The Ups and Downs of Life on the Edge: The Influence of Water Level Fluctuations on Biomass Allocation in two Contrasting Aquatic Plants

Plant communities at wetland edges typically exhibit strong zonation along water depth gradients, although community structure is not as simple as this common observation may suggest. Water levels fluctuate at many temporal scales and in varied patterns. Understanding of how water regime controls plant community structure requires information on how individual species are affected by water regime. The allocation of biomass to roots, tillers, stolons and inflorescences was measured over 16 weeks in two species of aquatic plants with contrasting life histories. Juncus articulatus L. produces large numbers of small seeds while Glyceria australis C. E. Hubb produces small numbers of large seeds. Both species also reproduce vegetatively. Changes in biomass allocation were measured in response to duration, frequency and amplitude of water level fluctuations. The fluctuations in water level were achieved by suspending plants in pots from chains and raising and lowering the pots in and out of water in replicate tanks. There were complex responses to water regime components. As is common to many emergents, both species increased allocation to above ground vegetative structures in response to increasing water depth. Differences between the species in overall patterns of allocation reflected differences in life history. Juncus articulatus showed greater overall biomass production and greater proportional resource allocation to inflorescences than Glyceria australis. There were other differences in response to components of water regime that distinguished the species, mainly in allocation to reproduction in response to the range of depths (amplitude) and extended dry periods. These different functional responses to aspects of water level fluctuations indicate it may be possible to manipulate species composition if water levels are controllable. The most important characteristics of water regime were threshold events such as extremes of depth and dry periods. These may not be captured in measures of water regime such as average water levels.     

Effect of wet and dry cycles in calcareous soil on mineral nutrient uptake of two grasses,Agrostis stolonifera L. and Festuca ovina L.

Shoot uptake of mineral nutrients (Ca, Cu, Fe, K, mg, Mn, P, S, Zn) by Agrostis stolonifera L. was compared with Festuca ovina L. under wet and dry cycles. Such conditions are typical for A. stolonifera sites, whereas F. ovina is growing mostly on consistently drier and better-drained soils. Plants were grown in a glasshouse, at controlled temperature and light conditions, using two moisture regimes, one constant at 60% WHC (water holding capacity), one wet/dry fluctuating between 35 and 100% WHC. Above ground and total biomass production was lower under wet/dry treatment than at constant water regime in F. ovina, but did not differ between regimes in A. stolonifera. Shoot uptake of most elements was severely reduced in F. ovina at the wet/dry regime. Shoot uptake and concentrations of most elements studied (Cu, K, Mn, P, S, Zn) were lower (p<0.05) under wet/dry treatment than at constant regime in A. stolonifera and tended to be lower also of Fe and Mg. Differences in biomass production observed are consistent with field evidence that A. stolonifera grows in sites which are periodically flooded but may become quite dry during other periods, and that F. ovina is limited to sites which are consistently drier and better drained. Evidence from the present study, however, does not support any view that alternating wet and dry cycles, as typical of A. stolonifera field sites, would be beneficial to nutrient acquisition of this species but that biomass production may develop normally at the lower uptake of most mineral nutrients measured under the wet/dry regime. Such regimes are decidedly unfavourable to both growth and nutrient acquisition of F. ovina.     

Germination potential,growth patterns and reproductive effort of Juncus articulatus and Glyceria australis in temporary shallow wetlands in Australia

