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

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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.     

城市化地区河岸带植被特征及其与河岸硬度的关系——以晋江市为例

确立了滨河河道硬度与滨河土地硬度两个指标,用以研究城市化地区河岸带的植物构成、多样性与均匀度、优势植物等特征与河岸硬度的关系。选定晋江市都市区12条主要河流作为研究对象,通过样地调查收集数据,并采用相应指数进行数据处理分析,结果表明:(1)调查河流的河岸带植被物种共计70科143属159种,其中乔木20科30属41种,灌木15科15属20种,草本35科98属98种,草本居主导地位,河流间种类分布不均衡;(2)河岸带植物乔灌草区系均以广义热带性成分居多,其次为世界性成分、温带性成分,无中国特有分布类型,乔木中广义热带成分占绝对主导,直观反映地带性特征,灌木与草本中的温带成分多于乔木中的温带成分,类型趋于多元,种类更丰富;(3)从河岸植被优势植物构成来看,主要优势乔木与灌木基本为本土植物,而主要草本中外来植物入侵严重,对地带性植物景观的指示显然不及乔木与灌木;(4)从河岸植被的人工美学属性来看,灌木优势植物中园林观赏植物的种类数显著多于乔木、草本优势植物,反映出晋江人工审美主导的滨河景观空间主要改变与塑造了灌木层植被景观;(5)从河岸植物的人居需求属性来看,龙眼、杨桃等多见于庭院林、水岸林的水果树种也在河岸植被中频繁出现,反映出人口密集区由于对植物生产用途的重视而对河岸植被产生的影响;(6)滨河土地硬度与河道硬度对河岸带灌木植物的影响最强烈,二者均对河岸带植物多样性造成威胁,且前者具有更大的影响力,而后者直接影响了河岸植物的分布形态,出于生产与审美目的的人为干扰对河岸带植物优势种的影响力随滨河硬度的增加而加强,低滨河硬度有助留存原生植被群落;(7)天然弯曲的河流形态对河流植被特征具有积极影响,有助于保留更多本土植被类型,并能在某种程度上丰富乔灌草的植物种类,但这种缓和作用无法根本扭转河岸硬化对植物多样性的胁迫影响。研究表明,城市化地区河流的岸带植被特征与沿线人为干扰类型和强度密切相关,恢复与塑造河岸生态景观要以乡土植被为主,通过乔木景观塑造地带性景观,并以灌木与草本丰富植被景观。     

Seed rain of fleshy and dry propagules in different habitats in the temperate rainforests of Chiloé Island,Chile

In temperate rainforests on Chiloé Island in southern Chile (42°S), most canopy trees bear fleshy, avian‐dispersed propagules, whereas emergent tree species have dry, wind‐borne propagules. In the present study, the following hypothesis was tested: regardless of species, fleshy propagules are deposited in greater numbers in canopy gaps and in forest margins and hence have a more heterogeneous seed shadow than wind‐dispersed propagules. To test this hypothesis, the seed rains of these two types of propagules were compared in the following forest habitats: (i) tree‐fall gaps (edges and centre); (ii) forest margins with adjacent pastures; and (iii) under closed canopy (forest interior). Seed collectors (30‐cm diameter) were placed in two (15 and 100 ha) remnant forest patches (n = 60–100 seed collectors per patch) distributed in the four habitats. Seeds were retrieved monthly from each collector during two reproductive seasons (1996, 1997). In both years, the seed rain was numerically dominated by two species with dry propagules (Laureliopsis philippiana and Nothofagus nitida) and three species with fleshy fruits (Drimys winteri, Amomyrtus luma, and Amomyrtus meli). The seed shadows of the two species with dry, wind‐dispersed seeds differed markedly. Seeds of L. philippiana were deposited predominantly in canopy openings, whereas N. nitida seeds fell almost entirely in the forest interior. The fleshy‐fruited species, Drimys and Amomyrtus spp., had similar seed deposition patterns in the various habitats studied, but the between‐year differences in seed rain were greater in Drimys winteri than in Amomyrtus spp. Although no more than 10% of fleshy‐fruited propagules reached the margins of the patch, approximately 7% of these were carried there by birds. Every year, canopy gaps (pooling data from edges and centres) concentrated approximately 60% of the total seed rain of both propagule types in both forest patches. Forest margins received less than 20% of the total seed rain, which was largely dominated by fleshy‐fruited species. Seed shadows were a species‐specific attribute rather than a trait associated with propagule type and dispersal mode.     

