Flow regulation affects temporal patterns of riverine plant seed dispersal: potential implications for plant recruitment

1. Changes to the natural flow regime of a river caused by flow regulation may affect waterborne seed dispersal (hydrochory), and this may be an important mechanism by which regulation affects riverine plant communities. We assessed the effect of altered timing of seasonal flow peaks on hydrochory and considered the potential implications for plant recruitment. 2. We sampled hydrochory within five lowland rivers of temperate Australia, three of which are regulated by large dams. These dams are operated to store winter and spring rains and release water in summer and autumn for agriculture. At three sites on each river, hydrochory was sampled monthly for 12 months using passive drift nets. The contents of the drift samples were determined using the seedling‐emergence method. 3. More than 33 000 seedlings from 142 taxa germinated from the samples. In general, more seeds and taxa were observed in the drift at higher flows. By altering the period of peak flows from winter–spring to summer–autumn, flow regulation similarly affected the period of peak seed dispersal. The effect of regulation on seed dispersal varied between taxa depending on their timing of seed release and whether or not they maintain a persistent soil seed bank. 4. Hydrochory in rivers is a product of flow regime and the life history of plants. By altering natural flow regimes and thus hydrochorous dispersal patterns, flow regulation is likely to affect adversely the recruitment of native plant species with dispersal phenologies adapted to natural flow regimes (such as many riparian trees and shrubs) and encourage the spread of non‐native (exotic) species. 5. Changes to hydrochorous dispersal patterns are an important mechanism by which altered flow timing affects riverine plant communities. Natural seasonal flow peaks (in this case spring) are likely to be important for the recruitment of many native riparian woody taxa.     

Impacts of artificial barriers on the connectivity and dispersal of vascular macrophytes in rivers: A critical review

1. Macrophytes play important functional roles in river ecosystems, providing habitat and food, as well as influencing flow, water chemistry, and sediment dynamics. They also represent an important component of river biodiversity.
2. Artificial river barriers have the potential to disrupt macrophyte dispersal, and compromise their distribution and persistence, but little information is available compared to barrier impacts on fish and macroinvertebrates. Here, we review the mechanisms supporting dispersal of river macrophytes in rivers and evaluate the nature of barrier impacts on macrophytes.
3. Hydrochory (dispersal of propagules by water) is the principal mechanism of downstream dispersal, while zoochory (dispersal of propagules by animals) is likely to be the most important vector of upstream dispersal and inter-catchment transport.
4. Most studies have focused on the impact of large structures such as dams, and the findings indicate the impact is highly context dependent. Slow-flowing habitats upstream of dams can act as traps to drifting propagules and thereby interrupt hydrochory. However, the consequences of interrupted hydrochory for downstream populations are unclear. River regulation can result in lower macrophyte diversity, although the lentic habitats associated with reservoirs can also favour an increase in the abundance and richness of macrophyte communities.
5. Instream barriers are unlikely to affect zoochory by birds directly, but barriers are well known to restrict fish movements, so there is considerable potential for barriers to disrupt zoochory by fish, although no empirical study has specifically examined this possibility.
6. There is a paucity of studies examining the impacts of low-head barriers on macrophyte dispersal. Given the influence of macrophytes on river processes, we call for further research into barrier impacts on macrophyte population dynamics in order to gain a better understanding of the consequences of river fragmentation for fluvial communities and ecosystem functioning.

     

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

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

Propagule input,transport and deposition in riparian environments: the importance of connectivity for diversity