Juncus articulatus is an exotic aquatic species occuring over large areas of Mother of Ducks Lagoon, a shallow and temporary wetland in northern New South Wales, Australia. The other dominant perennial species at the site is the native Glyceria australis. Little is known of the ecology of either species or of the potential of J. articulatus to invade wetlands. The germination potential, growth patterns and reproductive effort of J. articulatus and G. australis were compared in response to water regime, grazing and competition. Both species have persistent seed banks from which only a small percentage of seeds germinate in repeated wetting events and from sediment stored dry. J. articulatus germinated in very high numbers from Mother of Ducks Lagoon sediment, but was rare in other lagoons in the region. G. australis germinated in comparatively lower numbers from Mother of Ducks Lagoon sediment and was rare in other lagoon sediments despite being present in the extant vegetation in several lagoons. The introduced J. articulatus has several life cycle characteristics that differ from G. australis, notably a high allocation of biomass to inflorescences under all water regimes. Clipping stimulated above ground vegetative and sexual reproduction for J. articulatus and high numbers of tillers facilitate the sexual reproductive effort. Because J. articulatus is ubiquitous in the region, our results suggest that management of this exotic species should be directed towards minimizing recruitment opportunities and that this could be achieved by increasing variation in water regimes in combination with reduced disturbance from grazing.     

Expression of Tolerance for Meloidogyne graminicola in Rice Cultivars as Affected by Soil Type and Flooding

The effects of different water regimes on the pathogenicity of Meloidogyne graminicola on six rice cultivars were determined in two soil types in three greenhouse experiments. Two water regimes, simulating continuous flooding and intermittent flooding, were used with five of the cultivars. All cultivars were susceptible to the nematode, but IR72 and IR74 were more tolerant than IR20 and IR29 under intermittent flooding. All were tolerant under continuous flooding. UPLRi-5 was grown under multiple water regimes: no flooding; continuous flooding; flooding starting at maximum tillering, panicle initiation, or booting stage; and flooding from sowing until maximum tillering or booting. In sandy loam soil, M. graminicola reduced stem and leaf dry weight, root dry weight, and grain weight under all water regimes. In clay loam soil, the nematode reduced root weight when the soil was not flooded or flooded only for a short time, from panicle initiation, or booting to maturity, and from sowing to maximum tillering. In clay loam soil, stem and leaf dry weight, as well as grain weight, were reduced by the nematode under all water regimes except continuous flooding or when the soil was flooded from sowing to booting stage. These results indicate that rice cultivar tolerance of M. graminicola varies with water regime and that yield losses due to M. graminicola may be prevented or minimized when the rice crop is flooded early and kept flooded until a late stage of development.     

Effects of a fluctuating temperature regime experienced by Daphnia during diel vertical migration on Daphnia life history parameters

Many freshwater zooplankton species perform a diel vertical migration (DVM) and spend the day within the lower, colder hypolimnion of stratified lakes. Trade-offs that arise from this migration have already attracted much attention and the cold temperature in the hypolimnion is thought to be the main cost of this behaviour. In this study we additionally looked at the extra costs daphnids have from being exposed to a fluctuating temperature regime (cold during the day and warm during the night) which is less well studied until today. In our experiment Daphnia hyalina Leydig and Daphnia magna Straus either spent 24 h in constant warm water (19 °C), 24 h in constant cold water (12 °C), or spent 12 h in warm and 12 h in cold water in an alternating way (fluctuating temperature regime). We expected the values of the life history parameters of Daphnia in the fluctuating temperature regime to be exactly halfway between the values of the life history parameters in the warm and cold treatments because the daphnids spent exactly half of the time in warm water, and half of the time in cold water. Concordant with earlier studies our results showed that age at first reproduction and egg development time were reduced at higher temperatures. In the fluctuating temperature regime the values of both parameters were exactly halfway between the values at permanently warm and cold temperature regimes. In contrast, somatic growth was higher at higher temperatures but was lower in the fluctuating temperature regime than expected from the mean somatic growth rate. This suggests that a fluctuating temperature regime experienced by migrating daphnids in stratified lakes involves additional costs for the daphnids.     