Species Recruitment following Flooding,Sediment Deposition and Seed Addition in Restored Riparian Areas

In recent years an increasing number of streams have been restored to improve conditions for natural fen and meadow vegetation to develop in the associated riparian areas, but with modest success. Here we apply a controlled and replicated approach to investigate the role of flooding, sediment deposition, and seed addition for species recruitment in riparian areas with different types of standing vegetation. We expect that species recruitment is restricted in areas where the vegetation is dominated by fast‐growing productive species because competition for light will make the establishment of new species difficult, especially for low‐productive target species. We found that the naturally recruited species were few, mostly common, and widely distributed species. A majority of the recruited species, including target species added as seeds into the sediments, emerged in all areas independent of the characteristics of the standing vegetation. We observed significant temporal changes in compositional patterns throughout the experimental period (May to October). These changes were especially pronounced in areas with fen/fen‐meadow vegetation and were closely associated with the emergence and growth of species originating from the deposited sediments (e.g. Equisetum pratense, Poa trivialis, Urtica dioica), including the seeded target species (Lychnis flos‐cuculi and Lotus pedunculatus) and with a decline in fen‐associated mosses and small sedges. Compositional changes were also associated with shifts toward more productive species in areas previously dominated by low and intermediately productive species. We infer that flooding and sediment deposition play a limited role for recruitment of target species in riparian areas and that sediment deposition may entail a risk of losing diversity in riparian areas.     

Seed bank,seed dispersal and vegetation cover: Colonization along a newly‐created river channel

Question : What is the relative importance of the initial seed bank and subsequent seed dispersal for floristic composition of bank vegetation two years after creation of a newly‐cut reach of a river channel? Location : River Cole, West Midlands, United Kingdom. Methods : We took bank and bed sediment samples from a 0.5‐km reach of a new river channel cut into intact flood‐plain. After river diversion, seed samples deposited on artificial turf mats placed on the river banks and flood‐plain edge were taken in summer and winter 2002 and 2003. Seed rain samples from funnel traps were taken during summer 2002 and 2003. We undertook greenhouse germination trials to assess viable seed species within these samples. In summer 2004, we surveyed river bank vegetation. Agglomerative cluster analysis was used to investigate floristic similarity between seed bank, seed rain, seed deposition samples and final bank vegetation cover. DCA was used to explore contrasts between the samples and to assess whether these reflected interpretable environmental gradients. Results : Seed rain samples contained a small subset of species in the summer depositional samples. 38 species were found within the final vegetation, the seed bank, and at least one of the four sets of depositional samples; a further 30 species not present in the seed‐bank samples were present in at least one of the four sets of depositional samples and the final vegetation. Floristic composition of the vegetation was most similar to the depositional samples from winter 2002 and 2003 and summer 2003. DCA axis 1 reflected a time sequence from seed‐bank samples through depositional samples to the final vegetation. Conclusions : Newly cut river banks were colonized rapidly. Seed remobilization and hydrochorous transport from the upstream catchment are important for colonization. Species richness was highest in samples deposited during winter when high river flows can remobilize and transport viable seeds from upstream. This process would also have enhanced the species richness of seed production along the banks during the second summer (2003).     

Landscape-scale variation in the seed banks of floodplain wetlands with contrasting hydrology in China

