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Seed deposition in drift lines: Opportunity or hazard for species establishment?
Abstract
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.
... The frequency, extension, and duration of floods are determinant variables in flood-prone habitats (Guo et al., 1998; Bunn and Arthington, 2002; Scarano et al., 2003; Gomes et al., 2006), which may affect vegetation composition and structure in several systems (Nilsson, 1999; Hö lzel and Otte, 2004a; Vogt et al., 2007; Wittmann et al., 2007). For some plant species flooding may induce seed dormancy resulting in poor seed germination (Mollard et al., 2007). ...
... Therefore, seed transport by river can be important for the establishment and maintenance of wetland vegetation, as long as seeds are deposited at sites safe for the germination, establishment and development of plants (e.g. Andersson and Nilsson, 2002; Merritt and Wohl, 2006; Vogt et al., 2007). Although water regimes may significantly affect seed germination in swamp and riparian vegetation (Insausti et al., 1995; Brock and Rogers, 1998; Hroudová and Zá kravsky´,kravsky´kravsky´, 2003; Warwick and Brock, 2003; Gomes et al., 2006), information about seed tolerance after inundation is rather scarce for many tropical tree species. ...
... The viability after submersion suggests that M. bimucronata may present hydrochorous dispersal, a very important finding given that water may favor adequate seed dispersal for those plants living along riverbanks (e.g. Andersson and Nilsson, 2002; Merritt and Wohl, 2006; Vogt et al., 2007). Our results show that despite the lack of seed dispersion, M. bimucronata seeds may be able to germinate successfully even when plants are located in areas associated with lakes and rivers where seasonal flooding frequently occurs. ...
Mimosa bimucronata is a pioneering tree that occurs predominantly in moist lowlands, floodplains and on margins of rivers and lakes in Latin America. The effect of submergence on seed germination in M. bimucronata was firstly studied. Patterns of water absorption by M. bimucronata seeds were investigated thereafter to assess the imbibition phases of scarified and unscarified seeds. The germination percentage was significantly higher in scarified than in unscarified seeds, and the velocity of seed germination also increased considerably in scarified seeds. Submergence duration did not significantly affect germination percentages of scarified and unscarified seeds. Therefore, seed viability after submersion suggests that M. bimucronata may display hydrochorous dispersal and also that seeds are able to germinate successfully in areas with frequent seasonal flooding. With respect to imbibition phases, phase II was very short or even absent for scarified and unscarified seeds; therefore, a plateau, where water absorption by seeds is established, was not observed. Finally, we verified that the passage from phase I to III was very tenuous and took a long time in seeds without scarification.
... Some propagules happen to be temporarily stranded but become remobilized and continue dispersal before eventual germination takes place. In other similar cases, propagules die but retain their floating ability and can eventually make up a substantial proportion of the propagule drift (Nilsson & Grelsson, 1990; Vogt, Rasran & Jensen, 2007). Stranded, live propagules can germinate where stranded, or be further dispersed to uplands or across the riparian zone. ...
... When stranded, those packs of organic material may preserve moisture and facilitate seed germination and soil formation. These drift piles may provide opportunities for establishment, but may also be areas of high seed mortality due to fungal or bacterial infection and decomposition of organic material including seeds (Xiong & Nilsson, 1997; Vogt et al., 2007). Desiccation may also cause seed mortality in debris piles following water decline, depending on the composition of the debris piles and atmospheric conditions. ...
... In some cases, when the number of seeds is high enough for a sufficient period of time, species may be present in areas otherwise unsuitable for self-sustaining populations, a phenomenon known as sink populations (Pulliam, 1988) or spatial mass effects (Shmida & Ellner, 1984; Shmida & Wilson, 1985). A fourth factor may be that propagule stranding among excessive drift material can inhibit establishment (Vogt et al., 2007). Hydrochory has been shown to affect both the population dynamics and geographic distribution of species. ...
Hydrochory, or the passive dispersal of organisms by water, is an important means of propagule transport, especially for plants. During recent years, knowledge about hydrochory and its ecological consequences has increased considerably and a substantial body of literature has been produced. Here, we review this literature and define the state of the art of the discipline. A substantial proportion of species growing in or near water have propagules (fruits, seeds or vegetative units) able to disperse by water, either floating, submerged in flowing water, or with the help of floating vessels. Hydrochory can enable plants to colonize sites out of reach with other dispersal vectors, but the timing of dispersal and mechanisms of establishment are important for successful establishment. At the population level, hydrochory may increase the effective size and longevity of populations, and control their spatial configuration. Hydrochory is also an important source of species colonizing recruitment-limited riparian and wetland communities, contributing to maintenance of community species richness. Dispersal by water may even influence community composition in different landscape elements, resulting in landscape-level patterns. Genetically, hydrochory may reduce spatial aggregation of genetically related individuals, lead to high gene flow among populations, and increase genetic diversity in populations receiving many propagules. Humans have impacted hydrochory in many ways. For example, dams affect hydrochory by reducing peak flows and hence dispersal capacity, altering the timing of dispersal, and by presenting physical barriers to dispersal, with consequences for riverine plant communities. Hydrochory has been inferred to be an important vector for the spread of many invasive species, but there is also the potential for enhancing ecosystem restoration by improving or restoring water dispersal pathways. Climate change may alter the role of hydrochory by modifying the hydrology of water-bodies as well as conditions for propagule release and plant colonization.
... All phases of hydrochorous dispersal function as strong ecological filters: The release from the maternal plant, the primary transport to the water body, the following drift within or on surface of the water body, as well as the deposition, germination, and establishment. In some phases, for example, deposition in drift lines after the drift, seed mortality can be extremely high (Vogt et al., 2007;Xiong & Nilsson, 1997). Previous studies (e.g., Vogt et al., 2007;Fraaije et al., 2015;Sarneel & Soons 2012;Sarneel et al., 2014) focussed on the establishment of plant diaspores after the transport and on their probability to use safe sites for germination after flooding. ...
... In some phases, for example, deposition in drift lines after the drift, seed mortality can be extremely high (Vogt et al., 2007;Xiong & Nilsson, 1997). Previous studies (e.g., Vogt et al., 2007;Fraaije et al., 2015;Sarneel & Soons 2012;Sarneel et al., 2014) focussed on the establishment of plant diaspores after the transport and on their probability to use safe sites for germination after flooding. In the present study we analyse the drift itself and its function as a filter. ...
We studied seed transport in the Upper Eider River (Northern Germany). Our main questions were: Diaspores of what species are transported in the river and which of them remain viable after the drift? Could functional species traits be used as predictors for the survival of plant propagules in course of water transport?
The water body of the Upper Eider River was sampled for plant diaspores at two bridges with each four traps changed weekly during the whole year. Samples were separated into two equal groups. Each two samples per bridge and week were dried and seeds were counted manually. The other two samples were spread on sterilized soil for germination. Species composition and community weighted trait means were compared for dried samples (total transport) and seedlings (germinated after drift). About half of the species and 1/10 of the seeds were able to survive hydrochorous transport. Species traits (community weighted means) were not reliable predictors for survival of species during the hydrochorous transport, but the majority of traits reflect the differences between the transport pools of seeds and seedlings. Small seed size, ruderal life strategy and high light preference correlate positively with germinability after the drift, while large-seeded species adapted to endozoochory tend to lose viability during hydrochory. Dispersal of terrestrial plants with running water in the studied small river system is a highly stochastic event. We didn’t find evidences that specific adaptations to hydrochory significantly contribute to its success. Nevertheless, a few functional traits can increase the probability for the species to pass through the ecological filter “hydrochory”. Among those traits are (i) small seed size (less vulnerability for mechanical stress), (ii) generalist dispersal mode, less dependent on other particular vectors, and (iii) ruderal life strategy.
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... These methods all result in bare soil, where the establishment of riparian plant communities is supposed to start from seeds (Ter Heerdt and Drost, 1994 1992; Boedeltje et al., 2003a). Dispersal is often not considered a major constraint for establishment of riparian plant species, as they generally disperse well by water, wind, birds or mammals (Boedeltje et al., 2003b;Soons, 2006;Vogt et al., 2007;Van Leeuwen et al., 2012;Stroh et al., 2012) and their seeds remain viable in the soil seed bank for several years (Ter Heerdt and Drost, 1994;Boedeltje et al., 2003a;Jensen, 2004). ...
... This applies to the re-establishment of riparian vegetation on the large areas of bare soil during the drawdown of lakes, on river floodplains after winter flooding, or on new build gently sloped banks (Coops and Hosper, 2002;Nijhof, 2002;Boedeltje et al., 2003a). Our results also apply to established riparian stands, where species richness depends on the creation and recolonization of open soil in gaps (Lenssen et al., 1999;Vogt et al., 2007;Kotowski et al., 2010). Our results are in line with older (Weiher and Keddy, 1995) and more recent studies (Engels et al., 2011), where species-specific responses during early establishment explain variation in community composition. ...
Restoration of riparian plant communities on bare soil requires germination of seeds and establishment of seedlings. However, species that are present in the soil seed bank do not always establish in the vegetation. Temperature, moisture conditions and soil type could play a major role in the establishment of riparian plant communities, through impacting seedling emergence. We studied the effects of temperature, combinations of temperature and moisture conditions, and soil type on seedling emergence and mortality of perennial reeds (Typha latifolia and Phragmites australis) and annual or biannual pioneer species (Senecio congestus, Rumex maritimus and Chenopodium rubrum). The responses to the environmental conditions were species-specific and resulted in context-dependent differences in proportions of species emerging from the soil seed bank. Typha latifolia and S. congestus preferred wet or very wet conditions, C. rubrum and R. maritimus preferred dry to very dry conditions. Phragmites australis was able to establish under all conditions. Both cold and very dry conditions resulted in low emergence and survival, which was not fully compensated for when conditions became favorable again. Senecio congestus, R. maritimus and C. rubrum benefitted from secondary seedling emergence when, after a very dry period, the weather became very wet again, while T. latifolia and P. australis remained absent. When the conditions remained wet, more seedlings emerged from sand than from clay. However, when the soil was drying out, fewer seedlings emerged from sand than from clay. We propose that using information on plant species-specific responses to abiotic environmental conditions during germination, emergence and establishment can help to restore different target riparian plant communities.
... Even though the chances for plant propagules deposited in drift lines to evolve into viable populations are low in most cases (Vogt et al. 2007), the occurrence of species in drift-line material and sediments can be used as an important indicator for the connectivity of the landscape as well as for the dispersal ability of species in the region (McAtee 1925, Stopp 1956, Nilsson & Grelsson 1990, Vogt et al. 2006. The study of drift lines (such as seed bank or seed rain sampling) can be seen as an additional method to monitor biodiversity, especially according to the importance of interactions between plant species and their surrounding as well as the correlation with plant strategies (e.g. ...
... Studies, in which drift-line material was laid out for germination (e.g. Boedeltje et al. 2003, Neff & Baldwin 2005, Vogt et al. 2007, showed a much lower species diversity and propagule density than comparable studies, which directly considered propagules in drift-line material or seed traps (e.g. Ryvarden 1971, Middleton 2000, Boedeltje et al. 2004, Vogt et al. 2004, Tabacchi et al. 2005. ...
... In contrast to other studies that focus either on diaspore deposition at river margins or floodplains (e.g. Merritt & Wohl 2006;Vogt et al. 2007) or on the quantification of the flow of propagules through a river (e.g. Boedeltje et al. 2003Boedeltje et al. , 2004, our experimental approach and the spatial configuration of the floodplain allowed us to distinguish between seed inflow and outflow and to evaluate separately the species traits and abiotic factors that influence each process. ...