Simulated Herbivory and Vegetation Dynamics in Coal Slurry Ponds Reclaimed as Wetlands

The biodiversity of coal slurry ponds can be inhibited, at least in part, by dense stands of Phragmites australis. In this study, we demonstrate that species richness can be increased in coal slurry ponds if the dominant species (P. australis and Typha latifolia) are removed and that underwater herbivory simulated by cutting will kill emergents. The study was conducted in the greenhouse and the field in both flooded and drawndown conditions. Stems of plants of P. australis and T. latifolia were cut in a greenhouse and the cut plants of both species showed a decline in survivorship (25 and 42% survival, respectively) whereas all uncut plants survived. In a reclaimed coal pond at Pyramid State Park, Illinois, neither P. australis nor T. latifolia survived cutting underwater, but all of the uncut plants survived. Regrowth measured as total biomass of stems was less among flooded versus freely drained plants (0.3 and 2.6 g biomass, respectively). Cut versus uncut plants, combining freely drained and flooded, had less below-ground biomass (99.4 and 254.4 g, respectively). In the greenhouse study, oxygen levels in rhizomes subsequent to cutting were measured using an oxygen electrode and millivolt meter. Oxygen levels in P. australis were lower in cut versus uncut plants both in flooded (15.0 vs. 16.3% ambient O₂, respectively) and freely drained conditions (14.5 vs. 15.0%, ambient O₂, respectively). Similar responses to cutting were demonstrated by T. latifolia. In an unreclaimed coal slurry pond with monospecific stands of P. australis, plant species richness increased in cut plots as compared to uncut plots (29 vs. 2 species, respectively) between March and September, 1995. This study demonstrated that species richness can be increased in coal ponds by mechanical cutting and this potentially by herbivory; however, the additional species were mostly exotics.     

Changes in biotic communities developing from freshwater wetland sediments under experimental salinity and water regimes

1. Reduction in diversity of both freshwater aquatic and terrestrial ecosystems has been attributed to salinity increase and such increases are a symptom of changes to land use. Hydrological alteration to ground and surface water are likely to be associated with salinity increase and its influence on biodiversity. However the combined effects of salinity and hydrology on aquatic biodiversity have not been elucidated fully in either field or experimental situations. 2. The effect of salinity and water regime on the biota in sediments from seven wetlands from inland south‐eastern Australia was tested experimentally using germination of aquatic plant seeds (five salinity and two water levels) and emergence of zooplankton eggs (five salinity levels). Salinity levels were <300, 1000, 2000, 3000, 5000 mg L^?1 and water regimes were damp (waterlogged) and submerged. 3. Aquatic plant germination and zooplankton hatching was not consistent for all seven wetland sediments. Four of the wetland sediments, Narran Lakes, Gwydir Wetlands, Macquarie Marshes and Billybung Lagoon showed similar responses to salinity and water regime but the other three wetland sediments from Lake Cowal, Great Cumbung Swamp and Darling Anabranch did not. 4. As salinity increased above 1000 mg L^?1 there was a decrease in the species richness and the abundance of biota germinating or hatching from sediment from four of the wetlands. 5. Salinity had a particularly strong effect in reducing germination from sediments in damp conditions when compared to the flooded conditions. In parallel, salts accumulated in the sediment in damp conditions but did not in flooded conditions. 6. There is potential for increasing salinity in freshwater rivers and wetlands to decrease the species richness of aquatic communities and thus of the wetland community as a whole, resulting in loss of wetland biodiversity. This reduction in diversity varies between wetlands and is at least partly related to hydrology. For aquatic plants the reduction in diversity will be more marked for plants germinating from seed banks at the edges of wetlands where plants are not completely submerged than for the same seed bank germinating in submerged conditions.     