Question: How important is hydrochory for dispersing propagules along riverbanks and to what extent do the quantity and species composition of deposited propagules reflect the riparian vegetation, represent “new” species that are not present in the vegetation, and vary with river flow and season? Location: River Frome, Dorset, UK. Methods: Over 13 consecutive 6‐week time periods, during which river water levels were continuously monitored, aerial inputs of propagules to riverbanks were sampled using funnels, hydrochorous propagule transport was sampled using drift nets, and deposition across the riverbanks was sampled using astroturf mats. A survey of the riparian vegetation enabled comparison between samples and the standing vegetation, so that “new” species could be identified. Differences in propagule abundance and diversity between sampling methods, time periods and locations were tested using Mann‐Whitney U‐tests and Kruskall‐Wallis ANOVA. DCA established contrasts in the floristic composition of all deposited propagules and “new” propagules between different sample types, time periods and locations. Results: Aerial seed fall generated few propagules of low species richness. Hydrochory introduced large numbers of propagules and new species, resulting in high propagule deposition on the riverbank. The number and diversity of deposited propagules was governed by seasonal patterns of seed release and the hydrological regime. Propagule deposition was significantly greater on the most frequently inundated parts of the riverbank and autumn floods were particularly important for transporting “new” species to the study site and for remobilizing previously released propagules. Conclusions: The abundance and diversity of propagules deposited along riverbanks is dependent upon high river flows, which facilitates connectivity between the channel and the riparian zone.     

Biological Invasions Across Spatial Scales: Intercontinental, Regional, and Local Dispersal of Cladoceran Zooplankton

The frequency of dispersal of invertebrates among lakes depends upon perspective and spatial scale. Effective passive dispersal requires both the transport of propagules and the establishment of populations large enough to be detected. At a global scale, biogeographic patterns of cladoceran zooplankton species suggest that effective dispersal among continents was originally rare, but greatly increased in the past century with expanded commerce. Genetic analysis allows some reconstruction of past dispersal events. Allozyme and mitochondrial DNA comparisons among New World and Old-World populations of several exotic cladocerans have provided estimates for likely source populations of colonists, their dispersal corridors, and timing of earlier dispersal events. Detecting the Old-World tropical exotic Daphnia lumholtzi early in its invasion of North America has allowed detailed analysis of its spatial spread. Twelve years of collection records indicate a rapid invasion of reservoirs in the United States, by both regional spread and long-distance jumps to new regions. Combining landscape features with zooplankton surveys from south-central US reservoirs revealed higher colonization rates of D. lumholtzi at lower landscape positions, a result which can be explained by either greater propagule load or by higher susceptibility of these downstream reservoirs. Because invaded reservoirs provide a source of propagules for nearby floodplain ponds, the rarity of this species in ponds suggests limitation by local environments. Such analyses of invading species over multiple spatial scales allow a better understanding of ecological processes governing invasion dynamics.     

Genetic evidence for predominantly hydrochoric gene flow in the invasive riparian plant Impatiens glandulifera (Himalayan balsam)

Background and Aims

Riparian systems are prone to invasion by alien plant species. The spread of invasive riparian plants may be facilitated by hydrochory, the transport of seeds by water, but while ecological studies have highlighted the possible role of upstream source populations in the establishment and persistence of stands of invasive riparian plant species, population genetic studies have as yet not fully addressed the potential role of hydrochoric dispersal in such systems.

Methods

A population genetics approach based on a replicated bifurcate sampling design is used to test hypotheses consistent with patterns of unidirectional, linear gene flow expected under hydrochoric dispersal of the invasive riparian plant Impatiens glandulifera in two contrasting river systems.

Key results

A significant increase in levels of genetic diversity downstream was observed, consistent with the accumulation of propagules from upstream source populations, and strong evidence was found for organization of this diversity between different tributaries, reflecting the dendritic organization of the river systems studied.

Conclusions

These findings indicate that hydrochory, rather than anthropogenic dispersal, is primarily responsible for the spread of I. glandulifera in these river systems, and this is relevant to potential approaches to the control of invasive riparian plant species.     

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

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

Can seed dispersal by human activity play a useful role for the conservation of European grasslands?

