Invasion legacy effects versus sediment deposition as drivers of riparian vegetation

Riparian zones are formed by interactions between fluvio-geomorphological processes, such as sediment deposition, and biota, such as vegetation. Establishment of invasive alien plant (IAP) species along rivers may influence vegetation dynamics, evidenced as higher seasonal or inter-annual fluctuations in native plant diversity when IAP cover is high. This could impact the overall functioning of riparian ecosystems. Conversely, fine sediment deposited in riparian zones after floods may replenish propagule banks, thus supporting recruitment of native species. The interactive effects of invasion and fine sediment deposition have hitherto, however, been ignored. Vegetation surveys across rivers varying in flow regime were carried out over 2 years to assess changes in community composition and diversity. Artificial turf mats were used to quantify over-winter sediment deposition. The viable propagule bank in soil and freshly deposited sediment was then quantified by germination trials. Structural Equation Models were used to assess causal pathways between environmental variables, IAPs and native vegetation. Greater variation in flow increased the cover of IAPs along riverbanks. An increased in high flow events and sediment deposition were positively associated with the diversity of propagules deposited. However, greater diversity of propagules did not result in a more diverse plant community at invaded sites, as greater cover of IAPs in summer reduced native plant diversity. Seasonal turnover in the above-ground vegetation was also accentuated at previously invaded sites, suggesting that a legacy of increased competition in previous years, not recent sediment deposition, drives above-ground vegetation structure at invaded sites. The interaction between fluvial disturbance via sediment deposition and invasion pressure is of growing importance in the management of riparian habitats. Our results suggest that invasion can uncouple the processes that contribute to resilience in dynamic habitats making already invaded habitats vulnerable to further invasions.     

Effect of light stress from phytoplankton on the relationship between aquatic vegetation and the propagule bank in shallow lakes

1. The way light stress controls the recruitment of aquatic plants (phanerogams and charophytes) is a key process controlling plant biodiversity, although still poorly understood. Our aim was to investigate how light stress induced by phytoplankton, that is, independent from the aquatic plants themselves, determines the recruitment and establishment of plant species from the propagule bank. The hypotheses were that an increase in light stress (i) decreases abundance and species richness both of established aquatic plants and of propagules in the bank and (ii) decreases the recruitment success of plants from this bank. 2. These hypotheses were tested in 25 shallow lakes representing a light stress gradient, by sampling propagule banks before the recruitment phase and when the lakes are devoid of actively growing plants (i.e. at the end of winter), established vegetation at the beginning of the summer and phytoplankton biomass (chlorophyll a) during the recruitment and establishment phase. 3. The phytoplankton biomass was negatively correlated with the richness and abundance of established vegetation but was not correlated with the propagule bank (neither species richness nor propagule abundance). The similarity between the propagule bank and established vegetation decreased significantly with increasing phytoplankton biomass. 4. The contrast in species composition between the vegetation and the propagule bank at the highest light stress suggests poor recruitment from the propagule bank but prompts questions about its origin. It could result from dispersal of propagules from neighbouring systems. Propagules could also originate from a persistent propagule bank formerly produced in the lake, suggesting strong year‐to‐year variation in light stress and, as a consequence, in recruitment and reproductive success of plants.     

Changes along a disturbance gradient in the density and composition of propagule banks in floodplain aquatic habitats

This study used germination methods to examine the density, species composition and functional composition of propagule banks in a series of riverine wetland aquatic habitats subject to varying degrees of hydrological and management-related disturbance. Under permanent inundation (the conditions prevailing at most sites during the growing season) propagule germination and species richness was low, with floodplain perennials and helophytes particularly affected. Densities of floodplain annuals were largely maintained through continued germination of a few flooding tolerant species. On damp mud (conditions associated with hydrological instability) total seedling number and species richness increased significantly, but species richness of germinating hydrophytes declined. Mean seedling density at 0–0.1m depth was 15450 ± 4400 m–2, reaching a maximum (162 050 m–2) in temporary backwaters. Annual (e.g., Lindernia dubia, Cyperus fuscus) and facultative ruderal species (e.g., Lythrum salicaria and Alisma plantago-aquatica) predominated. Vertical zonation of the propagule bank was weakly developed. The numbers of individuals and species germinating varied significantly between sites. The seasonal, most intensely disturbed sites (temporary backwaters) supported a numerically large, species-rich propagule bank based on floodplain annuals, while the permanent, less disturbed sites (ditches and an oxbow pond) had a small, species-poor propagule bank composed of hydrophytes and helophytes supplemented by allochthonous seed inputs. Sites intermediate on the gradient had a propagule bank dominated by facultative amphibious, ruderal hydrophytes. The composition of the seed bank and the established vegetation was most similar at the heavily disturbed sites where the seed bank was maintained by vigorously fruiting annuals and supplemented by inputs from temporary habitats upstream. At permanent sites much of the propagule bank composition could be accounted for by inputs of floodborne seed from the immediately adjacent floodplain. The established vegetation at such sites appeared to be maintained mainly by vegetative propagation with recruitment from the propagule bank likely only after severe disturbance. The potential contribution of functionally diverse propagule banks to sucessional processes within fluvially dynamic floodplain aquatic habitats is emphasised.     