1. At a local scale, the species composition, diversity and spatial variation of wetland plant communities are determined primarily by spatial and temporal heterogeneity in their environments. Less is known about variation at a landscape‐level. The floodplain of the Changjiang (Yangtze) River in China includes hydrologically connected, subtropical wetlands with different hydrological characteristics. 2. We examined seed‐bank species composition and richness in marshes of two contrasting hydrological types: permanent marshes, fed by local runoff, and lakeshore marshes more closely connected to the regulated river. Lakeshore marshes are flooded annually to depth of approximately 1 m and during flooding they support an alternate, aquatic vegetation type. The soil seed bank in March was a comparative estimator of species diversity. At the beginning of the growing season it included seeds from both phases of alternating vegetation types associated with the annual hydrological cycle. 3. A regional pool of 101 species was detected in the seed banks of six wetlands associated with the river and its tributaries: 56 occurred in permanent marshes and 59 in lakeshore marshes, with only 15 common to both. Species rarefaction curves indicated that more species occurred in permanent than lakeshore marshes at equal numbers of individuals sampled. However, the more heterogeneous lakeshore seed banks were estimated (Chao 2) to have greater total species richness (81) than permanent marsh (60). 4. Analysis using Sørensen's coefficient of similarity and DCA ordination revealed complex variation, with much greater differences between hydrological types than within them, irrespective of geographical distance. The types also differed significantly in the composition of four functional groups of species. 5. Despite the potential for dispersal of propagules via the annually pulsing river system (hydrochory), at a regional and landscape scale, diversity is maintained largely by large‐scale temporal hydrological heterogeneity and smaller scale spatial and topographic heterogeneity.     

Predicting potential impacts of environmental flows on weedy riparian vegetation of the Hawkesbury–Nepean River,south‐eastern Australia

Remnants of native riparian vegetation on the floodplain of the Hawkesbury–Nepean River near Sydney, have significant conservation value, but contain a large component of weeds (i.e. exotic species that have become naturalized). A proposal for the introduction of environmental flows required an assessment of potential impacts on 242 native and 128 exotic species recorded along 215 km of the river. The likely effects of frequency, season, depth and duration of inundation were considered in relation to habitat, dispersal season and tolerance to waterlogging. Overseas studies provided only limited information applicable to the study area; however, comparisons with similarly highly modified riparian habitats in New Zealand were instructive. Depth and season of inundation appear to be the variables with the greatest potential for differential effects on weeds and native plants. Because of likely spread of propagules and enhancement of growth under the present nutrient‐enriched conditions, environmental flows that would cause more frequent flooding to higher levels of the riparian zone were judged to be of more benefit to weed species than native species, unless supported by bushland management including weeding. Predictions were limited by incomplete data on Hawkesbury–Nepean species, but two types of environmental flow were judged to be potentially beneficial for native water‐edge plants, and worth testing and monitoring: first, flows that maintain continuous low‐level flow in the river, and second, higher level environmental flows restricted to the river‐edge habitat in autumn (the season in which a greater proportion of native species than weed species are known to disperse propagules). In summary, the presence of environmental weeds in riparian vegetation constrain the potential for environmental flows to improve river health. However, with ongoing monitoring, careful choice of water level and season of flow may lead to environmental flows that add to our knowledge, and benefit riparian vegetation along with other river system components.     

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.     

Many alien invasive plants disperse against the direction of stream flow in riparian areas

Propagule pressure plays an important role in the invasion of alien plants into riparian areas. In this study, we focused on propagule pressure from both neighboring riparian areas and anthropogenic land-use areas because propagules are likely to originate from both sources. We tested the effects of whether neighboring units contained the alien plant species, focusing on the direction of invasion by alien plant species into the focal unit, and how much anthropogenic land was contained within the unit, focusing on both farmland and urbanized areas, on alien plant occurrences in Hyogo Prefecture, Japan. We modeled the occurrence of 10 alien plants using generalized linear models to evaluate species invasions by both propagules from both neighboring units and anthropogenic land within a unit. We also investigated the biological and ecological plant attributes that are likely related to invasion success, such as seed dispersal methods, seed size, and clonality, and tested the relationships between the model results and each species’ attributes. Results showed that the occurrence of an affected neighboring unit was positively associated with the occurrence of all 10 alien plants. Note that two alien invasive species were influenced by upstream flow direction, six species by downstream flow direction, and in two species, propagule supply was not distinguished by direction. In short, the dominant direction of dispersal was against the stream current, while dispersal in the downstream direction was less common. Species attributes were associated with these directions of dispersal. In addition, anthropogenic land was positively associated with the occurrence of most alien plants, although this effect was weaker than the neighbor unit effects. These results indicate that alien plants spreading into riparian areas do not always follow the natural flow regime; rather, they spread against the flow regime in some cases. We discuss an ecological explanation for these results and provide perspectives for future river management of alien plants that invade the riparian zone.     