Objective: To review the recent research into human‐mediated dispersal (HMD) in the European rural landscape, and explore the potential positive aspect of HMD for grassland conservation, in contrast to it's common association with the spread of invasive species. Methods: A literature search was undertaken to identify HMD vectors in the rural landscape for discussion regarding dispersal potential past and present, implications for management, and the identification of future research needs. Results: Grazing animals are important propagule dispersers, but the reduced movement of livestock through the landscape has also meant a reduction in seeds dispersed in this way. Other, non‐standard human‐mediated dispersal vectors such as clothing and motor vehicles can also transport seeds of many species, and HMD vectors often transport seeds with a variety of dispersal specialisations. Recommendations: There should be a greater movement of grazing animals throughout the landscape, either within larger grazing areas or between existing grasslands. Where this is not possible, other, more directed dispersal of propagules from species‐rich communities to target sites should be considered. The potential of non‐standard HMD vectors to make a positive contribution to biodiversity should be considered, but more research into all types of HMD vectors is important if we are to fully understand their role in the dispersal of plant species in fragmented landscapes.     

Connectivity and propagule sources composition drive ditch plant metacommunity structure

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

The dispersal and deposition of hydrochorous plant seeds in drainage ditches

1. Surface water is an important dispersal vector for wetland plant species. However, most previous studies on hydrochory (i.e. water dispersal) have focused on ecosystems with relatively rapid water flow. Therefore, there is a need to study such dispersal in slow‐flowing or stagnant waterbodies, such as drainage ditches, which might act as dispersal corridors between habitat patches. 2. To gain insight into the mechanisms by which seeds are transported in drainage ditches, the effect of the velocity of wind and water on the rate of transport of floating seeds of three wetland species (Carex pseudocyperus L., Iris pseudacorus L. and Sparganium erectum L.) was investigated. Furthermore, in release and retrace experiments with painted C. pseudocyperus seeds, a number of factors potentially determining the probability of seed deposition were investigated. 3. Net wind speed was found to be the main factor determining the rate at which seeds are transported in drainage ditches. No relation between water flow at middepth in the ditches and seed transport was found. Wind speed and flow at the water surface were positively related. The effect of wind speed on the rate of transport of floating seeds was greater for S. erectum seeds, because a greater ratio of their volume protrudes from the water, than for C. pseudocyperus and I. pseudacorus seeds. 4. The principal factors that determine seed deposition were aquatic plant cover, ditch slope and indentations in the ditch bank. Seeds changed direction if the wind direction changed, or if there was a bend in the ditch. The final pattern of deposition was related to mean net wind speed. Mean transport distance after 2 days varied between 34 and 451 m. 5. Unlike in rivers, seed transport in ditches was determined by wind speed and direction, enabling multidirectional seed dispersal. We conclude that in slow‐flowing waters, wind is a more important driver for hydrochorous seed transport than the flow of water. This sheds a new light on hydrochory and has important consequences for the management of otherwise fragmented wetland remnants.     

Water dispersal as an additional pathway to invasions by the primarily wind-dispersed tree <Emphasis Type="Italic">Ailanthus altissima</Emphasis>

Long-distance dispersal is a key process in biological invasions. Previous research has emphasized the role of nonstandard dispersal vectors, but consequences of a change in dispersal vector for the establishment of invasive plant species have received less attention. We analyzed how water-mediated dispersal rather than the more expected wind-mediated dispersal can affect the establishment of the invasive tree Ailanthus altissima in riparian corridors by changing the germination rate and velocity and by providing the option of a new pathway of vegetative propagation. We analyzed the potential of different types of propagules (fruits that have floated or been submerged, current- and second-year stem fragments) to establish new individuals after contact with water for 0, 3, 10, and 20 days. Length and type of seed contact with water led to divergent germination responses. Seeds that had floated for 3 days had an increased level of seed germination (87%), while a 20-day stay in water water-curbed germination to 32% compared to 53% in control. After floatation, the maximum number of emerged seedlings was achieved more than 3 weeks earlier than in all other treatments. In general, the germination was enhanced in floating compared to submerged fruits. Experiments with stem fragments revealed the option of a novel pathway for long-distance dispersal in river corridors: Except for stem fragments that floated for 20 days, 33–75% of buried stem fragments produced adventitious shoots, 10% also set roots. The results suggest that both generative and vegetative propagules of A. altissima can be dispersed at regional scales in river corridors. Hence, water as an additional dispersal vector is expected to enhance invasions by species with wind-dispersed seeds. Our findings suggest the importance of control of initial colonizations in riparian habitats and emphasize the need to include consequences of secondary dispersal when modeling the spread of invasive species.     