Ditches as species‐rich secondary habitats and refuge for meadow species in agricultural marsh grasslands

Questions

Can drainage ditches in agricultural marsh grassland provide a suitable habitat for the persistence of fen meadow species? How does the ditch margin vegetation develop as a function of regular dredging? Is ornithologically oriented management also beneficial for plant biodiversity?

Location

Riparian marshes, Eider‐Treene‐Sorge lowland, Schleswig‐Holstein, Germany.

Methods

We performed vegetation surveys of drainage ditches along with their water body, slope and margin structures annually for 3 years. The data were analysed with respect to date and means of ditch dredging. In addition, we recorded vegetation of the surrounding agricultural grassland, measured nutrient status of the soil and the water body and sampled seed bank of the ditch slopes. We used ANOVA and multivariate methods to describe the development of the ditch vegetation and the persistence of target meadow species.

Results

Vegetation re‐development of ditch margins proceeds quite rapidly after disturbance from dredging. Dominance of mudbank species was observed only in the first year, followed by an increase of reed species and reduction of phytodiversity. Target species of wet meadow communities reach highest abundance in the second and third year and build a significant seed bank before being suppressed by reeds.

Conclusions

In heavily eutrophicated, intensively used marsh grassland, regularly disturbed ditch margins are important secondary habitats for pioneer and subdominant wetland species, which have nearly disappeared in a larger area. Current management cycles of ditch dredging every 3–4 years comply with the successional development, allowing the mudbank and wet meadow species to persist in the vegetation and seed bank. In contrast to the frequency, the form of dredging (ditch profile), which is crucial for bird protection, plays a minor role for plants. We recommend moderate disturbance (mowing of ditch margins) to suppress strong competitors in the years between dredging for additional support to target plant species.     

Restoration of ditch bank plant species richness: The potential of the soil seed bank

Abstract. Small‐scale landscape elements, such as ditch banks, play an important role in preserving plant species richness in agricultural landscapes. In this study, we investigate whether the seed bank might be useful for restoring the above‐ground plant species richness. We studied the vegetation and seed bank composition at six species‐rich and six species‐poor ditch banks, where agri‐environment schemes are running to maintain and enhance ditch bank plant diversity. We show that the number of species in the seed bank was low, regardless of the number of species in the established vegetation. Moreover, the seed bank was always dissimilar to the established vegetation. Target species for nature conservation were occasionally present in the seed bank at both species‐poor and species‐rich sites, but rarely so if the species was absent from the established vegetation. We conclude that the potential use of the seed bank for restoration of ditch banks is minimal. At present, plant species richness seems to be largely controlled by germination opportunities; high biomass and competition appear to hamper germination at species‐poor sites. We recommend continued nutrient reduction at such sites. Soil disturbance measures and deliberate sowing should also be considered.     

Propagule banks and regenerative strategies of aquatic plants

Abstract. The role of the propagule bank in aquatic plant maintenance was studied in two riverine wetlands. Four sites were selected, characterized respectively by flooding, drying up, both disturbances operating, and neither operating. Our hypothesis was that recolonization after drying up would mostly involve seeds and buds from the propagule bank, whereas recolonization after floods would mostly involve rhizomes. Dry sites were characterized by a high density of seeds, and a high similarity between seed species and established vegetation. Unspecialized fragments remaining in wet parts of the sediment probably also contribute to species maintenance. Species maintenance in sites subjected to flooding was highly dependent on deeply anchored rhizomes, as indicated by the strong floristic similarity between species that occur in the established vegetation and rhizomes in the bank. Regeneration of the community after scouring floods also involved seeds, some species being able to flower under water. When scouring flooding and drying up were superimposed, regenerative strategies exhibited in the bank did not simply result from the ‘addition’ of the two disturbance effects. When the disturbances did not occur too closely together in time, species were able to survive either by: (1) producing many propagules under aquatic conditions or (2) coping with the temporal variability by producing several types of propagules.     