... In particular, caution should be observed when drawing conclusions from the calculated annual seed numbers; these should be seen as rough estimates rather than precise figures. However, our sampling design was dense compared to other studies; we sampled about 10% of the river width, frequently, for 1 year, while many other studies only considered samples taken over a much shorter period of time, less frequently (Goodson et al. 2003;Moggridge et al. 2009;Moggridge & Gurnell 2010) or even only after one flooding event (Cellot et al. 1998;Andersson et al. 2000;Vogt et al. 2006Vogt et al. , 2007. ...
Question: How do species traits and abiotic factors influence the extent of hydrochorous dispersal into and out of a small floodplain area along a free-flowing river in The Netherlands?
Location: The Kappersbult nature reserve (53°07′28″N, 6°37′14″E), which is a floodplain along the Dutch River Drentsche Aa.
Methods: Seeds transported by the river were collected in fine mesh nets for 24 consecutive hours once or twice a week for 1 year, upstream and downstream of the studied floodplain. Data on the captured seeds were related to species traits and abiotic factors and species composition in the floodplain.
Results: The floodplain functioned both as a seed source and sink. High levels of river water seemed to promote seed transport to or from the floodplain. Seeds of riverbank species occurred significantly more often in the river water than expected. Net source species had significantly higher seed production, taller stature and higher seed buoyancy, but lower site elevation than net sink species. Seed weight was significantly higher for sink species than for other species.
Conclusion: Our study found that inundation, and therefore more natural river water management, is a prerequisite for seed transport to and from a floodplain. The restoration of target floodplain vegetation may be successful for common species that produce many seeds and grow in proximity to the river. Consequently, it is expected that the probability of restoring vegetation types that occur further from the river, such as wet grasslands, by hydrochorous dispersal is low.
... However, species' composition of the dispersal pool is dependent on numerous additional factors, including the composition of the surrounding vegetation; in addition, specific adaptations of involved plants are expected to be meaningful [20]. The entire dispersal process can be seen as an ecological filter, with significant values of seed mortality within single steps, such as drift in the running water, deposition in drift lines, and seedling establishment [21,22]. In these steps, species that are occasionally flushed into the river are especially filtered out, while the successful dispersal, the ability to pass through the ecological filter, is often dependent on functional species traits. ...
Hydrological restoration was carried out in a Lower Traisen, a small river within the floodplain of the Danube. The main goal was the reestablishment of typical riparian plant communities by using the potential of natural dispersal processes. We studied the transport of plant diaspores in the river water before and after the reconstruction of the riverbed. Aquatic seed traps were placed upstream and downstream of the restoration site. We identified the transported species and tested the viability of propagules. Functional species traits were analyzed to predict the probability of successful hydrochorous dispersal and changes in the transport pool due to the restoration. One-third of the local species pool was detected as being diaspores in the river. We observed a significant increase of ruderal species and neophytes, while the competitors and stress-tolerant competitors declined. Hydrochory is an important dispersal pathway for numerous plant species in the study area, including those without specific adaptations to this vector. Hydrochorous transport appears to be a sink for large-seeded species, primarily adapted to endozoochory. Follow-up management should be recommended to control the invasive species and to improve the structural and biological diversity of the Traisen Valley by supporting target species, which are also represented in the transport pool.
... The importance of deposited FM as a habitat is also determined by the degree of decay, moisture retention, physical orientation in relation to flow (for large wood), and the composition, density, and size distribution of FM components (Harmon et al. 1986;Loeser et al. 2006;Harris et al. 2014;Heerhartz et al. 2016;Brien et al. 2017). The formation of biofilms and detritus food webs on FM also attract fungal decomposers and larger animals such as small mammals and birds (Xiong and Nilsson 1997;Vadeboncoeur et al. 2006), which have feedback effects on the structure and composition of the FM deposits (Xiong and Nilsson (2000) 1997; Vogt et al. 2007). Decay further increases the complexity of FM surfaces, which again leads to an increase in the abundance and biomass of associated macroinverterbate assemblages (Loeser et al. 2006;Schneider and Winemiller 2008;Czarnecka et al. 2014;Harris et al. 2014). ...
Floating matter (FM) is a pivotal, albeit neglected, element along river corridors contributing to their ecological integrity. FM consists of particulate matter of natural (e.g. wood, branches, leaves, seeds) and anthropogenic (e.g. plastic, human waste) origin as well as of organisms that, due to its properties, is able to float on the water surface. In this paper, we provide a comprehensive overview of the FM cycle and the fundamental environmental functions FM provides along rivers. Indeed, FM serves as an important geomorphological agent, a dispersal vector for animals and plant propagules, a habitat, a resource, and a biogeochemical component. Furthermore, we collected data on the amount of FM accumulating at dams and in reservoirs, and related it to key characteristics of the respective catchments. River fragmentation truncates the natural dynamics of FM through its extraction at damming structures, alteration in the flow regime, and low morphological complexity, which may decrease FM retention. Finally, we identify key knowledge gaps in relation to the role FM plays in supporting river integrity, and briefly discuss FM management strategies.
... Floating seeds may be entrapped in riparian vegetation [67], or get stranded by retreating water levels [28], which frequently occurs along the average water line. Particularly species with long-floating seeds are known to get dominantly deposited around this fluctuating water line [60,68], or at higher elevations in drift lines [30,69]. Unfortunately, the quality of the buoyancy data does not allow us to detect patterns in the deposition of long-floating seeds; the data represent floating percentages after only one week in water, which are not sufficiently representative for long floating times (months, years). ...
Riparian ecosystems along streams naturally harbour a high plant diversity with many increasingly endangered species. In our current heavily modified and fragmented catchments, many of these species are sensitive to dispersal limitation. Better understanding of riparian plant dispersal pathways is required to predict species (re-)colonization potential and improve success rates of stream and riparian zone conservation and restoration. Dispersal by water (hydrochory) is an important mechanism for longitudinal and lateral dispersal of riparian species. Crucially for recruitment potential, it also influences the elevation along the riparian hydrological gradient where seeds become deposited. Due to the complex interplay between abiotic and biotic factors, however, it remains unclear how exactly patterns in seed deposition are formed. We compared hydrochorous and non-hydrochorous seed deposition, and quantified patterns of seed deposition along the bare substrate of newly created stream riparian gradients. Water levels were monitored and seed deposition was measured with seed traps along the full range of riparian hydrological conditions (from permanently flooded to never flooded). Average seed numbers and species richness were significantly higher in flooded than in non-flooded seed traps (5.7 and 1.5 times higher, respectively). Community-weighted trait means indicated that typically water-dispersed seeds were more dominant in flooded than in non-flooded seed traps and gradually decreased in concentration from the channel to the upland. Moreover, highly buoyant seeds accumulated at the average water line, and clear elevational sorting of non-buoyant seeds occurred within the floodplain. These results establish a critical role of flooding in shaping patterns of seed deposition along the riparian gradient, delivering many seeds of typical riparian species to riparian zones and depositing them at species-specific elevations as influenced by seed traits, suggesting species-specific dispersal pathways. This shows that hydrochory likely has important consequences for riparian vegetation development and that flooding forms a key process for successful restoration.
... Hydrochory, as a primary dispersal mode, was noted by Neff and Baldwin (2005) for an Anacostia River tidal freshwater marsh, and it was also important in other wetlands (e.g. Middleton 2000;Vogt et al. 2007;Gurnell et al. 2008). Propagules, especially during late autumn and early winter 1995-97, were distributed in seed-containing wrack that was deposited away from the channel edges, but annual floods were also a potential source. ...
Background and aims
A created tidal freshwater wetland, with no extant seed bank, was monitored to evaluate the colonization potential of a tidal river, vegetation and seed bank changes, and also species persistence.
Methodology
The 32.3-ha mitigation, adjacent to the Delaware River, was completed in 1994. Three sites, with differing completion dates, and three locations (channel edge, midpoint, upland edge) were sampled. From 1995 to 1999, surveys permitted a view of initial dynamics involving soil seed bank density or field vegetation cover, as well as species composition. Vegetation was monitored through 2011, and in 2011 a seed bank survey was also undertaken.
Principal results
Between 1995 and 1999, 177 species emerged from soil samples, with a maximum of 32.3 ± 1.8 (SE)/sample. Eight species were added in 2011, indicating continuing dispersal and input. Comparing species richness across three sites for midpoint locations, 2011 totals and species/sample were lower (61–90 % and 53–72 %, respectively) than 1998 values. In vegetation plots during the first 5 years, 72 cover species were present. In 2011, there were 44; 14 were new and the majority (13) were woody and part of an overhanging edge canopy. Dispersal was primarily via water. Overall, the trend was towards lower seed bank densities, except for Mikania scandens and Phragmites australis which had higher densities, as well as cover values. Species richness also decreased. Species varied in persistence in the seed bank and vegetation, and among sites and with location. Initial site colonization differences were still apparent in 2011 seed bank samples. While present, Lythrum salicaria did not reduce species richness; Phragmites, in contrast, had substantial impact.
Conclusions
The single most important change was an increase in Phragmites in the vegetation. Vegetation dynamics are now determined by this species, and the persistent seed bank and continuing dispersal input have little obvious future in vegetation development.
... Moreover, two observations suggest that factors additional to dispersal and deposition co-determined the shoreline seed bank composition: the number of seeds in the seed banks did not reflect the prevailing wind direction, and the mean buoyancy of the seeds in the seed bank was much lower than the mean buoyancy of seeds trapped in the mats. These differences between deposited seeds and seeds in the seed bank are likely to be the result of processes not investigated here, such as local bioturbation (Willems & Huijsmans 1994), predation (Wurm 1998;Fraser & Madson 2008), seed decay (Vogt et al. 2007), differential germination (Lenssen et al. 1998;Sarneel & Soons 2012) and removal of germinated seedlings by wave action (Sarneel & Soons 2012). Therefore, we may conclude that on time scales from days to seasons, the ecological effect of winddriven dispersal of floating seeds on seed transport and deposition in ponds and other lentic water bodies is large; however, over longer periods (more than several years), the effect of wind-driven dispersal is reduced by additional processes that play at least equally important roles (Sarneel & Soons 2012). ...
QuestionWhat is the role of wind in the dispersal of waterborne seeds in slow-flowing and stagnant water bodies at different temporal and spatial scales? (i) Is there a direct effect of wind on seed dispersal speed and distance? (ii) Are prevailing wind conditions reflected in the seed deposition patterns during a year? (iii) What are the long-term (multiple year) effects of prevailing wind conditions on the pattern and composition of shoreline seed banks? LocationThe Westbroekse Zodden (52˚10N; 5˚07E) and De Weerribben (52°46N; 5°55E) fen reserves in The Netherlands. Methods
Real-time seed movement tracking experiments were conducted at different wind speeds. Additionally, we performed a seed trap experiment using artificial grass mats and carried out seed bank analyses using a seedling emergence test. ResultsWind speed and direction strongly determined the dispersal process and the resulting deposition patterns of floating seeds in shallow lakes or ponds. Wind speed directly influenced dispersal speed and distance. Increasing wind speed increased dispersal speed but decreased dispersal distance. Over multiple seasons, more seeds were deposited at downwind shorelines than at upwind shorelines, showing that wind-driven hydrochory resulted in directional transport according to the prevailing wind direction. The species composition of deposited seeds was also affected, with proportionally more water-dispersed seeds being deposited at down-wind shorelines. These effects of wind speed and directionality will have consequences for the colonization of riparian zones in lentic systems and, therefore, also influence management and restoration. In the long term, local seed banks in riparian zones reflected the prevailing wind conditions poorly, showing that additional processes, such as differential germination and predation, also play important roles at longer time scales. Conclusions
Wind plays an important role in the dispersal of waterborne seeds in lentic systems and (prevailing) wind speed and direction are reflected in seed dispersal trajectories and deposition patterns.