不同水分条件下混种密度对芦苇和反枝苋种间竞争的影响

与本地植物的种间竞争是影响外来植物能否成功入侵的关键因素之一,该研究通过受控模拟试验研究了本地植物芦苇(Phragmites australis)和外来入侵植物反枝苋(Amaranthus retroflexus)在淹水和干旱两种水分条件下混种密度(6∶2、4∶4和2∶6)对其种间竞争的影响。结果表明:(1)芦苇和反枝苋的相对产量与相对产量总和均小于1,即两种植物存在种间竞争。(2)种间竞争使芦苇和反枝苋的生长均受到了不同程度的抑制,表现在两者的株高和生物量均随着竞争者密度的增加而降低。(3)植株地上部分和地下部分的氮浓度表现出与株高和生物量相同的趋势,且在不同水分条件下存在差异。(4)芦苇和反枝苋分别在淹水和干旱环境下具有较强竞争力,但在各自较高混种密度下亦具有较强竞争力。可见,芦苇和反枝苋的种间竞争受到了水分和混种密度的影响。因此,在有反枝苋分布的湿地中,植物生长初期可通过增加土壤水分和/或增加芦苇等本地植物的种群密度以降低反枝苋的种群密度来限制其竞争能力,防止反枝苋在湿地中生长建群和扩散入侵。     

Oviposition in Culicoides impunctatus under laboratory conditions

We present a laboratory-based examination of oviposition preference in the Scottish biting midge, Culicoides impunctatus (Diptera: Ceratopogonidae). A variety of oviposition substrates were screened in no-choice and choice bioassays for efficacy in eliciting an egg-laying response. Both upper-layer photosynthetic Sphagnum spp. moss and Juncus articulatus infusions were identified as producing increased oviposition under no-choice conditions. During choice trials against a control of damp cottonwool, upper-layer Sphagnum spp. moss produced a significantly greater egg-laying response. These conclusions are interpreted in terms of possible cues involved in oviposition site selection and assessed for future use in colonisation of this troublesome species.     

Effects of water regime and habitat continuity on the plant species composition of floodplain forests

Questions: 1. How big is the difference in the herbaceous layer composition between flooded and unflooded stands? 2. Are there species or species groups which have an affinity to ancient vs. recent forests in stands with different water regimes? 3. Are patterns of life history traits different between flooded and unflooded stands as well as between ancient and recent forests in stands with a different water regime? Location: Floodplain forests in the Middle Elbe region and district of Leipzig, Central Germany. Location: The herbaceous layer was studied in randomly selected quadrats of 9 m² in 2000 and 2001. Six ancient (n_plot=59) and six adjacent recent forests (n_plot=108) were investigated in flooded stands as well as three ancient (n_plot=41) and three recent forests (n_plot=70) in stands that have not been flooded for 50 years. The association of single species, species groups and life history traits were statistically tested for flooded vs. unflooded stands and for ancient vs. recent forests. Results: Interruption of flooding caused a complete species turnover in the herbaceous layer composition. Whereas in the still flooded stands typical alluvial species prevail, species composition in stands without flooding for 50 years showed a closed relation to the Stellario‐Carpinetum. Six herbaceous species in the flooded and five in the unflooded stands showed a preference for ancient forests. Only one species in the flooded and six herbaceous species in the unflooded stands are significantly associated with recent forests. Life history traits differ between flooded and unflooded stands but are similar in ancient and recently flooded stands, while unflooded ancient forests have more geophytes and myrmecochorous species than recent forests. Conclusions: The specificity of species composition in floodplain forests can only be maintained by regular flooding. Interruption of inundations lead to differences in the patterns of species composition and life history traits between ancient and recent forests.     

Leaf development and phenology of Triticum aestivum and T. durum under different moisture regimes

Wheat grain yield production in the rain-fed areas is limited by water deficits during crop growth. A greenhouse experiment was conducted during spring 1992 at ICARDA, Tel Hadya, Syria, with eight genotypes representing two Triticum species (Triticum turgidum var. durum and Triticum aestivum L.) under four soil-moisture regimes (95%, 75%, 55%, and 35% field capacity) to study the effect of water deficit on leaf development. The phyllochron was similar in the two species across the watering regimes. The range in variation in phyllochron among the genotypes was similar in the two species. Phyllochron response to water stress among genotypes was distinct in the driest regime in both species. Cham 6 (T. aestivum) and Gallareta (T. turgidum var. durum) had similar phyllochron across all moisture regimes whereas in other genotypes phyllochron was higher in the dries regime. Leaf area decreased with increasing moisture stress. Triticum turgidum var. durum genotypes were later in flowering as they had, on average, one leaf more than Triticum aestivum genotypes with similar leaf appearance rates.     