Connectivity and propagule sources composition drive ditch plant metacommunity structure

The fragmentation of agricultural landscapes has a major impact on biodiversity. In addition to habitat loss, dispersal limitation increasingly appears as a significant driver of biodiversity decline. Landscape linear elements, like ditches, may reduce the negative impacts of fragmentation by enhancing connectivity for many organisms, in addition to providing refuge habitats. To characterize these effects, we investigated the respective roles of propagule source composition and connectivity at the landscape scale on hydrochorous and non-hydrochorous ditch bank plant metacommunities. Twenty-seven square sites (0.5 km² each) were selected in an agricultural lowland of northern France. At each site, plant communities were sampled on nine ditch banks (totaling 243 ditches). Variables characterizing propagule sources composition and connectivity were calculated for landscape mosaic and ditch network models. The landscape mosaic influenced only non-hydrochorous species, while the ditch network impacted both hydrochorous and non-hydrochorous species. Non-hydrochorous metacommunities were dependent on a large set of land-use elements, either within the landscape mosaic or adjacent to the ditch network, whereas hydrochorous plant metacommunities were only impacted by the presence of ditches adjacent to crops and roads. Ditch network connectivity also influenced both hydrochorous and non-hydrochorous ditch bank plant metacommunity structure, suggesting that beyond favoring hydrochory, ditches may also enhance plant dispersal by acting on other dispersal vectors. Increasing propagule sources heterogeneity and connectivity appeared to decrease within-metacommunity similarity within landscapes. Altogether, our results suggest that the ditch network's composition and configuration impacts plant metacommunity structure by affecting propagule dispersal possibilities, with contrasted consequences depending on species' dispersal vectors.     

不同基质中水鳖对共存的穗花狐尾藻和金鱼藻的影响

研究设置了营养水平不同的3种基质,在有或无浮水植物水鳖共存的情况下,混合种植营养吸收方式不同的两种沉水植物（仅从水中吸收营养的金鱼藻和从水和基质都能吸收营养的穗花狐尾藻）,探讨在不同的基质中浮水植物对沉水植物的影响。结果显示:实验中水鳖的存在均未对共存的两种沉水植物的生长产生抑制作用。当水鳖覆盖水面时,与无根的金鱼藻竞争光照和水体营养,促使其增加分枝数和茎长,并增加叶生物量的分配,最终导致水鳖的共存促进了金鱼藻的生物量积累。但与金鱼藻不同,有根的穗花狐尾藻可通过基质吸收营养,与共存的水鳖之间只存在光照竞争,使穗花狐尾藻仅以改变植株形态来适应浮水植物的表层覆盖,生物量积累并未发生显著变化。与沙处理相比,水鳖在营养水平较高的泥处理和泥沙混合处理中生长更旺盛,基质没有显著影响沉水植物的生物量和分株数。因此,在退化湿地生态系统中恢复水生植物时,为了提高群落的物种多样性,可以将沉水植物与少量的浮水植物组合配置,少量的浮水植物不会对沉水植物的生长带来负面影响。     

Assessing the potential for biotic communities to recolonise freshwater wetlands affected by sulfidic sediments

1. The formation of sulfidic sediments in response to factors such as secondary salinisation and fertiliser usage is an emerging concern for the management of many freshwater wetlands. However, fundamental knowledge regarding the influence of sulfidic sediments on the aquatic biota is still lacking. 2. This study investigated the potential for biota to recolonise wetlands affected by sulfidic sediments, by assessing zooplankton hatching and aquatic plant germination following inundation with freshwater. Sediment samples were collected from 16 wetlands in the southern Murray‐Darling Basin, Australia, that ranged in condition from non‐impacted to possessing a known history of sulfidic sediments and/or acidification. 3. Principal Components Analysis indicated that the wetlands separated out into five different groups based on their sediment chemistry: non‐impacted, sulfidic, sulfidic and highly saline (sediment EC 46 800–209 000 μS cm^?1), sulfidic and potentially acidic (sediment pH 5.81–6.45 and ANC 0.07–0.31% CaCO₃), and sulfidic and acidic (sediment pH 4.37 and ANC 0.00% CaCO₃). 4. A viable dormant propagule bank was present in all wetlands, but the taxon richness of zooplankton and aquatic plants was significantly lower in wetlands affected by sulfidic sediments compared with those that were non‐affected. 5. This suggests that zooplankton and aquatic plants will be capable of recolonising wetlands that have accumulated sulfidic sediments via their propagule banks if the appropriate remediation measures are undertaken, although the communities developing are likely to be less diverse compared with those in non‐affected wetlands.     