... Therefore, both seed and establishment limitations may restrict new colonization events for colonizers. Both seed and establishment limitations have been reported in other wetland ecosystems, although it is generally hypothesized that establishment is the most crucial phase (Lenssen et al. 1998;Andersson et al. 2000;Clark et al. 2007;Vogt et al. 2007). However, in addition to recruitment from seeds, colonizer species might also depend on recruitment from vegetative propagules. ...
In Dutch fens, species that colonize open water and induce the formation of floating peat mats have become rare. Many such riparian pioneer species occur predominantly on shorelines sheltered from the wind, whereas the majority of seeds tend to be deposited on exposed shorelines, as seeds are dispersed via wind-driven waves and currents. Do differences in germination and seedling survival between sheltered and exposed shorelines explain this difference?
The fen reserve ‘De Westbroekse zodden’ (52 °10′ N; 5 °07′ E)
With a sowing experiment, the germination, seedling survival and overall recruitment of Berula erecta, Calla palustris, Comarum palustre, Glyceria maxima and Mentha aquatica were studied on sheltered and wind-exposed banks in eight fen ponds. Temperature, light availability, water level, wave impact, litter and seed deposition and vegetation height were recorded over 16 wk. The probability of washing away was quantified with small seed mimics. With a greenhouse experiment, we separately examined the effects of environmental differences between sheltered and exposed banks.
In the field, compared to sheltered shorelines, exposed shorelines had a higher wave impact, higher light availability in spring and more litter and seeds deposited on them. In the greenhouse experiment, only litter addition decreased germination. This effect was overridden in the field, where the higher light availability on exposed banks increased germination. In the field, the number of seedlings decreased strongly over time, and eventual recruitment was determined by the degree to which seeds and seedlings were washed away by wave action. The probability of being washed away was highest on exposed shorelines (where waves were larger), which resulted in higher recruitment on sheltered shorelines.
The recruitment of colonizing species to fen pond shorelines is limited by the probability that seeds and seedlings may be washed into the open water. This process can eventually cause more successful recruitment on upwind or lee-side shorelines despite lower seed inputs there.
... Jansson et al. (2005) counted 389 6 99.6 seedlings/L emerging from drift samples from margins of the Vindel River in 1997, with a concentration of 12.1 6 0.9 species per sample. Such a high concentration of seeds increases the likelihood of some seeds becoming established due to the mass effect alone (sensu Shmida and Wilson 1985), but such concentrations of seedlings (;1 seedling/cm 2 translated to our experimental plots) have been shown to run the risk of high mortality due to intra-and interspecific competition and mortality caused by fungal growth in drift (Vogt et al. 2007). Indeed, 80% of these seeds were from a single species, C. acuta, suggesting that interspecific competition would be a factor. ...
The spatial distribution and temporal availability of propagules fundamentally constrain plant community development. This study experimentally tested several hypotheses about the relative roles of wind and water dispersal in colonization and development of riparian communities along rivers. Through controlling the source of propagules (dispersed by wind, water, or both) reaching newly created, bare river margin sites, we isolated the relative roles of dispersal and other factors in plant community development over five years.
Replicated treatments were established at 12 sites spanning 400 km along two adjacent rivers in northern Sweden, one fragmented by a series of dams, the other free-flowing. Bare river margins receiving only water-dispersed propagules had significantly higher species richness compared to plots receiving only wind-dispersed propagules during the initial two years of colonization. Species richness increased annually throughout the study along tranquil and turbulent reaches of the free-flowing river but reached an asymptote at comparatively low richness after a single year on the impounded river. Propagule source strongly influenced
species richness during the initial establishment along both rivers, with richness being significantly higher in plots receiving water-dispersed seeds. This strong treatment effect continued to be important through time along the regulated river but diminished in importance along the free-flowing river where other factors such as soil moisture, light availability, and exposure of sites to fluvial disturbance overshadowed the influence of
dispersal pathway in mediating species richness. This suggests that hydrochory (plant dispersal by water) may be more important for maintenance of diversity in regulated systems where long-distance dispersal is absent or negligible, but that the rich local propagule source along free-flowing rivers supports high species richness. The number of unique species was higher in water-dispersal plots along both the regulated and free-flowing rivers. This result suggests that hydrochory may contribute to temporal variability of sites, may enhance richness over time, and may have an important role in meta-population and meta-community
dynamics of plant communities through long-distance (and local) dispersal and chance colonization. Our findings provide experimental evidence that water dispersal of plant propagules influences colonization dynamics and is important for long-term community development in riparian zones.
... (3) Flooding in boreal, free-flowing rivers results in erosion, transport and deposition of litter and sediments (Xiong et al., 2001). Accumulation of litter and sediments may influence vegetation through burial and changes in light availability (Tilman, 1993), changes in temperature and moisture (Green & Kauffman, 1995), transport and deposition of seeds (Nilsson & Grelsson, 1990;Vogt, Rasran & Jensen, 2007) and nutrient supply (Olde Venterink et al., 2006). Flood disturbance is generally thought of as the primary driver of riparian plant diversity and distribution (cf. ...
1. Differing responses in riparian species richness and composition to disturbance have been reported as a possible explanation for the differences along and between rivers. This paper explores the role of physical disturbance in shaping landscape-scale patterns of species distribution in riparian vegetation along a free-flowing river in northern Sweden.
2. To test whether sensitivity to disturbance varies across large landscapes, we experimentally disturbed riparian vegetation along an entire, free-flowing river by scouring the soil and the vegetation turf, cutting vegetation, applying waterborne plant litter, and after a period of recovery we measured vegetation responses. The experiment was repeated for two consecutive years.
3. We found no significant effect of disturbance on species composition, but all three forms of disturbance significantly reduced species richness. There was no downstream variation in community responses to disturbance but morphological groups of species responded differently to different kinds of disturbance. Graminoids were most resistant, suppressed only by litter burial. All forms of disturbance except cutting reduced the density of herbaceous species, and species density of trees + shrubs and dwarf shrubs was negatively affected by both scouring and cutting. We also evaluated the effects of disturbance in relation to varying levels of species richness. In nearly all cases, responses were significantly negatively correlated with control plot species richness, and relative responses indicated that species-rich plots were less resistant to scouring and cutting.
4. Our results suggest that although all disturbance treatments had an effect on species richness, variation in sensitivity to disturbance is not the most important factor shaping landscape-scale patterns of riparian plant species richness along rivers.
... In order to differentiate between responses of glycophyte and halophyte species, driftline material was added to the mesocosms containing seeds of both tidal freshwater and salt marsh species. Drift litter has been shown to contain high amounts of plant propagules and catch the majority of species dispersing via water in a wetland (Neff & Baldwin 2005;Vogt, Rasran & Jensen 2007). ...
1. Transplant experiments with adult plants showed that marsh plant zonation along estuarine salinity gradients develops according to species tolerances of abiotic factors and biotic interactions. Thus, glycophytes are restricted from salt marshes by abiotic stress, whereas halophytes are excluded from freshwater marshes by competition. We hypothesized that adult plant zonations along estuarine stress gradients are predefined by processes affecting plants during their seedling phase. In an experimental glasshouse study, we tested how a mixed community of glycophytes and halophytes developed during the first months of early establishment affected by different flooding and salinity regimes.
2. We set up two factorial mesocosm experiments that tested the effects of three tidal regimes (daily tide, spring tide, no tide) and three salinity levels (fresh, brackish, salt) on the emergence and early establishment of glycophyte and halophyte seedlings. In the Emergence Experiment seedlings were identified and removed once per week. In the Establishment Experiment species cover was estimated every 2 weeks, and after 15 weeks final above‐ground biomass of each species was determined.
3. Both glycophytes and halophytes showed highest seedling emergence under freshwater conditions. Emergence of halophytes was less impaired by increased salinities than that of glycophytes. Emergence and establishment of glycophytes were extremely reduced under brackish and salt conditions, whereas under freshwater conditions, glycophytes showed a consistent increase in cover over time. Cover of halophytes showed a similar increase in brackish and salt treatments; however, in freshwater treatments it significantly decreased after week 9. While daily tides predominantly decreased seedling emergence and establishment, spring tides had no significant effects on seedlings in freshwater and negative effects in saltwater treatments.
4. Synthesis . The ability of a species to germinate and emerge under conditions representing a particular position along an environmental gradient is necessary but not solely responsible for determining plant zonation patterns along estuarine marsh gradients. Our results show that competitive exclusion of halophytes by glycophytes as an important driver of marsh plant zonations along estuarine salinity gradients takes effect already during the early seedling establishment phase of the co‐occurring plants.
In this article, we track the evolution of fluvial biogeomorphology from the middle of the 20th century to the present. We consider the emergence of fluvial biogeomorphology as an interdisciplinary research area that integrates knowledge drawn primarily from fluvial geomorphology and plant ecology, but with inputs from hydrology and landscape ecology. We start by assembling evidence for the emergence of the field of fluvial biogeomorphology with a keyword search of the Web of Science and a detailed analysis of papers published in two scientific journals: a geomorphology journal—Earth Surface Processes and Landforms; a multidisciplinary river science journal—River Research and Applications. Based on this evidence, we identify three distinct time periods in the development of fluvial biogeomorphology: the ‘early years’ before 1990; the transitional decade of the 1990s; and the period of rapid expansion and diversification in themes, methods and investigation scales since 2000. Because the literature is vast, we can only summarize developments in each of these time periods, but we refer to recent in‐depth reviews and conceptual perspectives on relevant topics. Thus, rather than a full and deep review, we present an annotated bibliographic overview of the development of fluvial biogeomorphology, whereby the text describes broad trends but is supported by tables of citations that can deliver greater detail. We end with a brief consideration of likely future developments.
Most propagule bank research has been conducted in rural areas and has emphasised agricultural soils, although research on
rural riparian propagule banks has expanded rapidly over the last decade. This paper presents observations of the species
abundance of plant propagules in urban riparian soils along the River Brent, Greater London, UK. The data are analysed to
explore changes in species abundance of the propagule bank with depth (0–5 cm compared with 5–10 cm soil layer), with distance
from the river (0–1 m, 1–2 m, 2–3 m), and at different locations on the river network. No significant differences were found
in propagule abundance with soil depth or distance from the river’s edge, but significantly more species were found in the
propagule bank close to the river’s edge. The species composition of the Brent’s propagule bank is then compared with riparian
propagule banks from the rural rivers Dove, Frome and Tern, UK. Species richness of the Brent propagule bank samples was generally
lower than was found in the three rural rivers and, in total, 21%, 4%, 5% and 4% of observed species were aliens in the Brent,
Dove, Frome and Tern soil samples, respectively. When the species present in the Brent’s propagule bank were compared with
published data on urban garden soils in another UK city, the latter were found to support a higher number of alien species,
suggesting that other propagule sources to urban river systems support lower numbers of alien species in riparian zones than
domestic gardens.