Timing is everything: priority effects alter community invasibility after disturbance

Theory suggests that communities should be more open to the establishment of regional species following disturbance because disturbance may make more resources available to dispersers. However, after an initial period of high invasibility, growth of the resident community may lead to the monopolization of local resources and decreased probability of successful colonist establishment. During press disturbances (i.e., directional environmental change), it remains unclear what effect regional dispersal will have on local community structure if the establishment of later arriving species is affected by early arriving species (i.e., if priority effects are important). To determine the relationship between time‐since‐disturbance and invasibility, we conducted a fully factorial field mesocosm experiment that exposed tundra zooplankton communities to two emerging stressors – nutrient and salt addition, and manipulated the arrival timing of regional dispersers. Our results demonstrate that invasibility decreases with increasing time‐since‐disturbance as abundance (nutrient treatments) or species richness (salt treatments) increases in the resident community. Results suggest that the relative timing of dispersal and environmental change will modify the importance of priority effects in determining species composition after a press disturbance.     

Importance of landscape context for post‐restoration recovery of riparian vegetation

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The dispersal capacity of vegetative propagules of riparian fen species

Flowing water can disperse a high number of seeds and vegetative propagules over long distances and is therefore a very important dispersal vector in wetland habitats. Although the dispersal of seeds is relatively well studied, the dispersal of vegetative propagules has received less attention. However, in riparian and aquatic systems where many species have clonal growth forms, it can be very important. The relative importance of vegetative propagules in the dispersal of fen species was assessed first by determining their relative abundance in the field and second, by determining the buoyancy of plant fragments of ten fen species experimentally. On average, vegetative propagules made up 3.2–58.9% of the total propagule number (mainly Elodea nutallii). Buoyancy of the tested species ranged from 25 days to over 6 months. Surprisingly, the propagules of Stratiotes aloides and Hydrocharis morsus-ranae increased buoyancy when spring started (after ca. 100 days). The results demonstrate that vegetative propagules of riparian and aquatic fen species have a high capacity to disperse over long distances via water and are therefore likely to play an important role in the colonisation of new habitats. Especially because in nine out of the ten species tested, over 50% of the propagules were still viable after 6 months of floating.     

Biodiversity and ecosystem functioning: It is time for dispersal experiments

The experimental study of the relationship between biodiversity and ecosystem function has mainly addressed the effect of species and number of functional groups. In theory, this approach has mainly focused on how extinction affects function, whereas dispersal limitation of ecosystem function has been rarely discussed. A handful of seed introduction experiments, as well as numerous observations of the effects of long‐distance dispersal of alien species, indicate that ecosystem function may be strongly determined by dispersal limitation at the local, regional and/or global scales. We suggest that it is time to replace biodiversity manipulation experiments, based on random draw of species, with those addressing realistic scenarios of either extinction or dispersal. Experiments disentangling the dispersal limitation of ecosystem function should have to take into account the probability of arrival. The latter is defined as the probability that a propagule of a particular species will arrive at a particular community. Arrival probability depends on the dispersal ability and the number of propagules of a species, the distance a species needs to travel, and the permeability of the matrix landscape. Current databases, in particular those in northwestern and central Europe now enable robust estimation of arrival probability in plant communities. We suggest a general hypothesis claiming that dispersal limitation according to arrival probability will have ecosystem‐level effects different from those arising due to random arrival. This hypothesis may be rendered more region‐, landscape‐ or ecosystem‐specific by estimating arrival probabilities for different background conditions.     