Seed deposition in drift lines: Opportunity or hazard for species establishment?

Arrival at suitable sites for germination and establishment is crucial for hydrochorous plants. Seed deposition in riparian landscapes occurs mainly within litter in drift lines. We investigated whether conditions within drift lines are suitable for seed survival and germination and whether drift lines act as seed sources or sinks for species diversity in riparian fen grasslands.We analysed drift litter in a small river valley in Northern Germany (Upper Eider valley) after typical late winter floods in 2000, 2002 and 2004. The content of viable seeds in drift lines was studied via a germination trial with drift litter samples taken from five fen grassland sites representative of the vegetation of floodplain. Further, seedling recruitment of both abandoned and moderately grazed fen grasslands affected by drift line deposition was compared with equivalent sites without drift lines. Additionally, the decline of viable seeds after the deposition of drift litter was studied. Finally, the relative importance of seed-containing drift litter and microsite availability for seedling recruitment was investigated in a field experiment with the factors disturbance (mowing, large gaps, small gaps, undisturbed control) and addition of (seed-containing) drift litter (yes, no) in an abandoned riparian fen.Seedlings which recruited from drift litter collected at different sites differed in species composition. A nearly 50% reduction of the number of viable seeds in drift litter was observed already during 2 weeks after deposition. Naturally deposited drift lines had a positive effect on the number of seedlings, although an increase of seedling recruitment in areas with drift litter was only observed at the grazed site. Adding drift litter reduced the number of seedlings in the field experiment, while creating gaps enhanced seedling recruitment.These results indicate that in riparian fen grasslands, drift lines can affect species diversity either negatively by hampering germination and/or establishment or positively by increasing seed availability. As the studied river system is comparatively species poor and flooding extension is limited, the role of drift lines as seed source might be of minor importance in the study area.     

Plant recolonization of reclamation areas from patches of salvaged forest floor material

Question

Understorey development is a great challenge in the restoration of many forest sites, particularly when sources of vegetation propagules are scarce. Can placement of propagule‐rich soil patches within reclaimed landscapes otherwise covered with propagule‐poor material promote the dispersal of vegetation from the patches into the surrounding areas?

Location

Large reclamation site in the Canadian (Alberta) boreal forest.

Method

Patches of propagule‐rich forest floor material were placed within a matrix of propagule‐poor peat material. Vegetation assessments (cover estimates, seed rain) were done surrounding these patches in the third and fourth growing seasons.

Results

There was significant egress of species from the patches into the peat after four growing seasons, and overall species associated with the patches had higher cover in the peat than species that were associated with the peat itself. While wind‐dispersed herbaceous species from the patches were found at the leading edge of the egressing community, most species used vegetative propagation, resulting in short egress distances. Several patch‐associated species were found in seed rain collected on the peat areas but were not observed in this material, suggesting seedbed limitations.

Conclusion

Despite the relatively short distance of egress, this experiment suggests that placement of propagule‐rich soil material within reclaimed landscapes will promote egress into adjacent propagule‐poor soil material.     

河道生态护坡关键技术及其生态功能

在上海4个生态河道示范区,开展了以生态修复和河岸侵蚀控制为目标的河道生态护坡技术研究。在不同河段分别采用了土壤生物工程护坡、全系列生态护坡以及复合式生物稳定等三类生态护坡技术。护坡工程完成后,以浦东新区机场镇生态河道示范区为重点,对主要护坡工程进行持续3a（2004~2006年）的生态监测,从坡岸的结构稳定性和生态稳定性两方面对护坡工程进行生态功能研究。3a来,护坡植物生长良好,新生枝叶和根系的护坡作用明显,土壤抗剪强度明显增加,河岸土壤侵蚀得到有效控制,坡岸的结构稳定性增强;同时河岸生境得到改善,本地植物快速恢复,生物多样性增加,河岸植物群落结构由单一结构向复杂结构转变,生态稳定性逐渐增强。     

Urban riparian systems function as corridors for both native and invasive plant species