The dispersal of seeds through hydrochory can be an important driver of community dynamics and play an integral role in the
colonization of restored wetlands. We assessed sources of seeds on the shoreline and in adjacent waters of the Potomac River
to Dyke Marsh in Virginia. Drift-line samples were taken at 40 random points four times during 2005–2006, and water surface
net trawling took place on the Potomac River from 2003–2005 using six, 200-m transects around the perimeter of the marsh.
Seed supply through hydrochory and species richness was low at Dyke Marsh when compared to other regional tidal freshwater
marshes. We discovered distinct temporal patterns, where high species richness and seed density were found in the fall for
the water trawl samples but in the spring for the drift-line samples. High fall dispersal in the water trawls may exist owing
to peak plant senescence and seed release, while high seed germination in the spring along shorelines may arise due to sufficient
cold stratification of marsh seeds.
Fen landscapes in The Netherlands are characterised by high biodiversity. Especially the ponds that have been created by peat excavation in former centuries inhabit numerous species. Under relatively nutrient-poor conditions, those fen ponds were rapidly colonised and often, species-rich floating mats developed. However, since the peat excavation stopped, fens have lost species typical for the colonisation of open water and the succession towards floating mats. Besides, most fens became fragments in an intensively used landscape with barriers for dispersal and an impoverished habitat quality. New ponds have been created to restore opportunities for succession towards floating mats. Unfortunately, the characteristic species have often remained absent and the formation of floating mats has hardly ever been observed. In order to increase the effectiveness of restoration measures, this thesis aims to determine which mechanisms and conditions are important drivers for the colonisation of open water by plant species growing from the bank. Colonisation starts with the arrival and germination of seeds, but for fens, vegetative propagules appeared to be just as important as seeds. Wind speed and direction turned out to be the major determinants of the dispersal of propagules via water. Even over longer periods of time, wind resulted in a directional transport following the main wind direction. This directionality is likely to have consequences for the colonisation of banks with different orientations to the prevailing wind direction, and will determine the effectiveness of connections (e.g. ditches) between water bodies. The wind-induced water currents that drive the dispersal of seeds also cause differences in recruitment conditions between more exposed (downwind) and more sheltered (upwind) banks. This affected germination and establishment there and recruitment was highest at sheltered banks. Most likely, wave action determined this difference. This could imply that the common practice to minimise natural water table fluctuations could impede the establishment of colonisers. With natural water level fluctuations, water tables are higher in the fall, when the majority of seeds disperse, and drop in spring, so that the seeds can germinate and become established without the risk of being washed away. The vegetation development in restored ponds will also depend on the habitat characteristics. A large controlled mesocosm experiment showed that nutrient availability (of N and P) in both the surface water and the bank soil can accelerate the growth of colonisers and their expansion into the water. A large field survey on 62 ponds showed that a high nutrient availability, high sulphate concentrations in the surface water, high turbidity, deep water, a high muskrat density and the absence of the keystone species Stratiotes all impede the development of bank vegetation in Dutch fen ponds. However, the effects of degraded habitat characteristics seemed to overrule the effects of dispersal limitations, as colonisation often failed even in highly connected ponds or when sufficient propagule sources were present. Overall, successful restoration of species-rich plant communities that colonise the water and induce the formation of floating peat mats requires a multifaceted approach to address the various negative effects of land use changes.
Rivers are important corridors for movement, migration, and dispersal of aquatic organisms as well as for dispersal of the seeds and vegetative propagules of riparian plants. In this investigation, the relationships between flow regime, channel morphology, dispersal phenology, and seed deposition patterns were evaluated using experimentation in a flume. A channel with geomorphic features common to a wide range of stream morphologies was constructed in a 1.8 X 20 m experimental flume through which three hydrologic regimes (one natural and two typical of dam releases) were routed in replicated trials. Relationships between dispersal phenology and hydrologic regime were examined using color-coded Betula fontinalis seeds released over each 10-min trial. Spatial patterns of seed deposition along stream margins were then compared to determine the individual and combined effects of flow regime, fluvial feature, and timing of seed release. Reynolds number, Froude number. Weber number, flow velocity, and a dimensionless recirculation index were then used to develop predictive models of patterns of seed dispersal. Fluvial feature. flow regime. and timing of seed release all described significant proportions of the variability in concentrations of seeds deposited. Fewer seeds were deposited during the ascending flow releases than under descending and stepped regimes, indicating that ascending flows during seed release may have a significant negative effect on hydrochoric seed deposition. Ascending flows entrain and transport rather than deposit seeds. Retention and recirculation of seeds in fluvial environments with low recirculation indices extend the period over which plants with buoyant seeds are able to disperse, possibly to the advantage of such species. Systematic delivery of seeds to specific types of fluvial environments in repeated trials and under varied regimes suggests that hydrochory may serve as a form of directed dispersal for some riparian plant species. Froude number explained a large proportion of the variation in seed deposition, followed by flow velocity, Reynolds number, and Weber number. The recirculation index, which quantifies the strength of recirculation in the fluvial environment, accounted for the greatest proportion of the variability in seed dispersal patterns. Poisson regression models of seed deposition as a function of dimensionless hydraulic parameters provide an empirical basis for incorporating spatial patterns of seed dispersal into riparian vegetation models, In addition, through incorporating dispersal phenology and information regarding the regeneration niche of species of special concern, specific hydrograph characteristics may be managed below dams to accommodate or inhibit species using hydrochory.
Extreme Überflutungsereignisse können in unserer heutigen fragmentierten Landschaft von großer Bedeutung sein. Im Sommer 2002 haben wir nach Rückgang der “Jahrhundert-Flut” der Elbe hydrochore Nah- und Fernausbreitung pflanzlicher Diasporen untersucht. Zusätzlich wurde der Einfluss unterschiedlicher artspezifischer Charakteristika (Schwimmfähigkeit der Diasporen, Ausbreitungs-Typ, Ellenberg Feuchte-Werte) auf die Ausbreitung analysiert und die Relevanz hydrochorer Ausbreitung während extremer Überflutungen für den Naturschutz diskutiert. Entlang eines Transekts von insgesamt ca. 400 Flusskilometern der Mittleren Elbe (Ost-Deutschland) haben wir von jeweils drei Habitaten (Acker, Auen-Grünland, Flussufer der Elbe) an fünf Lokalitäten Spülsaummaterial entnommen. Wir untersuchten dessen Diasporengehalt und verglichen die Artenzusammensetzung des Spülsaums mit derjenigen charakteristischer Vegetationstypen derselben Region. Ein Teil der Spülsaumproben wurde visuell auf Diasporen durchsucht, während der übrige Teil zur Keimung ausgebracht wurde. Insgesamt liefen ca. 1500 Keimlinge von mehr als 70 Arten aus den Proben auf. Am häufigsten kamen Deschampsia cespitosa, Urtica dioica, Ranunculus repens und Lycopus europaeus vor. Das manuelle Sortieren des Spülsaummaterials ergab, dass nur ca. 1.5% der in den Proben enthaltenen Diasporen in den Keimungsuntersuchungen aufliefen. Obwohl die meisten Diasporen und Keimlinge aus den Spülsaumproben zu weit verbreiteten Arten gehörten, wurden einige Diasporen seltener und gefährdeter Arten gefunden. Die Artenzusammensetzung der Spülsaumproben unterschied sich nur geringfügig zwischen den untersuchten Habitaten und Lokalitäten, was als Hinweis sowohl auf Nah- als auch Fernausbreitungsprozesse gewertet wird. Im Gegensatz dazu unterschied sich die Vegetation der untersuchten Habitate signifikant. Die analysierten artspezifischen Charakteristika unterschieden sich nicht signifikant zwischen Spülsaummaterial verschiedener Habitate wohl aber zwischen den betrachteten Vegetationstypen. Wir schließen aus den Ergebnissen, dass während extremer Überflutungen Diasporen von Arten unterschiedlicher Habitate und Lokalitäten zusammen transportiert werden und dass derartige Überflutungen den normalen Ausbreitungsradius von Pflanzenarten ausweiten können.
umans have long been fasci- nated by the dynamism of free-flowing waters. Yet we have expended great effort to tame i rivers for transportation, water sup- ply, flood control, agriculture, and ; f
The Eider valley pasture landscape - nature conservation by extensive grazing and marshland rewetting Abstract: The nature conservation project "Eider valley pasture landscape" combines aims of marshland restoration with aims of species conservation. The rewetting of the fen soils aims at increasing the potential of river marshes for accumulating nutri- ents in the peat, whereas the implementation of large-scale extensive grazing systems should promote habitat diversity and habi- tat conditions for typical species of open and semi-open ecosystems. The governmental nature conservation agency has drafted contracts for the landowners combining ecological and economical aspects and permitting the inclusion of areas of different land- owners in large grazing systems. - The Eider valley is characterized by an mosaic of different types of river marsh, most of which have been drained. In the transition area from the marshes to tha adjacent valley sides numerous spring-fed marshes of varying nutrient status are to be found. More intensive land use and the drainage of former wetlands on the one hand, and the wide- spread termination of land use on the other hand have led to the predominance of mostly species-poor used and fallow wet graz- ing land. Succession on fallow grasslands is analysed by means of vegetation mapping and the monitoring of permanent plots. Vegetation development on fallow grasslands is characterized by the increasing abundance of highly competitive species, whereas weak competitors of low productivity suffer from succession and their species number decreases in the course of vegetation development. - Large-scale moderate cattle grazing systems were introduced to stop the decline. The grazing behaviour of the cattle and the influence the herbivores had on the vegetation were analysed in grazed and ungrazed plots in the summer of 1999. Both the hydrological conditions of the stands and the successional status of the vegetation were identified to be the key fac- tors controlling the grazing behaviour of the cattle. Some ecological and economical aspects of large-scale grazing systems are presented and the applicability of the concept of this nature conservation project to other geographical areas is discussed.
The hypotheses that litter deposition on riverbanks would depend on riverbank features and that litter mass would affect riverbank vegetation were tested by sampling vegetation, litter, and soil along a seventh-order stretch of the unregulated Vindel River in northern Sweden. Sampling was carried out within a horizontal litter gradient at the top of the riverbank where most of the litter deposition occurs. The amount of leaf litter left on the riverbank after flooding was significantly correlated (P < 0.05) with soil organic matter. In contrast, wood litter mass and total litter mass were not correlated (P > 0.05) with riverbank features such as soil organic matter, riverbank slope, or stem density. Species richness, species diversity, and the proportions of stemmed and nonstemmed species showed quadratic relationships (P < 0.05) with leaf litter mass, whereas ground vegetation cover decreased monotonically (P < 0.05) with increasing leaf litter mass. Tall, stemmed species were most frequent at eroded sites with low amounts of leaf litter and at sites where leaf litter had accumulated. In contrast, the highest proportion of low-growing species was found at sites with intermediate leaf litter masses. Community characteristics such as species richness, species diversity, and the proportions of stemmed and nonstemmed species (but not ground vegetation cover) were less strongly correlated with wood litter mass and total litter mass. Linear equations explained about half of the variation in ground vegetation cover along gradients of wood and total litter masses. Density and richness of seeds increased with increasing litter mass. We discuss the litter effects in terms of productivity and disturbance and conclude that flooding in pristine rivers may exert a considerable influence on riverbank vegetation as a result of litter displacement.