How does the propagule bank contribute to cyclic vegetation change in a lakeshore marsh with seasonal drawdown?

Lakeshore marshes around Liangzi Lake, in the middle reach of the Yangtze River, China, experience annual changes of water level of c. 1.5 m. During the drawdown period, the vegetation is structured by helophytes and emergents; during the rainy season when the dams are closed (June–September) the marshes are flooded and their vegetation rapidly changes to be come dominated by submerged, floating-leaved and tall emergent species. The species composition and abundance of both the marsh seed bank and the vegetative propagule bank were compared with those of the drawdown and flooded vegetation types. These data provided a test of the predictive power of van der Valk's model of northern temperate seasonal vegetation change in a subtropical, freshwater wetland with cyclic vegetational change. The abundant species were detected in the propagule bank. The seed bank was found to determine the species richness of both types of the vegetation, whereas the vegetative propagule bank consisted of the dominants of the drawdown vegetation. Water depth conditions, and the composition of seed and vegetative propagules banks together determine the structure of the standing vegetation during drawdown and flooding. van der Valk's succession model was found to predict the seasonal vegetation change reasonably well. The Chi-square test showed no significant difference between predicted vegetation and actual vegetation in both drawdown and flooding periods.     

Digging deep for diversity: riparian seed bank abundance and species richness in relation to burial depth

相似文献   

The possibility of submerged macrophyte recovery from a propagule bank in the eutrophic Lake Mikołajskie (North Poland)

Changes in submerged macrophyte communities in the eutrophic Lake Mikołajskie have been studied for the last 40 years. As the recent commissioning of a number of sewage treatment plants within the complex of the Mazurian Great Lakes has led to an improvement in water transparency, it was expected that species composition would start to return to the state present before intensive eutrophication. The role of seed and oospore banks in the reconstruction of submerged macrophytes is analysed on the basis of laboratory experiment. Cores of sediments (9 cm long) were collected from the littoral of Lake Mikołajskie. The cores were divided in 3 cm layers and were incubated under laboratory conditions. Five species of submerged macrophytes were germinated from the propagule banks. The richest in viable propagules was the deepest layer and poorest in active propagules was the shallowest layer of sediment. The recolonization of the littoral zone of Lake Mikołajskie by species, which occurred at a earlier period in the lake’s history is possible because viable propagules have persisted in deeper sediments.     

Passive internal transport of aquatic organisms by waterfowl in Doñana, south-west Spain

Aim Waterbirds may play an important role in the maintenance of aquatic ecosystem biodiversity by transporting plants and invertebrate propagules between different wetlands. The aim of this study is to provide the first quantitative analysis of the transport of plant and animal propagules by a community of waterbirds. Location Doñana marshes in south‐west Spain. Methods We quantified the number of intact seeds and invertebrate eggs in 386 faecal samples from 11 migratory waterfowl species (10 ducks and coot), collected from 3 November to 3 December 1998 (when birds were arriving from further north), and 22–25 February 1999 (when birds were leaving Doñana). Results Intact seeds of at least 7 plant genera, and invertebrate eggs (ephippia of at least 2 crustacea, statoblasts of at least 2 bryozoans and eggs of Corixidae) occurred in 65.6% of the faecal samples in early winter and 67.8% in late winter. Main conclusions The abundance of different propagule types varied between waterfowl species in a seasonal and species specific manner, probably owing to differences in foraging strategies, bill and gut morphology, and seasonal shifts in propagule availability or distribution. Lamellar density was positively correlated with the abundance of intact propagules. Our results confirm that waterfowl play an important role in the dispersal of organisms in aquatic environments by internal transport. Wherever there is a propagule bank accessible to waterbirds, transport can occur even when propagule production and waterfowl movements do not overlap in time.     