Riparian areas are often the only green areas left in urban and suburban landscapes, providing opportunities for conservation and connectivity of both aquatic and terrestrial organisms. While city planners and land managers often tout the importance of riparian networks for these uses, it is not well established if urban riparian plant communities are actually functioning as connected assemblages. Furthermore, urban riparian zones are well known to be highly invaded by non-native plant species and may be functioning to increase the spread of non-native species across the landscape. Here we examine connectivity of plant assemblages in riparian networks within an extensively urbanized landscape. We sampled riparian plant communities at 13 sites along three second-order streams of the Rahway River watershed, New Jersey. We also characterized propagule dispersal at each site by sampling litter packs on the river banks five times between March–October 2011 and identifying germinants from litter packs after cold stratification. Species turnover of both riparian and litter vegetation was more strongly associated with flow distance, particularly for native species, indicating that riverine systems are important for promoting connectivity of native plant assemblages in urban landscapes. However, non-native germinants significantly dominated propagule dispersal along the stream reaches, particularly early in the growing season, suggesting spread utilizing the river system and preemption may be an important mechanism for invasion success in this system. Our data show that management of invasive species should be planned and implemented at the watershed scale to reduce spread via the river system.     

The role of riparian trees in maintaining riverbank stability: A review of Australian experience and practice

Riverside vegetation is a significant factor influencing the occurrence and progress of streambed and riverbank erosion. Recent riparian management practice in Australia has focussed on re-establishing or maintaining native riparian vegetation in order to control or prevent erosion as well as regenerate or preserve the complex variety of in-stream and riverside habitats. This work presents an integrated review of field and experimental studies conducted in eastern Australia that evaluate native vegetation's role in mass failure of riverbanks. Several results of these studies have general application and include the following: (1) The presence of riparian forest on riverbanks significantly reduces the likelihood of erosion by mass failure due to reinforcement of riverbank soils by tree roots and this reduced likelihood of mass failure enables a narrower channel cross-section than would otherwise be the case for many Australian coastal streams. (2) A number of Australian tree species have apparently evolved roots that seek the permanent, summer water table in order to survive prolonged dry spells, these root systems are particularly effective in mass failure mitigation due to rooting depths that are commonly greater than 5 m and are sometimes well in excess of 20 m. (3) The so-called “Root-Area-Ratio method” of calculating the shear strength of root-reinforced soil using root tensile strength data and Waldron's [Waldron, L.J., 1977. The shear resistance of root-permeated homogenous and stratified soil. J. Soil Sci. Soc. Am. 41, 843–849] and Wu et al.’s [Wu, T.H., McKinnell, W.P., Swanston, D.N., 1979. Strength of tree roots and landslides on Prince of Wales Island, Alaska. Can. Geotech. J. 16, 19–33] simple root model leads to significant overestimation of the actual root-reinforcement due to (a) breakage or pull-out of roots that taper and narrow beneath the shear plane such that individual roots do not achieve the tensile strength calculated on the basis of root diameter at the shear plane; and/or (b) the fact that the soil mass fails progressively along the length of potential shear plane rather instantaneously across the entire shear plane.     

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.     

Off-channel temporary pools contribute to native riparian plant species diversity in a regulated river floodplain

Off-channel temporary pools on riverbanks have characteristic seasonal wet–dry cycles resulting from direct inflow of river water or hyporheic flow seepage. This study addressed two questions regarding the role of temporary pools in supporting diversity of riparian vegetation in a regulated river in Japan: (1) do temporary pools maintain high native riparian plant species diversity? And, (2) how do physical environmental factors affect the pattern of plant species distribution? The study was conducted on a 1-km section of the Hayade River alluvial fan in Niigata Prefecture, central Japan. Two to five transect belts consisting of 20 contiguous plots (1 m × 1 m) were laid out in two off-channel temporary pools and six other sites classified by physiognomy. Vascular plant presence, relative elevation, and substratum type were recorded in all plots. Species richness, Simpson’s reciprocal index (1/D), and Shannon–Wiener function (H′) were calculated by life form to estimate plant species diversity. Two conclusions could be drawn from the results. First, the temporary pools maintained high native riparian plant species diversity in this regulated river floodplain. Second, several environmental factors (seasonal wet–dry cycle, low elevation, complex micro-topography, and fine substrata) created spatial and seasonal heterogeneity of moisture conditions in the temporary pools, supporting plant species diversity. For sustainable maintenance of riparian plant species diversity, progressive river-management should restore original riverine dynamics to generate a shifting mosaic of diverse geomorphology.     