The ecology of species of intermitlently flooded nitrophilous annual communities (class Bidentetea) is investigated intensively at the banks of the river Elbe since 1993, some aspects even since 15 years. The investigations show clearly, that area, development and divcrsity vary year by year. The main reasons are different weathers which cause time and dimensions of the high waters. The period of investigation however is still to short to decide whether it is a fluctuation or a straightened development, especially because the yearly variations are overlapped by the irregulär spreading of alien species. The analyses of the drift lines and similar Sediments show that high waters are of great importance for the spreading of Bidentetea species at the river Elbe. After floods it was even possible to find germinable diaspores in the Sediments deposited in Elm-Oak woods. The invasive plants fit till now well in the existing plant communities, a displacement of native species was not obeserved.
To understand the influence of dams on connectivity of riparian plant communities along rivers, we examined plant dispersal by water (hydrochory) and riparian plant community attributes upstream and downstream from dams on two rivers in the southern Rocky Mountains, Colorado, USA. Drifting plant propagules were collected from the water column along reaches upstream and downstream from dams to examine the longitudinal and temporal variation in seed‐pool species composition and concentration of water‐transported seeds. Similarities between species composition of the hydrochoric seed pool and local standing riparian vegetation were used to evaluate the degree of longitudinal connectivity along river corridors and to isolate the relative contributions of local versus regional species pools to hydrochoric species composition. Furthermore, several synthetic attributes (longevity, origin, life‐form and dispersal mode) and species composition of riparian plant communities were examined to explore the effects of interrupted propagule dispersal on standing vegetation.
We estimated that as many as 120 million seeds were transported via hydrochory along free‐flowing reaches of the Rocky Mountain streams in a single growing season. Seed concentration (seeds/m ³ ) in the water column was reduced by 70–94% along reaches downstream from dams compared to free‐flowing reaches. The similarity in species composition of hydrochoric seeds and local standing vegetation was nearly two times greater downstream from reservoirs compared to upstream. This suggests that hydrochory complements local species pools by importing seeds from throughout the upstream catchment area along free‐flowing river reaches, but that hydrochoric seeds are derived primarily from local sources along regulated river reaches. Species richness recovers as a function of downstream distance from contributions of standing vegetation and seeds from tributary streams. Hydrochory may extend the period over which viable seeds of a parent population are dispersed. Even after dispersal of parent populations has terminated, seeds may continue to be available due to residence time in water transport. This extension of the ‘effective dispersal window’ of some species may exceed two weeks or more and may influence the likelihood of successful establishment.
In this study, synthetic attributes of riparian vegetation did not differ significantly between free‐flowing and regulated reaches, whereas formal statistical comparisons of community composition upstream and downstream from reservoirs indicate that there are differences in community composition upstream and downstream from dams. These findings suggest that the consequences of 50 to 100 years of fragmentation result in community‐wide effects along Rocky Mountain streams and that these effects may be partially explained by dam‐caused disruption in connectivity of plant populations. Copyright © 2005 John Wiley & Sons, Ltd.
Summary 1 The diversity and abundance of viable diaspores trapped at the downstream end of a 15-km lowland stream were quantified and related to five potentially predicting vari- ables: species' occurrence in the species pool, distance to the nearest stand and the life-history traits seed buoyancy, seed production and plant height. 2 From 126 samples, 106 614 individuals of vascular plants developed, 95.8% from vegetative diaspores and 4.2% from seeds. Among these plants, three free-floating, 12 submerged, 22 emergent (aquatic) and 70 riparian (semi-aquatic and terrestrial) species were recorded, respectively, accounting for 24.3%, 71.9%, 1.2% and 2.6% of the total number of viable diaspores trapped. 3 Of the free-floating, submerged and emergent species, 100%, 98.9% and 23.7% of the diaspores were vegetative, respectively, whereas it was 2.9% for riparian species. 4 Diaspores of 79% of the total number of aquatic species and 40% of riparian species observed in the established vegetation were trapped. Minimal dispersal distances ranged from 0 to 6 km. 5 Multiple regression analysis conducted for submerged species, showed that 71% of the variation in the diaspore pool could be predicted by occurrence of species in the vegetation. For emergent species, seed production and occurrence of species explained 54% of the variation, seed production being most important. Mean seed buoyancy of emergent species was higher than that of the other groups. For riparian species, seed production, occurrence and buoyancy explained 48% of the variation in the diaspore pool. Seed production per plant was the most important variable. Linear regression revealed a negative relationship between distance and abundance of the diaspore pool for submerged and riparian species. 6 We conclude that the occurrence of species in the species pool is a significant pre- dictor for the dispersal of free-floating and submerged aquatics that rely on vegetative propagation. Seed production and buoyancy are of additional importance with regard to emergent aquatics. Riparian species with a limited terrestrial dispersal capacity may largely extend their range by hydrochory. That is, if they produce large amounts of (small) seeds and provided that these can reach the water body. Buoyancy and high frequency and abundance in the established vegetation promote this dispersal capacity as well.
Summary • We carried out a factorial experiment to examine how groundwater availability (low and high sites with intermediate or rare flooding), vegetation canopy, leaf litter and seed availability interacted to determine the species richness of a productive wet grassland community in Wicken Fen National Nature Reserve, Cambridgeshire, UK. Seeds of 18 species were added to half the plots in each of eight combinations of elevation, canopy and litter, and seedling emergence was observed for two growing seasons. • Both individual and interactive effects on plant diversity and colonization were determined for all four examined factors. Interactive effects explained 41–63% of the total variation in both species richness and numbers of individuals growing from added seeds. • Neither elevation nor vegetation canopy had significant individual effects on total species richness, but their interaction was significant. Litter addition limited seedling emergence at the low elevation but favoured it at the high elevation. • The relative importance of vegetation canopy and plant litter in affecting plant community composition varied with the community parameter considered (species richness or number of seedlings), elevation and stage of vegetation development. In general, plant litter was more important in determining species richness, whereas the vegetation canopy was more important in determining seed germination and seedling emergence. Plant litter was also more important than vegetation canopy at an early stage of vegetation development and at low elevation. • Seed availability was the most important factor in determining overall species richness in the studied community. The influence of the local seed bank was very limited. Seedling emergence and seedling species richness were generally enhanced by lower elevation and seed addition, but depressed by vegetation and litter addition. • The complex relationships observed have considerable implications for ecological modelling and ecosystem restoration. Manipulation of one factor may produce unexpected effects on other factors, which may induce a series of consequences for the whole community. Further knowledge on how natural communities are organized and maintained is needed to guide the management of ecosystems. Journal of Ecology (2003) 91, 976–986
We discuss the dynamics of plant litter, the effects of litter on the chemical and physical environment, the direct and indirect effects of plant litter on plant populations and communities, and different adaptative traits that may be related to litter accumulation. The production of litter depends primarily on the site productivity, but other properties of the environment, as well as chance, may introduce important variation. The existence of time lags between the production of plant organs and their transformation into litter appears as a relevant character of litter dynamics seldom included in models. Herbivory, and other processes that destroy biomass or reduce productivity, may reduce the amount of litter produced. The destruction of litter encompasses a complex of interactions. The main processes, including physical and chemical degradation, consumption by invertebrates and decomposition, are differentially affected by the environment and by the physical and chemical characteristics of the litter itself. The relative importance of those processes varies among systems.
Although seed dispersal is assumed to be a major factor determining plant community development in restored wetlands, little
research exists on density and species richness of seed available through dispersal in these systems. We measured composition
and seed dispersal rates at a restored tidal freshwater marsh in Washington, DC, USA by collecting seed dispersing through
water and wind. Seed dispersal by water was measured using two methods of seed collection: (1) stationary traps composed of
coconut fiber mat along an elevation gradient bracketing the tidal range and (2) a floating surface trawl net attached to
a boat. To estimate wind dispersal rates, we collected seed from stationary traps composed of coconut fiber mat positioned
above marsh vegetation. We also collected a small number of samples of debris deposited along high tide lines (drift lines)
and feces of Canada Goose to explore their seed content. We used the seedling emergence method to determine seed density in
all samples, which involved placing the fiber mats or sample material on top of potting soil in a greenhouse misting room
and enumerating emerging seedlings. Seedlings from a total of 125 plant species emerged, during this study (including 82 in
river trawls, 89 in stationary water traps, 21 in drift lines, 39 in wind traps, and 10 in goose feces). The most abundant
taxa includedBidens frondosa, Boehmeria cylindrica, Cyperus spp.,Eclipta prostrata, andLudwigia palustris. Total seedling density was significantly greater for the stationary water traps (212±30.6 seeds/m2/month) than the equal-sized stationary wind traps (18±6.0 seeds/m2/month). Lower-bound estimates of total species richness based on the non-parametric Chao 2 asymptotic estimators were greater
for seeds in water (106±1.4 for stationary water traps and 104±5.5 for trawl samples) than for wind (54±6.4). Our results
indicate that water is the primary source of seeds dispersing to the site and that a species-rich pool of dispersing propagules
is present, an interesting result given the urbanized nature of the surrounding landscape. However, species composition of
dispersing seeds differed from vegetation of restored and natural tidal freshwater marshes, indicating that planting is necessary
for certain species. At other restoration sites, information on densities of dispersing seeds can support decisions on which
species to plant.
The total production of plant litter and the proportion of leaf litter are higher in riparian corridors than in upland ecosystems throughout the world. Periodical water-level fluctuation is believed to be the major cause of these differences. During flood periods, much plant litter is redistributed locally and between regions, following erosion, transport, and deposition of litter. The importance of litter redistribution varies with factors such as flood regime, topography, and vegetation. Litter from the riparian corridor is usually a major constituent of the litter transported by the river. The decomposition of litter is faster in riparian corridors than in upland systems due to a higher rate of leaching and a higher decomposer activity. Relative warmth and soil fertility may also enhance litter decomposition in riparian corridors. In general, accumulated litter affects plants physically by burying them, chemically by adding nutrients and phytotoxins, and biologically by adding diaspores. The physical impact of a certain amount of litter may be weaker in riparian corridors than in uplands because the rapid decomposition reduces the time that litter is present. In other words, higher amounts of litter are needed to affect riparian vegetation than are needed to affect other types of vegetation. The nutrient content of riverborne litter is reduced by leaching, but dissolved nutrients from litter might still reach the riparian vegetation, e.g., by adsorbing to inorganic particles. Phytotoxins are probably unimportant in riparian systems. The input to the riparian corridor of plant diaspores, borne by litter packs in the river, may be large. Indirect biological effects of litter, including its diaspores, are the attracting of animals and microbes that may influence the plant community, and the creation of bare soil for plant colonization.
Water-borne seed transport and seed deposition during flooding were studied in the Upper Eider river (N-Germany) by direct sampling of the rivers seed content with aquatic seed traps and by analysing the number of deposited seeds on sedimentation mats which were exposed near the river on the soil surface during a flooding period of approx. three weeks.The number of seeds which were transported at the surface of the river Eider was continuously analysed by four aquatic seed traps for a period of 20 weeks (July–December 2000). To test the capture rate of these traps, a recapture experiment with colour marked seeds of Helianthus annuus L. was carried out. During the investigation period approx. 9000 seeds of 76 species were captured by the four aquatic seed traps. The number of trapped seeds varied both spatially (across the river profile) and temporally. Considering this variation and the capture rate of the traps, the water-borne seed transport was estimated to be 3139 seeds per week and meter of the river profile.The seed deposition during a flood in early spring 2002 was analysed by using 20 sedimentation mats. To distinguish effects of seed dispersal into patches from outside from seed rearrangement within patches, the water-borne seed transport was excluded from one half of the mats by fencing them with a woven fabric which was permeable for water but not for floating seeds. Outside of the exclosures 152 viable seeds of 26 species were deposited on the sedimentation mats while only one single seedling was found on mats from which water-borne seed transport was excluded.The results demonstrate that hydrochorous dispersal processes might play an important role in connecting otherwise fragmented populations in periodically flooded habitats along rivers.