Centennial‐scale changes to the aquatic vegetation structure of a shallow eutrophic lake and implications for restoration

1. We investigate long‐term (>200 years) changes to the composition and spatial structure of macrophyte communities in a shallow, eutrophic lake (Barton Broad, eastern England) and consider the implications for lake restoration. 2. Historical macrophyte data were assembled from a variety of sources: existing plant databases, museum herbaria, journal articles, old photographs and eyewitness accounts. Additionally, two types of sediment core sample were analysed for plant macro‐remains and pollen; bulk basal samples from multiple core sites analysed to provide information on ‘pre‐disturbance’ macrophyte communities and two whole cores analysed to determine historical change. 3. Prior to the late 1800s, macrophyte communities were diverse and included a multilayered mosaic of short‐stature submerged taxa and taller submerged and floating‐leaved species. With the progression of eutrophication after around 1900, the former community was displaced by the latter. Diversity was maintained, however, since an encroaching Schoenoplectus–nymphaeid swamp generated extensive patches of low‐energy habitat affording refugia for several macrophytes otherwise unable to withstand the hydraulic forces associated with open water conditions. When this swamp vegetation disappeared in the 1950s, many of the ‘dependent’ aquatic macrophytes also declined leaving behind a sparse, species‐poor community (as today) resilient to both eutrophication and turbulent open waters. 4. The combination of historical and palaeolimnological data sources offers considerable benefits for reconstructing past changes to the aquatic vegetation of lakes and for setting restoration goals. In this respect, our study suggests that successful restoration might often be better judged by reinstatement of the characteristic structure of plant communities than the fine detail of species lists; when nutrients are low and the structure is right, the right species will follow.     

The trade-off between vegetative and generative reproduction among angiosperms influences regional hydrochorous propagule pressure

Aim Local communities are subject to spatiotemporal contingencies of landscape processes; community assembly is thus often considered to be unpredictable and idiosyncratic. However, evolved trade‐offs of species’ life histories may set distinct constraints on the assembly of species communities. In plants, the recruitment and invasion success of species into communities depend primarily on the number of propagules available and on their generative or vegetative character. Life‐history trade‐offs prevent individual plants from producing large numbers of both generative and vegetative propagules, but it is not clear whether this constrains their availability at the landscape scale. We thus tested whether: (1) the observed relationship between generative and vegetative propagules deviates from the null expectation stating that species contributing the bulk of generative propagules to the propagule rain should also contribute the bulk of vegetative propagules; and (2) whether vegetative and generative propagule pressures are negatively correlated once species abundance in the regional pool is accounted for. Location A large riparian landscape in the Netherlands. Methods Analyses were based on an extensive trapping of floating propagules (214,049 propagules of 47 species), and a rough proxy of species abundance across the entire pool. We used both species and phylogenetically independent contrasts as data points, and accounted for variation in size of generative propagules. Results Both hypotheses were confirmed. Numbers of generative and vegetative propagules trapped per species were significantly negatively correlated (r = ?0.33; t₄₅ = ?2.61, P = 0.006) and thus strongly deviated from the null expectation. This was confirmed by analyses accounting for variation in species abundance across the species pool, and in the size of generative propagules. Main conclusions The results indicate that plant recruitment and community assembly across streams may be influenced by the way individual plants allocate their resources between competing life‐history functions. Life‐history evolution across angiosperms might thus have constrained the present‐day assembly of local communities.     

三江平原沟渠土壤种子库特征及其与地上植被的关系

为了探究三江平原沟渠土壤种子库在湿地植物保护中的作用及其在湿地恢复中的潜力,该文采用幼苗萌发法与样方调查相结合的方法,对三江平原不同开挖年限沟渠的土壤种子库特征及其与地上植被的关系进行了研究。结果表明:沟渠具有较大规模的土壤种子库,边坡种子库显著大于底泥种子库,边坡种子库密度为8 973–25 000 seeds·m–2,底泥种子库密度为506–1 488 seeds·m–2。开挖10年、20年和30年的沟渠土壤种子库共有50种植物萌发,隶属于20科41属。开挖10年、20年和30年的沟渠土壤种子库萌发物种数分别为37种、34种和33种,地上植被物种数分别为25种、33种和22种。土壤种子库和相应地上植被的相似性系数分别为38.7%、35.8%和32.7%。随着植物群落演替的进行,地上植被的Simpson指数、Shannon-Wiener多样性指数和Pielou均匀度指数均逐渐增大。三江平原沟渠土壤种子库和地上植被中保存了大量湿地植物,表明沟渠具有保护植物物种多样性的作用,沟渠土壤种子库具有湿地恢复的潜力。随着沟渠开挖年限增加,沟渠植物群落呈现退化特征,建议对沟渠系统加强管理。     