Multiple-scaled spatial genetic structures of <Emphasis Type="Italic">Fraxinus mandshurica</Emphasis> over a riparian–mountain landscape in Northeast China

Landscape genetics increasingly focuses on the way in which landscape features cause the fragmentation of lineages of terrestrial organisms. However, landscape features can also provide functional connectivity or corridors, enhancing the dispersal of plant populations, particularly the case in riparian habitat. Unfortunately, recent research in tree genetics has paid little attention to this role. To examine the possible effects of landscape connectivity on the current population genetic distribution of Fraxinus mandshurica and to provide insights into conserving the local genetic diversity for this endangered tree species, we used nine nuclear microsatellite loci to examine the spatial genetic structure of F. mandshurica at multiple-scales over a riparian–mountain landscape in Northeast China. F-statistics indicated that the magnitude of among-population genetic differentiation was significantly higher between the riparian and mountain habitats than within the riparian habitat. Spatial analysis of molecular variance and principal coordinate analysis consistently revealed that this species exhibited a clear landscape genetic structure between the riparian and mountain habitats, despite no significant isolation by distance pattern being identified by the Mantel test. Spatial autocorrelation analysis further demonstrated significant, positive fine-scale spatial genetic structure among individuals over short distances (<80 m) in each mountain population. Conversely, no spatial genetic structures were identified within and among the riparian populations. Overall, the results suggest that seed dispersal is very low among mountain populations; however seed transport is probably enhanced by a secondary phase of hydrochory (water-dispersal) among riparian populations during flooding. Despite this, there was no significant accumulation of genetic diversity in downstream populations along the main channel. This result suggests that hydrochory is not sufficient to produce a clear unidirectional gene flow along the water course, although it may impede the development of spatial genetic structuring within and among riparian populations.     

The importance of seasonal flow timing for riparian vegetation dynamics: a systematic review using causal criteria analysis

1. Whilst it is widely recognised that a natural flow regime is important for sustaining riverine ecosystems, the relative importance of the various components of flow regime for riparian vegetation dynamics is poorly understood. We sought to determine the current extent of knowledge on the importance of seasonal flow timing for riparian plants by conducting a systematic review of the literature using causal criteria analysis. 2. Using a definition of ‘riparian’ that included riverine, wetland and floodplain systems, we found sufficient evidence to provide strong support for the existence of causal relations between seasonal flow timing and a number of riparian plant processes, namely rates of waterborne dispersal (hydrochory), germination and growth, as well as riparian community composition. There was insufficient evidence to infer a causal relationship between flow timing and the reproduction or survival of riparian plants. 3. Thus, we argue that seasonal flow timing is important for many of the processes that generate and sustain riparian vegetation communities. River regulation, and/or flow management aimed at restoring ecological values, should consider flow timing and its implication for riparian flora. Because of regulation, many of the rivers of south‐eastern Australia have inverted seasonal flow patterns. Whilst direct evidence of the effects of this inversion on the flora of these rivers is lacking, the results of our causal analysis allow us to predict how these plant communities may have been affected. 4. However, these predictions must be treated with caution because of the reliance of some of the causal analyses on wetland studies. For riverine flora, further research is particularly needed on the effects of seasonal flow timing on hydrochory, survival and reproduction. 5. Causal criteria analysis provides a defensible and efficient means for assessing the extent of evidence for or against ecological hypotheses of this kind. In this case, systematic review of the literature provided strong evidence to support a number of causal links between seasonal flow timing and riparian vegetation dynamics, whilst also efficiently identifying knowledge gaps.     