The flow regime is regarded by many aquatic ecologists to be the key driver of river and floodplain wetland ecosystems. We have focused this literature review around four key principles to highlight the important mechanisms that link hydrology and aquatic biodiversity and to illustrate the consequent impacts of altered flow regimes: Firstly, flow is a major determinant of physical habitat in streams, which in turn is a major determinant of biotic composition; Secondly, aquatic species have evolved life history strategies primarily in direct response to the natural flow regimes; Thirdly, maintenance of natural patterns of longitudinal and lateral connectivity is essential to the viability of populations of many riverine species; Finally, the invasion and success of exotic and introduced species in rivers is facilitated by the alteration of flow regimes. The impacts of flow change are manifest across broad taxonomic groups including riverine plants, invertebrates, and fish. Despite growing recognition of these relationships, ecologists still struggle to predict and quantify biotic responses to altered flow regimes. One obvious difficulty is the ability to distinguish the direct effects of modified flow regimes from impacts associated with land-use change that often accompanies water resource development. Currently, evidence about how rivers function in relation to flow regime and the flows that aquatic organisms need exists largely as a series of untested hypotheses. To overcome these problems, aquatic science needs to move quickly into a manipulative or experimental phase, preferably with the aims of restoration and measuring ecosystem response.
The Beals smoothing function, also known as the sociological favorability index, is a data transformation for heterogeneous ecological community data. After Beals smoothing, virtually any ordination method can successfully extract the dominant patterns in the data. Presence–absence (1,0) values in the raw data matrix are replaced by continuous probabilities. The transformed value in a given cell represents the probability of a particular species occurring in a particular sample unit, based on the other species that were present in that sample unit. These probabilities are constructed from a matrix of joint occurrences between species. Beals smoothing is appropriate for heterogeneous community data sets with a large number of zeros in the data matrix (i.e. most samples contain a fairly small proportion of the species). The method may also be useful for smoothing out differences due to varying sampling intensity or where sample quality is uneven. Beals smoothing is also desirable when the data set c...
The nature conservation project "Eider valley pasture landscape" combines aims of marshland restoration with aims of species conservation. The rewetting of the fen soils aims at increasing the potential of river marshes for accumulating nutrients in the peat, whereas the implementation of large-scale extensive grazing systems should promote habitat diversity and habitat conditions for typical species of open and semi-open ecosystems. The governmental nature conservation agency has drafted contracts for the landowners combining ecological and economical aspects and permitting the inclusion of areas of different landowners in large grazing systems. - The Eider valley is characterized by an mosaic of different types of river marsh, most of which have been drained. In the transition area from the marshes to the adjacent valley sides numerous spring-fed marshes of varying nutrient status are to be found. More intensive land use and the drainage of former wetlands on the one hand, and the widespread termination of land use on the other hand have led to the predominance of mostly species-poor used and fallow wet grazing land. Succession on fallow grasslands is analysed by means of vegetation mapping and the monitoring of permanent plots. Vegetation development on fallow grasslands is characterized by the increasing abundance of highly competitive species, whereas weak competitors of low productivity suffer from succession and their species number decreases in the course of vegetation development. - Large-scale moderate cattle grazing systems were introduced to stop the decline. The grazing behaviour of the cattle and the influence the herbivores had on the vegetation were analysed in grazed and ungrazed plots in the summer of 1999. Both the hydrological conditions of the stands and the successional status of the vegetation were identified to be the key factors controlling the grazing behaviour of the cattle. Some ecological and economical aspects of large-scale grazing systems are presented and the applicability of the concept of this nature conservation project to other geographical areas is discussed.
1. In-stream dispersal and floating capacity of 18 pioneer plants (nine chamaephytes, eight hemicryptophytes and one shrub) from alluvial floodplains on the upper Isar (Bavaria, Germany) were studied using field and laboratory experiments between 1994 and 1996. 2. Drift collection of seeds with nets at the water surface proved unsuitable because of the great amount of invertebrates drifting along the river, though it allowed an estimation of the amount of drifting seeds. In August and September we found over 9,000 drifting seeds per meter stream width and day or over 120,000 seeds over the whole stream width in 24 h. Sediment-filled baskets proved to be better traps but were often destroyed during flooding periods. Although drift collection was of limited value for sampling seeds in a highly dynamic floodplain, water transport of viable seeds could be demonstrated for nine of the 18 species. 3. In laboratory experiments, in which seeds were placed into a beaker, stirred up to five days and then used for germination tests, seeds of 16 of the 18 species showed a moderate to good floating capacity. Only two species had seeds that could not float longer than a few minutes (Gypsophila repens and Silene vulgaris ssp. glareosa). 4. We hypothesised that dispersability decreases with increasing seed mass, but the assumption that heavy seeds are not dispersed over great distances is valid only for environments without the possibility of in-stream dispersal.
The paper focusses on the dispersal of plants by running water in the floodplain areas of the rivers Weser, Aller and Leine (Northern Germany). To get an impression of the dispersal potential of rivers, various methods were applied to seize plant diaspores at high tides: Trapping with drift nets as well as collections of drift litter and river sediments. Countings of the samples just as germination experiments were used to determine the assemblages of diaspores.
.1 Around Lake Nussbaumen in northern Switzerland, 50 ha of former agricultural area will be restored to semi-natural wet grasslands similar to those before drainage in 1943. This involves re-establishment of the characteristic plant species. In previous experiments, the establishment of target species by seeding or planting was more effective when the topsoil was removed. For practical reasons, removal of the topsoil can hardly be applied on a large scale, the question being whether this measure could not be replaced by one with less environmental implications. 2. The three main factors influenced by topsoil removal are relative water table (distance from soil surface), nutrient conditions and competing plant species from the soil seed bank. If a single factor could be identified as inhibiting - directly or indirectly - the establishment of the target species, it might be possible to manipulate only this one factor and thus avoid topsoil stripping. 3. In a cross-factorial field experiment we tested these three factors, i.e. relative water table, nutrients and competitors, for their effects on the growth of four wetland species (Carex flava, Filipendula ulmaria, Ranunculus flammula, Selinum carvifolia) after planting on peat soil. Both the mean above-ground biomass after one growing season and the coefficient of variation in biomass (as a measure of asymmetric competition) were considered. 4. The mean biomass of the planted species was severely reduced by competitors, but enhanced by nutrient addition. The effect of competitors tended to be stronger when nutrients were added, probably because of additional shading. However, this nutrients x competition interaction was only marginally significant (P= 0.07). A lower relative water table tended to enhance the mean biomass of target species when no nutrients and no competitors had been added (the effect of water table was only examined under these conditions and only marginally significant). 5. The coefficient of variation was higher with a lower relative water table, indicating more asymmetric competition, and thus a higher importance of above- than below-ground competition under drier conditions. No other factor had a significant influence on the coefficient of variation. 6. When the topsoil cannot be removed, depletion of the soil seed bank to reduce competitors seems to be the most effective method for the restoration of fen grasslands, at least in the short term. However, long-term studies with more levels of the factors are needed to substantiate this recommendation for wetland restoration.
Until the late fifties of this century species rich wet meadows were characteristic of the swampy alluvial plains in SchleswigHolstein (north-western Germany). Today many of these meadows undergo successional changes due to abandonment. The vegetation development after abandonment can be characterised as a sequence of different successional stages. After an initial phase(successional stage I) follows a phase of clonal expansion of highly competitive species (successional stage II) and a phase of immigration and establishment (successional stages III and IV). In the course of succession species richness decreases and highly productive vegetation-stands develope. Species contributing to successional changes can either be present in the initial aboveground vegetation, in the soil seed bank or in the seed rain.
The detritus standing crop, microbial respiration, and macroinvertebrate biomass were examined in monthly samples from the riffle sections of a first-order woodland stream. Total detritus was remarkably constant; the average (with 95% CL) ash-free dry mass standing crop was 426.4 + 85.9 g/m^2 over the 14 mo of the study. Throughout the year benthic detritus was dominated by fine particulate detritus (<1 mm), which made up 68.9% of the total ash-free dry mass. Woody debris made up 8%, whole leaves 3.5%, and leaf fragments and other coarse particulate detritus accounted for 19.7% of the total standing crop. Decreases in standing crop were attributable to microbial respiration, macroinvertebrate assimilation, and downstream export. Microbial respiration annually removed 150% of the average standing crop, with the major effect on the smallest particle size category. Macroinvertebrate assimilation, defined as the sum of respiration and growth, removed 11.6% of the detritus standing crop annually. Shredders accounted for 20% of total animal assimilation, with the remaining 80% attributable to collectors and grazers. Based on monthly changes, it appears that total detritus standing crop is the result of the past discharge regime, which determines the overall amount of detritus present, and the rate of biological (microbial and invertebrate) processes, which determine the size and quality of the detritus particles. This suggests that detritus in streams, while strongly affected by both biotic and abiotic factors, may be in equilibrium within physical and biological constraints such that an annual steady-state system exists, similar to that for soil systems.
The importance of hydrochory, or seed dispersal by water, to the regeneration of Taxodium distichum (bald cypress) and Nyssa aquatica (water tupelo) as examined in a forested floodplain of the Savannah River in South Carolina. Seedfall and dispersal by water were quantified for 2 yr using floating seed traps. Water depth, surface velocity, and flow direction were monitored over the same period. Seedfall for bald cypress and water tupelo occurred primarily from early fall throughout the winter, when water levels were rising in the swamp. Extended buoyancy periods for newly released seeds and fruits (bald cypress: 42 @+ 37 d; water tupelo: 85 @+ 36 d) prolonged dispersal for both species. Flowing water transported experimentally released seeds long distances, but in a uniform direction, and concentrated them nonrandomly against logs, trees, knees, and other emergent substrates. An examination of the soil seed banks in five microsite types supported the results of the seed transport study. Lowest woody seed densities occurred in the open areas, and highest seed densities occurred in sediments adjacent to emergent substrates such as logs. Additionally, elevated water levels of 1-2 m caused by short-term, high-discharge floods scoured seeds of Nyssa sylvatica var. biflora, Quercus spp., Liquidambar styraciflua, Pinus taeda, and other species from adjacent bottomland hardwood communities and transported them into the bald cypress-water tupelo forest. We conclude that elevated water levels, which occur during late fall when seeds are released, influence the distribution of bald cypress and water tupelo seeds and their availability for recruitment. Short-term, deeper floods may also be important for seed transport among bottomland hardwood communities that are spatially separated or differ in species composition.
Floods in rivers can deposit large quantities of organic debris in the riparian corridor and cause considerable change to its vegetation. We studied the importance of leaf-litter accumulation on riparian vegetation in different climates by constructing gradients ranging from complete litter removal to addition of up to 6000 g/m2 (35-40 cm deep) of leaf litter and recorded the consequent changes in vegetation. Experiments were located along rivers in northern and southern Sweden, Italy, and Romania. Plant biomass decreased with increasing litter mass but did not vary with site-dependent variables such as climate and soil. Species density for various plant life traits varied with litter mass, summer temperature, and summer precipitation. Litter mass was the most important variable for all life traits except rhizomatous and stoloniferous species, which were most dependent upon summer precipitation. These findings are useful for the prediction of responses in riparian vegetation following future climatic changes.