Revegetation following experimental disturbance in a boreal old-growth Picea abies forest

Abstract. We studied revegetation patterns after experimental fine-scale disturbance (e.g. uprooting) in an old-growth Picea abies forest in southeastern Norway. An experimental severity gradient was established by manipulation of the depth of soil disturbance; two types of disturbed areas were used. Species recovery was recorded in the disturbed patches in three successive years after disturbance. The cover of vascular plants and, even more so the cover of bryophytes and lichens, recovered slowly after disturbance. The least severe treatments (removal of vegetation and removal of vegetation and the litter layer) was followed by the fastest recovery. The mean number of vascular plant species was usually higher three years after disturbance than before disturbance, while the opposite was true for bryophytes. Several vascular plant species that were abundant in intact forest floor vegetation (Vaccinium myrtillus, V. vitis-idaea and Deschampsia flexuosa) recovered during a three-year period primarily by resprouting from intact rhizomes and clonal in-growth. Other important recovery mechanisms included germination from a soil-buried propagule bank (e.g. Luzula pilosa, Plagiothecium laetum agg., Pohlia nutans and Polytrichum spp.) and dispersal of propagules into the disturbed patches (e.g. Betula pubescens and Picea abies). Microsite limitation seemed to occur in several species that were abundant in the soil propagule bank (e.g. the ferns Athyrium filix-femina, Gymnocarpium dryopteris and Phegopteris connectilis) but which did not appear in disturbed patches. Disturbance severity influenced revegetation patterns, recorded both as trajectories of vegetation composition in a DCA ordination space and as change in floristic dissimilarity. The length of the successional path (compositional change measured in β-diversity units) increased with increasing disturbance severity, and was also influenced by the area of the disturbed patch and the distance to intact vegetation. The rate of succession depended on the method by which it was measured; decreasing year by year in floristic space, while first decreasing and then increasing in ordination space. The reason for this difference is explained.     

The role of emergent vegetation in structuring aquatic insect communities in peatland drainage ditches

Availability of macrophyte habitat is recognized as an important driver of aquatic insect communities in peatland drainage ditches; however, eutrophication can lead to the decline of submerged vegetation. While emergent vegetation is able to persist in eutrophicated ditches, vegetation removal, carried out during ditch maintenance, can reduce the availability of this habitat. In this study, we applied the landscape filtering approach to determine whether the absence of emergent vegetation is a habitat filter which structures aquatic insect communities in peatland drainage ditches under different trophic conditions. To this end, a field study was carried out in one mesotrophic (Naardermeer) and one eutrophic (Wormer and Jisperveld) peatland in the province of North Holland, The Netherlands. We assigned life history strategies to insect species and applied linear mixed models and redundancy analyses to taxonomic and functional aquatic insect community data. Our results indicate that while differences between peatlands primarily determine the species pool within each wetland, emergent vegetation acted as a secondary filter by structuring functional community composition within ditches. The eutrophic peatland was dominated by insects adapted to abiotic extremes, while species with good dispersal abilities were strongly related to emergent vegetation cover. This study demonstrates the applicability of life history strategies to provide insight into the filtering of species due to availability of emergent macrophyte habitat. To ensure greater diversity of insect communities in ditch habitats, it is recommended that some vegetation be spared during maintenance to leave patches from which insect recolonization can occur.     

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.     

The role of the permanent soil seed bank in early stages of a post-agricultural succession in the Inland Pampa,Argentina

We studied the soil seed bank composition in four old fields of different ages, after abandonment from agriculture. Complete seed bank composition was assessed by direct seed separation from soil samples and identification to species. Most species found in the seed bank were not important in the present seral communities. Seed of the species that dominated the early succession were generally not found. Additionally, there were very few propagules rather than on the germination of in situ propagules. We suggest that pampean grasses evolved under that the course of post-agricultural succession will depend strongly on the pattern of arrival of exogenous propagules rather than the germination of in situ propagules. We suggest that pampean grasses evolved under disturbances of low intensity and/or a disturbance regime dominated by small gaps, in which open areas could be rapidly colonized from the edges and/or by remnant vegetative propagules. The changes produced by the introduction of agriculture triggered the invasion by exotic species adapted to the new disturbance regime.