality and leaf abscission over the same 20-day flooding period. Flooded red alder exhibited an initial decline in root RGR, but recovered between days 10 and 20 with the formation of adventitious roots. Furthermore, initiation of adventitious roots in red alder coincided with an increase in stomatal conductance without a similar recovery of carbon dioxide exchange rate. Sitka alder formed few adventitious roots, lost much of its root and leaf biomass, and showed no restoration of growth during flooding or recovery. Different responses of red and sitka alder to flooding serve as a partial explanation for the different patterns of distribution of these species and suggest some adaptations of red alder that permit flood tolerance.     

Alterations in flood frequency increase exotic and native species richness of understorey vegetation in a temperate floodplain eucalypt forest

The delivery of environmental flows for biodiversity benefits within regulated river systems can potentially contribute to exotic weed spread. This study explores whether exotic plants of a floodplain forest in Victoria, Australia, are characterised by specific functional groups and associated plant traits linked to altering hydrological conditions over time. Permanently marked 20 × 20 m² plots from five wetland sites in Eucalyptus camaldulensis floodplain forest were sampled twice, first in the early 1990s (1993–1994) and then 15 years later (2007–2008). Species cover abundance data for understorey vegetation communities were segregated by season and analysed using ordination analysis. Exotic species richness was modelled as a function of site flooding history and native species richness using general linear models. Site ordinations by detrended correspondence analysis showed differential community compositions between survey dates, but native and exotic species were not clearly differentiated in terms of DCA1 scores. Most exotics belonged to functional groups containing annual species that germinate and reproduce under drier conditions. Exotics reproducing under wetter conditions were in the minority, predominantly perennial and capable of both sexual and asexual reproduction. Site flooding history and native species richness significantly predicted exotic species richness. Vegetation changes are partially structured by reduced flood frequency favouring increased abundance of exotic, sexually reproducing annuals at drier sites. Sites of low flood frequency are more sensitive to future exotic weed invasion and will require targeted management effort. Flow restoration is predicted to benefit propagule dispersal of species adopting dual regeneration strategies, which are predominantly natives in this system.