For the regeneration niche to contribute to the maintenance of species diversity interspecific differences in sensitivity of seedling recruitment to environmental conditions is assumed. We experimentally tested differences between meadow species for the response of seed germination to chilling, and sensitivity of seedling recruitment to microscale heterogeneity. We also compared the dynamics of seedling recruitment in gaps. Seed germination was tested in standard laboratory germination tests, comparing control seeds with seeds chilled at +4 °C, and at –14°C for one month. Species responses varied from significant increases in germinability after chilling (e.g. Cirsium palustre, Betonica officinalis, Angelica sylvestris) to significant decreases (e.g. Hieracium umbellatum, Succisa pratensis, Selinum carvifolia). In some species, chilling at + 4 °C has a similar effect to chilling at –14 °C, in others the effect of chilling at + 4°C was intermediate, and in some, there was no effect of chilling at + 4°C, but an effect of chilling at –14°C. Different chilling temperatures also affect timing and speed of seed germination under greenhouse conditions.
Aspects of seed dispersal, particularly mobility of viable seeds, were investigated in two sites at the lower river Dalälven, southeastern central Sweden, from 1980 to 1985. Three different approaches were used to assess the seed rain. (i) Seed traps were placed at ground level. This component of the seed rain was dominated by seeds from the local vegetation, particularly Betula spp. A total of 30 species were identified, (ii) Drift material was collected in spring and autumn and the seed content was identified through germination trials. Seeds dispersed by drift mainly belonged to species abundant in the study sites, but included a minor element not represented in the local vegetation. A total of 23 species were identified, of which seeds of Potentilla palustris, Carex canescens, and other Carex spp. were most abundant. In the field, seedlings growing on fresh accumulations of drift were dominated by Potentilla palustris. (iii) Samples of snow were collected and the seed content determined visually. Of the seven species represented in the snow samples, Betula spp. dominated. In particular, flooding extended the seed rain beyond the boundaries of many species. It is concluded that the distribution of wet meadow species is not limited by dispersal.
Recent decades have witnessed a rapid growth in man's understanding of environmental changes over the 10,000-15,000 year timescale and in the 10-50 year timescale. Substantial efforts have been made to link space and time but greater effort is needed to consider changes over the historical timescale of 100-500 years - the period of intensifying human impact culminating in the creation of truly manmade rivers. The synthesis of information from a wide range of sources has enabled general statements on the history of the large alluvial rivers of western Europe to be made. Some of these statements have been compiled into a book which examines the sequence of early river engineering works; cartographic data analyses and applications to channel change; a French case study; the use of geochemical analyses and flood plain stratigraphy in deriving pollution histories; paleoecological methods using fossil invertebrates from alluvial sediments; a case study of the Lower Rhine; reconstruction of fish faunas; historical development of fisheries of the Elbe; Douro, and Ebro; and the variability of historical models of river development as applied to the Garonne, Meuse, Weser, and Rhone rivers. This book marks the first step in the development of a European Network of Scientific Cooperation for the improved management of large rivers, ideally encouraging and advancing the historical analysis of rivers not only in western Europe but worldwide. Such studies will contribute to more rational approaches to river management, to further resource development and to better conservation practice. Separate abstracts have been processed for two papers for inclusion in the appropriate data bases.
This study tested whether the frequency of flood disturbances was able to slow down or stabilize vegetation succession in former braided channels over a decade. According to the Patch Dynamics Concept and to succession theory, species richness and diversity should be high but stable in the frequently (40 days/year) flooded channel, and should change over time in the infrequently (1 day/year) flooded one. Within the frequently disturbed channel, composition of vegetation as well as species richness and diversity appeared stable through dynamic equilibrium over the decade. Only one zone, because of particular geomorphological features that decreased disturbance intensity, developed highest diversity and richness as expected from the Intermediate Disturbance Hypothesis. The highest disturbance effect decreased species richness and was related to a higher spatial heterogeneity of the substrate (number of grain-size classes). In the other zones, richness and diversity appeared to be lowest where disturbance frequency was lowest or disturbance intensity was highest. From 1981 to 1987, the infrequently flooded channel underwent succession, and species richness increased in the major part of the channel, whereas diversity increased only in its extreme parts.
We studied the effects of litter of the annual grass Setaria faberii, the perennial herb Solidago spp. (mostly S. canadensis), and leaves of the hardwood tree Quercus alba on a successional plant community. We also assessed light interception by these litter types in the laboratory. Light extinction followed the Beer-Lambert exponential law. Solidago litter had the highest transmittance constant and Quercus litter the lowest. The three types of litter produced different light mosaics at the microsite (0.8 cm diameter) scale. In the field, all three litter types affected community structure, but the effect of Quercus was the strongest. Litter reduced the density of the two dominant grasses, Setaria faberii and Panicum dichotomiflorum. Quercus and Setaria litter resulted in biomass compensation (i.e., fewer but larger individuals) by S. faberii but not by P. dichotomiflorum, which probably was always outcompeted by S. faberii. Solanum carolinense, the main dicot in the community, was unaffected by litter addition. Setaria and Solidago litter enhanced the establishment of Erigeron annuus, but Quercus litter reduced it. Litter reduced the number of flowering individuals of S. faberii and the number of seeds per plot; Quercus litter increased the production of seeds per individual. We conclude that species-dependent effects of litter on plant populations may significantly alter interspecific interactions and change plant community structure through direct and indirect effects.
Previous studies of the influence of hydrochory on plant dispersal have focused on relationships with existing vegetation patterns or litter deposits. River sediment deposition studies have assessed sediment accretion rates, particle size and quality. The link between seed and sediment transport and deposition has been overlooked and never quantified.
This paper presents observations of over‐winter viable seed and sediment deposition on three river margin sites along the River Dove, England. At these sites, 105 paired artificial turf mats were installed on the river bank top, face and toe from October 1999 to March 2000 to collect river‐deposited material. From each pair, one mat was used to determine the dry weight; median particle size; percentage sand, silt, clay and organic content of deposited sediment. The other was used in germination trials to determine the number and species of deposited viable seeds. Topographic surveys and water level measurements supported the estimation of the hydrological characteristics of each mat location.
Few seeds and little sediment were found on mats not inundated by river water. For these inundated, sediment characteristics varied primarily with elevation (relative to local mean river water level during the study period), with less marked between‐site contrasts. The species composition of viable seeds also varied with elevation. Detrended correspondence analysis (DCA) revealed associations between river/wetland‐related seed species and the bank toe samples. Regression analysis revealed significant relationships between seed number, sediment properties and mat elevation. Redundancy analysis (RDA) revealed associations between the species and abundance of viable seeds and elevation, sediment weight and organic matter content. These analyses suggest that sediment and seed deposition by rivers are closely related processes.
The implications of these results for seed remobilization and dispersal and hydrochory within riparian zones are discussed, as are the implications of changes in sediment/seed delivery mechanisms, river flow regimes, and the hydraulic properties of river margins for seeds, sediments and riparian vegetation patterns. Copyright © 2003 John Wiley & Sons, Ltd.
Litter layers have a negative effect on seedling recruitment in many ecosystems. Successional fen grasslands are characterized by the built-up of litter layers which might be a key-factor influencing species composition and diversity during succession. Seed size is thought to be an important feature for the regeneration niche of species. Large seed size has been shown to be of advantage for germination and establishment under shaded conditions and beneath litter layers and thus for late-successional species.We investigated the effects of litter (control, litter layer of 3 cm and 8 cm) on seedling recruitment in 35 grassland species varying in seed size and successional status and on the abiotic conditions temperature, light quantity and light quality. Furthermore, we analyzed the species' light requirement for germination and its relation to seed size, successional status and establishment beneath litter.Light quantity was significantly lower beneath litter (reduction of 78% and 91% beneath 3 cm and 8 cm litter, respectively). The R/FR-ratio decreased from 1.2 without litter to 0.8 and 0.6 in presence of 3 cm and 8 cm litter. Seedling recruitment of 33 species was hampered by a litter layer. Establishment in presence of litter was positively related to seed size (r = 0.62, p < 0.001 and r = 0.46, p < 0.01 for 3 cm and 8 cm litter, respectively). In addition, establishment of early-successional species was hampered by litter to a significantly greater extent than recruitment of late-successional species independent of seed mass (ANCOVA: F = 7.6, p < 0.01). We found a positive relation between seed mass and the proportion of germination in darkness (r = 0.57, p < 0.001) indicating that small-seeded species had a higher light requirement for germination. We also found positive relationships between the proportion of germination in darkness and establishment in the 3 cm and the 8 cm litter treatment (r = 0.4, p < 0.05 and r = 0.3, p < 0.05 for 3 cm and 8 cm litter, respectively).We conclude that species-specific effects on seedling establishment are mainly due to differences in seed mass, successional status, and light requirement for germination and that these factors thus influence species composition and diversity during abandoned fen grassland succession.Nach der Nutzungsaufgabe ist die Sukzession auf Feuchtgrünland-Brachen durch die Bildung mächtiger Streuauflagen gekennzeichnet. Durch Effekte auf Keimung und Etablierung beeinflusst die Streuauflage die Artenzusammensetzung und -vielfalt und wird deshalb als ein Schlüsselfaktor für die Vegetationsentwicklung angesehen.In einem Gewächshaus-Experiment wurde der Einfluss unterschiedlicher Streumächtigkeiten (Kontrolle, 3 cm und 8 cm) auf die abiotischen Faktoren Temperatur, Lichtintensität und Lichtqualität sowie auf die Keimlingsetablierung von 35 Grünlandarten untersucht. Die ausgewählten Arten unterscheiden sich hinsichtlich ihres Samengewichts (von 0,1 bis 4,8 mg) sowie ihres Verhaltens im Sukzessionsverlauf (Arten früher sowie später Sukzessionstadien). Weiterhin wurden die Zusammenhänge zwischen der Lichtbedürftigkeit der Keimung und dem Samengewicht, dem Verhalten im Sukzessionsverlauf und der Keimlingsetablierung unter Streuauflagen analysiert.Unter den Streuvarianten war die Lichtintensität signifikant niedriger als in der Kontrolle (Reduktion um 78% und 91% unter 3 und 8 cm Streuauflage). Das Hellrot/Dunkelrot-Verhältnis sank von 1,2 ohne Streuauflage auf 0,8 und 0,6 unter 3 bzw. 8 cm Streumächtigkeit. Die Keimlingsetablierung von 33 Arten wurde durch die Streu signifikant verringert. Die Etablierung von Arten mit einem hohen Samengewicht wurde weniger durch die Streuauflagen beeinflusst als diejenige kleinsamiger Arten (r = 0,62, p < 0,001 und r = 0,46, p < 0,01 für 3 cm und 8 cm Streumächtigkeit). Zusätzlich wurde (auch bei Berücksichtigung des Samengewichts als Kovariable) die Keimlingsetablierung von Arten früher Sukzessionsstadien stärker verringert als die von Arten später Sukzessionsstadien (ANCOVA: F = 7,6, p < 0,01). Weiterhin wurde ein positiver Zusammenhang zwischen dem Samengewicht und dem Keimungsprozentsatz im Dunkeln gefunden (r = 0,57, p < 0,001), was zeigt, dass kleinsamige Arten eine höhere Lichtbedürftigkeit für die Keimung aufweisen. Auch zwischen dem Anteil der Keimung im Dunkeln sowie der Keimlingsetablierung unter Streuauflagen wurden positive Zusammenhänge nachgewiesen (r = 0,4, p < 0,05 und r = 0,3, p < 0,05 für 3 cm bzw. 8 cm Streumächtigkeit).Die Ergebnisse zeigen, dass die artenspezifischen Unterschiede hinsichtlich der Keimlingsetablierung unter Streuauflagen im Wesentlichen durch die Faktoren Samengewicht, Lichtbedürftigkeit für die Keimung sowie das Sukzessionsverhalten erklärt werden können und dass diese Eigenschaften somit die Artenzusammensetzung und -vielfalt während der Sukzession auf Feuchtgrünland-Brachen beeinflussen.