Interaction between Water and Wind as a Driver of Passive Dispersal in Mangroves

Although knowledge on dispersal patterns is essential for predicting long-term population dynamics, critical information on the modalities of passive dispersal and potential interactions between vectors is often missing. Here, we use mangrove propagules with a wide variety of morphologies to investigate the interaction between water and wind as a driver of passive dispersal. We imposed 16 combinations of wind and hydrodynamic conditions in a flume tank, using propagules of six important mangrove species (and genera), resulting in a set of dispersal morphologies that covers most variation present in mangrove propagules worldwide. Additionally, we discussed the broader implications of the outcome of this flume study on the potential of long distance dispersal for mangrove propagules in nature, applying a conceptual model to a natural mangrove system in Gazi Bay (Kenya). Overall, the effect of wind on dispersal depended on propagule density (g l^-1). The low-density Heritiera littoralis propagules were most affected by wind, while the high-density vertically floating propagules of Ceriops tagal and Bruguiera gymnorrhiza were least affected. Avicennia marina, and horizontally floating Rhizophora mucronata and C. tagal propagules behaved similarly. Morphological propagule traits, such as the dorsal sail of H. littoralis, explained another part of the interspecific differences. Within species, differences in dispersal velocities can be explained by differences in density and for H. littoralis also by variations in the shape of the dorsal sail. Our conceptual model illustrates that different propagule types have a different likelihood of reaching the open ocean depending on prevailing water and wind currents. Results suggest that in open water, propagule traits (density, morphology, and floating orientation) appear to determine the effect of water and wind currents on dispersal dynamics. This has important implications for inter- and intraspecific variation in dispersal patterns and the likelihood of reaching suitable habitat patches within a propagule''s viable period.     

The Role of Propagule Banks from Drainage Ditches Dominated by Free‐Floating or Submerged Plants in Vegetation Restoration

Dominance by free‐floating plants results in a loss of plant species in many waters. An important source for re‐establishment of non‐floating aquatic plants can be the propagule bank. This study focuses on whether the propagule bank of free‐floating plant–dominated ditch sediments can serve as potential source for recovery of a diverse plant community. The first objective was to determine differences in propagule germination from sediments of ditches in the Netherlands that differ in vegetation composition through a seedling‐emergence experiment. The second objective was to analyze the effect of sediment disturbance on the number of germinating propagules. The results show that, compared to sediments from ditches with submerged vegetation, sediments from free‐floating plant–dominated ditches produced significantly lower numbers of individuals and species of submerged and emergent plants, while numbers of individuals and species of free‐floating plants were higher. These results suggest that sediments from free‐floating plant–dominated ditches have lower potential to recover a diverse plant community probably resulting from positive feedback mechanisms caused by the vegetation present, maintaining the free‐floating plant–dominated state. Sediment disturbance strongly favors the germination of free‐floating plant propagules, especially from free‐floating plant–dominated ditch sediments. Ditch maintenance activities such as mowing and dredging will therefore likely favor persistence of the free‐floating plant–dominated state. To shift from dominance by free‐floating plants to a more diverse plant community, alternative maintenance methods should be considered that cause less sediment disturbance together with measures that promote colonization such as temporary drawdown or re‐introduction of species.     

Lichen colonization patterns show minor effects of dispersal distance at landscape scale

The role of dispersal in controlling the distribution of species at landscape scale (10²–10⁴ m) is still a matter of dispute. Here, we use the early colonization pattern of 23 epiphytic lichen species in a former tree‐less heathland landscape (170 km²) to test three hypotheses on how a landscape is colonized: A) mainly by long‐distance dispersal (LDD), B) by rare LDD events followed by limited local dispersal, and C) mainly by limited dispersal, resulting in a colonization front. The study system consisted of a chronosequence of 94 habitat patches constituting 0.4% of the landscape area, with a minimum inter‐site distance of 0.2 km. We used generalized linear mixed models with Bayesian inference to test predictions from the hypotheses. When age of sites and habitat area were accounted for, additional effects of geographical position of sites (distance from old sites, distance‐dependent relative propagule pressure, and distance from border of study area) on the probability of colonization by lichen species were small. Furthermore, species richness of sites did not depend on geographical position, either. Our results support a colonization process mainly governed by LDD at landscape scale, and that local stepwise colonization was not important. We argue that passively dispersed species with numerous small propagules tend to exhibit patchy populations with extensive dispersal at the landscape scale, rather than behaving like classical metapopulations.