A study of seed dispersal by flood flow in an artificially restored floodplain

Riverine floodplains play many important roles in river ecosystems. However, many floodplains have suffered degradation or loss of ecological function due to excessive river improvements or through changes in agricultural systems. As a result, many floodplain restoration projects are being conducted worldwide. One of the many methods being implemented to restore floodplain vegetation is flood water seed dispersal. In this technique, precisely estimating the effect of seed dispersal by flood water is important in order to achieve successful floodplain revegetation. Here, we focus our attention on sediment transport by flood water into the Azamenose Swamp, a restored floodplain. We attempt to estimate the function of seed deposition in the restored floodplain and explain how the seeds are deposited in the floodplain by flood water. The result suggests that the restored floodplain functions as a more appropriate deposition site for seeds than the riverbanks of the main river. It was also found that the distance from the inflow site and the weight of the sediment were related to seed deposition.     

Riparian vegetation’s responses to the new hydrological regimes from the Three Gorges Project: Clues to revegetation in reservoir water-level-fluctuation zone

The new hydrological regime brought about by the Three Gorges Project is the absolute opposite of the natural flood rhythms of the Yangtze River. Flooding timing, duration, frequency, rate of change and magnitude in the Three Gorges Reservoir (TGR), as important factors in affecting riparian vegetation, are completely different from the natural Yangtze Rivers’. The riparian vegetation plays important roles in stabilizing riverbanks, maintaining local biodiversity and improving water quality. Pre-dam and postdam riparian vegetations along the Yangtze River basin in the Three Gorges Reservoir Area (TGRA) were investigated in 2001 and 2009, respectively. The investigation of pre-dam riparian vegetation in 2001 showed that about 405 vascular plant species belonging to 240 genera of 83 families were distributed in the natural water-level-fluctuation zone (WLFZ) of the Yangtze River. However, only 231 vascular plant species belonging to 169 genera of 61 families were found in the WFLZ of the TGR in 2009. The changes in riparian vegetation resulted in the formation of a bald WLFZ with a 30 m magnitude in the TGRA. Although certain perennial species could be repeatedly detected in both field surveys, these are unsuitable for revegetation in the WFLZ of the TGR. This unsuitability is due to their survival via seed germination, and death in subsequent winter flooding. We propose a list of local plants that may be used to recover and re-establish riparian vegetation in the WFLZ of the TGR. We suggest that numerous physiological experiments be performed to assess how the adaptable mechanisms of riparian plants from the TGRA will respond to winter flooding. Endangered wetland shrub species such as Rhamnus tzekweiensis and Buxus ichangensis, which are confined to the riverbanks of the Yangtze River in the TGRA, should be conserved.     

Propagule morphology and river characteristics shape secondary water dispersal in tree species

Plant migration is a multi-stage process often driven by multiple dispersal vector systems. Water-mediated dispersal (hydrochory) is known to move propagules of nonaquatic species over long distances, but whether propagule morphology affects floating processes is an open question. We used a multi-species approach to assess the role of propagule morphology in the dispersal of primarily wind-dispersed tree species in different urban rivers; the impact of hydraulic structures (locks, spillways) on floating was also considered. We released tagged propagules of eight tree species (Acer platanoides, Acer negundo, Acer saccharinum, Ailanthus altissima, Fraxinus excelsior, Robinia pseudoacacia, Tilia platyphyllos, Ulmus glabra) in the main lowland Spree River and in the small tributary Panke River (Berlin, Germany) and directly observed the fate of the floating propagules over river sections of 1,200 m. Our results demonstrate the following: (1) Water is an effective dispersal agent for wind-dispersed tree species, extending typical wind-related transport distances by several times. (2) Interspecific differences in transport distances reflect propagule characteristics (dry weight, maximum wing width) and river system. (3) Propagule morphology also affects deposition patterns as it was generally the large propagules that were trapped along semi-natural embankments in slow flow areas. (4) Hydraulic structures hampered but did not entirely stop water-mediated dispersal and diminished the effects of propagule morphology on floating processes. These results provide novel insights into the functioning of hydrochory as an important dispersal vector of tree species in river systems and as a driver of plant invasions.