Shade does not ameliorate drought effects on the tree fern species Dicksonia antarctica and Cyathea australis

We examined the responses of two tree fern species (Dicksonia antarctica and Cyathea australis) growing under moderate and high light regimes to short-term water deficit followed by rewatering. Under adequate water supply, morphological and photosynthetic characteristics differed between species. D. antarctica, although putatively the more shade and less drought adapted species, had greater chlorophyll a/b ratio, and greater water use efficiency and less negative δ¹³C. Both species were susceptible to water deficit regardless of the light regime showing significant decreases in photosynthetic parameters (A _max, V _cmax, J _max) and stomatal conductance (g _s ) in conjunction with decreased relative frond water content (RWC) and predawn frond water potential (Ψ_predawn). During the water deficit period, decreases in g _s in both species started one day later, and were at lower soil water content, under moderate light compared with high light. D. antarctica under moderate light was more vulnerable to drought than all other plants as was indicated by greater decreases in Ψ_predawn, lowest stomatal conductance, and photosynthetic rates. Both tree fern species were able to recover after a short but severe water stress.     

Is hydrological manipulation an effective management tool for rehabilitating chronically flooded,brackish‐water wetlands?

1. Reinstating more natural water regimes is often a priority intervention to rehabilitate wetlands that have been degraded through anthropogenic changes to their natural wetting and drying cycles. Hydrological interventions are often made in chronically desiccated wetlands but less commonly in wetlands that have been permanently inundated and that require a drawdown phase for rehabilitation. Reports on the effectiveness of reinstating a drawdown phase in chronically inundated wetlands are particularly rare. 2. We undertook a landscape‐scale, experimental drawdown of water levels at Dowd Morass, a large, Ramsar‐listed, brackish‐water wetland in south‐eastern Australia that had been artificially flooded for 30+ years. During the hydrological manipulation, c. 500 ha of the wetland was drawn down and re‐flooded, and the remaining c. 1000 ha was used as a control site. Fringing areas with a fluctuating water regime were used as a reference site. Results were analysed in terms of gradient analysis, by classifying the different water regimes created by the hydrological interventions. The response of wetland vegetation was measured along replicated transects over a 4‐year period, before, during and after drawdown. Wetland plants were assigned to plant functional groups for analysis. Assembly theory and knowledge of life‐history traits were used to predict that drawdown would promote recruitment of plant species that required exposed sediment for germination and seedling establishment. 3. Within‐wetland microtopography interacted with the hydrological interventions to generate three distinct water regimes, which were differentiated by the spatial extent of exposed sediment and duration of the dry period. Drawdown promoted limited recruitment of some plant species, and the survival of cohorts then depended strongly on the extent and duration of the dry period. Species richness and vegetation cover (understorey and overstorey) continued to decline in constantly flooded areas of the wetland. Increased salinisation of sediments and surface waters reduced the effectiveness of the drawdown and dramatically affected species richness and cover of aquatic vegetation, which did not recover fully when fresher conditions returned. 4. The capacity of vegetation to respond to the reinstatement of a drawdown cycle following chronic inundation was constrained by abiotic (e.g. salinity) and biotic (e.g. depauperate seedbanks) factors. Reinstating a dry phase in chronically inundated, brackish‐water wetlands is complex and risky and may not effectively improve vegetation condition in the short term. In the case of Dowd Morass, rehabilitation was most successful in sites that had been shallowly flooded prior to drawdown and that remained dry for longest.     