The effects of different forms of land use on germination and establishment of the rare fen species Succisella inflexa were investigated in seed introduction experiments in a mown and an abandoned fen meadow in SE Germany. Treatments included abandonment, mowing in fall and mowing with creation of gaps in the moss and litter layer. Floating capacity of seeds was tested in order to estimate potential dispersal by water.
On the mown meadow, gaps had a slightly positive effect on germination rates, while greatly increasing seedling survival until the next spring. At the abandoned site, litter inhibited germination, whereas mosses had a negative effect on germination and a positive effect on survival rates during the first year after germination. Both germination and seedling establishment were negatively affected by the presence of slug herbivores. On the abandoned site, no seedlings at all survived until the next spring. Even though seeds of Succisella inflexa were capable to float for several weeks and to germinate thereafter, the situation at the field sites indicates that longdistance dispersal is highly unlikely. Our results showed that not only direct effects of abandonment, such as accumulation of litter, may have led to poor germination and poor seedling establishment of the species. Additionally, indirect consequences of changes in land use, such as higher seedling herbivory by slugs and successional vegetation changes due to abandonment, were important in determining habitat quality and availability of microsites for seedling recruitment. Furthermore, early mowing imposed seed limitation on plant populations.
SUMMARY 1. Rivers are linear ecosystems across landscapes with an effective transport of organisms, sediment and organic matter. Dispersal is studied mostly during single events and for single species, and there is little knowledge on how the drift of plant litter and propagules varies within and between years for entire communities.
2. We used floating traps for collecting waterborne plant litter and propagules in a small boreal river over 2 years. We installed the traps at four different locations along the river, and emptied them at least once a week during the ice-free season. We analysed propagule content by sorting and identifying species and through germination tests on bare soil.
3. In total, we recorded at least 54 taxa in the samples, and the highest density recorded in one sample was 5000 propagules per 100 g litter (dry weight). Large temporal variations in litter and propagule transport were revealed, both within and between years.
4. The longitudinal pattern was consistent between years, with an increasing mass of litter and number of propagule taxa downstream. The results highlight the importance of the temporal and longitudinal dimensions in river management.
Seed inputs were studied from January 2001 to August 2002 along a tranverse successional gradient of a riparian zone within the mid-reaches of the Garonne River (SW France). Seeds were trapped monthly at ten sites distributed along the gradient. In total, 117 000 seeds belonging to 296 species were collected, among which 145 were exotic species. Seed inputs were overall dissimilar to the extant vegetation; however, similarity among seed input samples was higher than similarity among the extant vegetation samples. Exotic seed inputs resulted from distinct modes of dispersal in comparison to the native ones. Exotic guilds also comprised more short-lived species. A significant positive linear relationship was observed between exotic and native species richness for both seed inputs and extant vegetation. Flood events were responsible for species richness peaks in seed inputs. They also contributed towards peaks in exotic species and individuals. However, most exotic seeds were recruited during low water period, with a significant lag in time compared to native inputs. Species composition, densities of species and individuals as well as the percentages of exotic species distinguished the infrequently agricultural zone from the frequently disturbed riparian zone. The latter zone was the most invaded both in terms of seed input and extant vegetation. Copyright © 2005 John Wiley & Sons, Ltd.
Following the environmental sieve concept, the setting in which the recruitment of Taxodium distichum occurs in, becomes increasingly restrictive from the seed to seedling stage in an impounded forested wetland. Although a wide elevational band of dispersing seed moves across the boundary of a swamp-field in the water sheet, the zone of germination is relegated to that portion of the forested wetland that draws down during the growing season. Seedling recruitment is further restricted to the uppermost zone of the winter water sheet. These patterns are likely applicable to other species of dominant swamp species, e.g., Cephalanthus occidentalis crossed the boundary of a forested wetland and abandonded field in winter flooding (November–December and November–March, respectively) in Buttonland Swamp. The elevation of the boundary was 101.3m NGVD. While the seeds of at least 40 swamp species were dispersed across the boundary, few viable seeds were dispersed after the winter season. Kriged maps showed seeds of T.distichum and C.occidentalis dispersed in patches in the water depending on the position of the water sheet. Most species of both water- and gravity-dispersed species had a localized pattern of seed distribution (either spherical or exponential) and this indicated that seeds may not be dispersed for great distances in the swamp. Water-dispersed T.distichum and C.occidentalis had larger dispersal ranges (A
0=225 and 195m, respectively) than Bidens frondosa and B.discoidea (A
0=14 and 16m, respectively). Seed dispersal varied with season depending on the availability of seeds. In Buttonland Swamp, viable seeds typically were dispersed for T.distichum in November–June, and for C.occidentalis in November-July. Low water occurred in August 1993 and high in February 1994 (99.8 and 101.6m NGVD, respectively). The seed banks along the landscape boundary varied in species composition according to elevation (r
2=0.996). While the similarity of species richness between water-dispersed seeds and the seed bank at elevations that flooded (during June 1993 through May 1995) was high (10–17%), it was low between water-dispersed seeds and the seed bank at elevations that did not flood (5%). T.distichum seeds had a short germination window in that seeds germinated within a year following their production in zones that were flooded in the winter followed by drawdown during the next growing season. After 1 year, less than 5% of the T.distichum seeds remained viable on the surface of the soil. Germination of T.distichum was confined to specific elevations (above 99.3 but below 101.6m NGVD) during this study with 4.1% of the seedlings surviving for more than 2 years at a mean of 101.4 m NGVD. All seedlings below this elevation died. To maximize natural regeneration along the boundaries of swamps in abandoned farm fields targeted for restoration, this study suggests a flood pulse regime consisting of high water in the winter to maximize dispersal of live seeds followed by low water in the summer to facilitate seed germination and seedling recruitment. Hydrologic restoration could assist in the natural recovery of damaged wetlands if a seed source exists nearby.
Snails and earthworms affected the dynamics of a simple, three-species plant community, in the Ecotron controlled environment facility. Earthworms enhanced the establishment, growth and cover of the legume Trifolium dubium, both via the soil and interactions with other plant species. Worms increased soil phosphates, increased root nodulation in T. dubium, and enabled T. dubium seedlings to establish in the presence of grass (Poa annua) litter, by increasing soil heterogeneity. Worms also buried the seeds of Poa annua and Senecio vulgaris, reducing the germination of new seedlings. Snails reduced nitrogen-fixing Trifolium dubium and increased cover of plant litter, thereby reducing ammonia-nitrogen concentrations in the soil. These effects and their interactions demonstrate that the detritivore food chain, and earthworms in particular, cannot be ignored if we are to understand the spatial and temporal dynamics of plant communities.
Soil is defined here in terms of arbitrary boundaries rather than of functions of a soil body. Soil animals are defined in relation to their effects on the soil body. Animals living in the soil body and intimately related to it are indeed part of the soil. Animals living above the soil make contributions to it. Many animals are amphihabitant, that is, they live for a time in the soil and then in environments outside the soil. Exopedonic (outside the soil) and endopedonic (inside the soil) animals are considered with respect to twelve activities: mounding, mixing, forming voids, back-filling voids, forming and destroying peds, regulating soil erosion, regulating movement of water and air in soil, regulating plant litter, regulating nutrient cycling, regulating biota, and producing special constituents. Animals participate in numerous processes of soil formation and effect the usefulness of soils.
Restoration projects in previously intensively used wet grassland areas in Europe showed that the re-establishment of target plant species can be strongly limited by propagule dispersal. In a large wet grassland area in Northwest Germany (Borgfelder Wümmewiesen, 677 ha) with relatively intact habitat conditions and extensive inundations it was tested whether the re-establishment of grassland species during restoration succession depends on population density in the established vegetation, the ability to survive in the soil seed bank, or on long-distance dispersal through inundations. For this purpose species recruitment was recorded in 58 permanent plots over a period of 6 years. In a multiple regression, species frequencies in the established vegetation (a measure for population density), frequencies in the drift line (dispersal capacity by winter inundation) and seed persistence in the soil were used as predictors for recruitment rates. Recruitment was mainly controlled by population density in the established vegetation. Long distance dispersal by means of winter floods and a long seed bank persistence contributed less (but still significantly) to the recruitment rates. Propagation by means of stolons and adaptations to hydrochory had a positive effect on recruitment rates and frequencies in the drift line. High recruitment rates of target species in the test area, compared with other grassland areas of a more intensive land use history, emphasize the importance of a big species pool and the spatial interconnection of species-rich (source-) and species-deficient (sink-) habitats. Many target species, however, exhibited low recruitment rates because their ability to disperse in time and space is low. Restoration management for wet grasslands should, therefore, focus on large areas with a short intensification history and remnants of target species populations, which are connected with sink habitats by inundations.
Although soil fungi are likely to be a major cause of mortality for buried seeds, few ecological studies have examined the role these pathogens play in natural systems. In particular, few studies have investigated whether losses of seeds to soil fungi are habitat-dependent. We used fungicide treatments to investigate whether losses of buried seeds of four grasses (Bromus inermis, Danthonia spicata, Glyceria striata, and Poa pratensis) to soil fungi differed among meadows differing in soil moisture. We also applied water to some treatments, to determine whether this increased losses of seeds to fungi. For all four grasses, fungicide additions improved one or more measures of seed viability, though this effect was small. For Danthonia and Glyceria, fungicide was less likely to improve viability in dry meadows than in wet and/or mesic meadows. Adding water reduced some measures of viability of seeds of Danthonia and Poa in dry meadows, but fungicide partly counteracted these negative effects, suggesting that adding water reduced performance by increasing fungal attack. These results indicate that fungi represent a hazard for buried seeds of these species, particularly in wetter soils, and potentially may contribute to the reduction of populations of vulnerable species in wetter sites.
The leaf litter decomposition of Typha latifolia L., Phragmites australis (Cav.) Trin. ex Steud., Acacia longifolia Willd., Populus alba L., and Salix alba L. along the Porsuk River, Eskisehir, Turkey was studied using the fibreglass bag technique. After 180 days, soft-leaved species such as Acacia longifolia decomposed more quickly (0.52% loss · day-1) than the tougher leaved species such as Typha latifolia and Phragmites australis (0.31 and 0.37% loss · day-1, respectively). Typha latifolia had the toughest while Acacia longifolia had the softest leaves among the five species as measured with a penetrometer (428.0 versus 128.2 g). Both leaf toughness and time were related to percent loss per day (F = 4.4, p < 0.01 and F = 37.0, p < 0.01, respectively). World trends in percent loss per day for Typha latifolia do not fit predictions based solely on latitude.