Growth rates of selected Australian tropical rainforest tree species under controlled conditions

Controlled environment treatments were applied to assess the effects of temperature on the seedling mortality and growth rates of Toona australis and Flindersia brayleyana, two tropical rainforest tree species from northeast Queensland, Australia. Past workers have assigned these two species to the same ecological niche in terms of their response to canopy disturbance and gap-phase regeneration; however, their geographic ranges are very different. The hypothesis was that the species confined to the warm tropics (F. brayleyana) would have higher seedling mortality and a slower growth rate at lower temperatures than the species that occurs over a wide latitudinal range from the warm tropics to cooler temperate environments (T. australis). Significant differences were found in the growth rates of these two species in the warm (29/22° C) and cool (22/10°C), but not the intermediate (24/16° C), day/night temperature regimes. Their growth rates both decreased with decreasing temperature, but the decrease was significantly less for F. brayleyana which had the faster growth rate and lower seedling mortality in the cool regime. These results led to the rejection of the hypothesis and a test of the assignment of these two species to the same ecological niche. The test involved monitoring their growth to sapling-size in the intermediate temperature regime together with four other co-occurring tropical rainforest tree species belonging to different ecological niches. The growth rates and proportions of above-ground biomass allocated to woody tissue distinguished T. australis and a fast-growing pioneer species from F. brayleyana and three primary forest species. The stem heights and aboveground biomass of T. australis and the pioneer species exceeded the other four species by factors ranging from two to five. It is concluded that T. australis does not belong to the same ecological niche as F. brayleyana, and it is recommended that more research be conducted on the ecotypic temperature responses of the taxon T. australis.     

Regeneration potential of <Emphasis Type="Italic">Taxodium distichum</Emphasis> swamps and climate change

Seed bank densities respond to factors across local to landscape scales, and therefore, knowledge of these responses may be necessary in forecasting the effects of climate change on the regeneration of species. This study relates the seed bank densities of species of Taxodium distichum swamps to local water regime and regional climate factors at five latitudes across the Mississippi River Alluvial Valley from southern Illinois to Louisiana. In an outdoor nursery setting, the seed banks of twenty-five swamps were exposed to non-flooded (freely drained) or flooded treatments, and the number and species of seeds germinating were recorded from each swamp during one growing season. Based on ANOVA analysis, the majority of dominant species had a higher rate of germination in non-flooded versus flooded treatments. Similarly, an NMS comparison, which considered the local water regime and regional climate of the swamps, found that the species of seeds germinating, almost completely shifted under non-flooded versus flooded treatments. For example, in wetter northern swamps, seeds of Taxodium distichum germinated in non-flooded conditions, but did not germinate from the same seed banks in flooded conditions. In wetter southern swamps, seeds of Eleocharis cellulosa germinated in flooded conditions, but did not germinate in non-flooded conditions. The strong relationship of seed germination and density relationships with local water regime and regional climate variables suggests that the forecasting of climate change effects on swamps and other wetlands needs to consider a variety of interrelated variables to make adequate projections of the regeneration responses of species to climate change. Because regeneration is an important aspect of species maintenance and restoration, climate drying could influence the species distribution of these swamps in the future.      

Natural attenuation of weathered oil using aquatic plants in a farm in Southeast Mexico

An experiment was conducted in field for three years to assess the sustainability of aquatic plants Leersia hexandra, Cyperus articulatus, and Eleocharis palustris for use in the removal of total hydrocarbons of weathered oil in four areas contaminated with 60916–119373 mg/kg of hydrocarbons. The variables evaluated were coverage of plant, dry matter, density of plant growth-promoting rhizobacteria, and the removal of total weathered oil. The variables showed statistical differences (p = 0.05) due to the effects of time and the amount of oil in the soil. The three aquatic plants survived on the farm during the 36-month evaluation. The grass L. hexandra yielded the greatest coverage of plant but was inhibited by the toxicity of the oil, which, in contrast, stimulated the coverage of C. articulatus. The rhizosphere of L. hexandra in control soil was more densely colonized by N-fixing bacteria, while the density of phosphate and potassium solubilizing rhizobacteria was stimulated by exposure to oil. C. articulatus coverage showed positive relationship with the removal of weathered oil; positive effect between rhizosphere and L. hexandra grass coverage was also identified. These results contributed to the removal of weathered oil in Gleysols flooded and affected by chronic discharges of crude oil.     

Effects of soil fertility and flooding regime on the growth of Ambrosia trifida

Understanding the effects of abiotic environmental factors on invasive plants species traits is of importance for practical prevention. To examine the effects of soil fertility and flooding regime on the growth of Ambrosia trifida L., a mesocosm experiment was conducted for 18 weeks. Two levels of soil fertility (high and low) and three types of flooding regime (non-flooded, flooded, and periodically flooded) were prepared. Shoot height and dry weight of each plant were measured. We found both individual and interactive effects of soil fertility and flooding regime on the overall growth performance of A. trifida (p?<?0.05). The highest shoot height (154.7?±?4.4 cm) and total dry weight (TDW, 13.0?±?1.4 g) were obtained under high fertility and non-flooded condition. Height and weight were relatively low under flooding conditions (flooded and periodically flooded). In particular, shoot height (102.3?±?3.2 cm) and TDW (3.2?±?0.3 g) were the lowest under low fertility and periodically flooded condition. On the other hand, the ratio of above- to below-ground dry weight was relatively high under flooded conditions, showing the adaptive phenotypic plasticity. Adventitious root formation and more biomass allocation to shoots were a flooding-adaptive mechanism of A. trifida, well developed under high fertility condition. We suggest maintaining appropriate water regime and avoiding eutrophication in wetlands would be necessary to prevent A. trifida from invading. These findings will contribute to the conservation of biodiversity in wetlands by effective management of A. trifida.

     

RELATIONSHIPS BETWEEN FLOODING TOLERANCE,LIFE HISTORY,AND SHORT-TERM COMPETITIVE PERFORMANCE IN THREE SPECIES OF POLYGONUM

The objective of this study was to compare the growth and short-term (single season) competitive performance of three species of Polygonum known to differ in flooding tolerance and life history. Polygonum amphibium is a perennial with low sexual reproductive effort and a relatively high degree of flooding tolerance, P. lapathifolium is an annual species with a high sexual reproductive effort and a low tolerance to flooding, and P. hydropiperoides is intermediate to the other two in terms of sexual reproductive effort and flooding tolerance. In order to determine the relative growth and competitive abilities of these species, mixtures and monocultures of plants were grown in pots and maintained under three flooding regimes: 1) flooded, 2) partially drained, and 3) well drained. Both P. hydropiperoides and P. amphibium grew best under flooded and partially drained conditions with reduced growth in the drained treatment. Polygonum lapathifolium, in contrast, grew as well in the drained treatment as in the more flooded treatments. Results from competition experiments were consistent in showing the relative competitive abilities to be P. lapathifolium > P. hydropiperoides > P. amphibium regardless of flooding regime. Thus, short-term competitive performance was found to trade off with flood tolerance rather than with sexual reproductive effort.     

The effect of inundation and salinity on the germination of seed banks from wetlands in South Australia

Three wetlands from the Upper South East of South Australia were chosen to investigate how a past history of drought (dry since 2002, 2004 and 2005) and salinity (2800 to >20,000 mg L⁻¹) influenced the response of the seed bank to two water regimes (drained and flooded) and four salinities (500, 1000, 3000 and 5000 mg L⁻¹). The maximum number of germinants (1270 ± 850 m⁻²) and species richness (7 ± 2.4) was greatest under the fresher drained treatment compared with the flooded more saline treatment under which there was no germination at one site. There were significant interactions between water regime and wetland previous history for two wetlands, but not the third which was the most saline and had experienced the longest drought. This indicated that the previous drought and salinity conditions experienced by a wetland affected seedling emergence but in the two less impacted wetlands the imposition of fresher drained conditions mitigated against these impacts. This suggests that if drought conditions continued with repeated exposure to elevated salinities the number of seeds and the species diversity of the seed banks would continue to decline.