Seed dispersal by rivers in tropical dry forests: An overlooked process in tropical central Mexico

Aims

Rivers are important corridors for the movement, migration and dispersal of aquatic organisms, including seeds from riparian plants. Although tropical dry forests (TDF) are among the most extensive and floristically rich ecosystems of tropical habitats, and the most globally endangered ecosystem, less attention has been given to riparian corridors within this ecosystem. Although most TDFs manifest peak seed dispersal during dry seasons, we hypothesized that riparian corridors may show a dispersal peak during the rainy season, due to an anticipated ‘sweep or drag effect’, resulting from river overflow and bank erosion. Our main aims were to investigate whether there were any differences in the seed communities transported by the river to sites in rainy as opposed to dry seasons, and to evaluate any possible relationship between the riparian seed community and river flow.

Location

Amacuzac River, drainage of the Balsas basin, State of Morelos, Mexico.

Methods

To evaluate the above assumption, we associated Amacuzac River flow with the number of species and seeds dispersed by water. We also characterized and evaluated differences between seed communities transported by the river during the rainy and dry seasons, and between four different sites located along the river. We used univariate and ordination NMDS techniques to evaluate patterns between seasons at the community level.

Results

Forty‐five plant species were identified from 909 seeds collected from the river. The composition of riparian seed communities was markedly different between seasons but not between sites. Seed abundances were significantly higher in the rainy than in the dry season and varied between sites. Seed species diversity in the river (H’ = 1.6–1.9) showed no significant differences between seasons or sites, but species assemblages and dominance varied according to season. Ordination techniques and subsequent fitting analyses showed that seed species composition was positively associated with river flow.

Conclusions

Seed dispersal patterns generated by rivers are significant mechanisms for structuring the composition and distribution of the riparian plant community in Mexican TDF. Varying species assemblages and seed abundance dispersed by the river throughout the year is a relevant and until now unknown consequence that may affect the dynamics and composition of riparian plant communities in this region. This study initiative will promote new avenues of research regarding plant establishment and succession.     

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.     

Plant colonization of bare peat surface - relative importance of seed availability and soil

A field survey on two former peat harvesting sites of similar successional age revealed a marked difference in species composition and a 30-fold difference in biomass of the established vegetation Based on this observation, a field experiment in which the substrate was changed between sites was conducted to examine whether the differences in revegetation were mainly a consequence of differences in seed supply or in substrate quality After three growing seasons, a many hundred-fold difference in plant biomass existed between the transplanted and control plots with the same substrate, but only a small difference between the plots with a different substrate within the site Biological activity in the substrate of the slowly revegetating site (site 1) increased when transported to the more rapidly revegetating site (site 2), while the opposite was true when substrate from site 2 was transported to site 1 The sites differed in the rate of N mineralization, and particularly in nitrification Despite the differences in substrate quality between the two sites, seed supply appeared to be the major factor controlling colonization The impact of the soil factors on plant establishment was, however, considered to be large enough to act as an additional controlling factor     

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.     

Influence of an ecosystem engineer,the emergent macrophyte Sparganium erectum,on seed trapping in lowland rivers and consequences for landform colonisation

1. Plant physical ecosystem engineers can influence vegetation population and community dynamics by modifying, maintaining or creating habitats. They may also have the potential to act upon biotic processes, such as seed dispersal. 2. Examples exist of reduction in seed dispersal distances in vegetated compared to unvegetated terrestrial environments, and concentration of seed deposits associated with plant patches. Such effects in aquatic environments have been little studied, but the engineering effect of plant patches on patterns of flow velocity and sediment deposition in streams suggests that they may play a similar role. 3. In this study, we assess the potential of an emergent aquatic species, Sparganium erectum, to play a role in physically modifying river habitats and trapping seeds by examining patterns of seed deposition and substrate type in 47 river reaches across England and southern Scotland, U.K. 4. Areas of the river channel within or adjacent to S. erectum patches harboured more plant seeds and more species than unvegetated areas and had finer, sandier substrates with higher organic matter, total nitrogen and total phosphorus content. Most seed species were competitive, indicating that they were well suited to colonise the competitive environment of an S. erectum patch, and could potentially further stabilise accumulated sediments and contribute to landform development. 5. We demonstrate that S. erectum patches influence both the physical environment and the retention of seeds, in consistent patterns across the channel bed, for a range of lowland rivers that vary in stream power and geology and which can be expected to vary in levels of supply of fine sediment and seeds. 6. Our findings support the hypothesis that the fundamental influence of a riverine ecosystem‐engineering species on slowing fluid flow links the habitat creation process of sediment sorting and retention to seed trapping. We suggest the process is applicable to a wide range of aquatic and riparian vegetation. We also suggest that the mono‐specific and competitive growth, which is typical of these engineering species, will strongly influence the recruitment of trapped seeds.     

Seed Dispersal and Seedling Emergence in a Created and a Natural Salt Marsh on the Gulf of Mexico Coast in Southwest Louisiana, U.S.A.

Early regeneration dynamics related to seed dispersal and seedling emergence can contribute to differences in species composition among a created and a natural salt marsh. The objectives of this study were to determine (1) whether aquatic and aerial seed dispersal differed in low and high elevations within a created marsh and a natural marsh and (2) whether seedling emergence was influenced by marsh, the presence of openings in the vegetation, and seed availability along the northern Gulf of Mexico coast. Aerial seed traps captured a greater quantity of seeds than aquatic traps. Several factors influenced aquatic and aerial seed dispersal in a created and a natural salt marsh, including distance from the marsh edge, cover of existing vegetation, and water depth. The natural marsh had a high seed density of Spartina alterniflora and Distichlis spicata , the low-elevation created marsh had a high seed density of S. alterniflora , and the high-elevation created marsh had a high seed density of Aster subulatus and Iva frutescens . The presence of adult plants and water depth above the marsh surface influenced seed density. In the natural marsh, openings in vegetation increased seedling emergence for all species, whereas in the low-elevation created marsh, S. alterniflora had higher seedling density under a canopy of vegetation. According to the early regeneration dynamics, the future vegetation in areas of the low-elevation created marsh may become similar to that in the natural marsh. In the high-elevation created marsh, vegetation may be upland fringe habitat dominated by high-elevation marsh shrubs and annual herbaceous species.     

丹江口库区滩涂与入库支流植被与土壤种子库: 水传播潜力探讨

为了探索库区滩涂与入库支流土壤种子库与植被的关系, 以及物种通过水传播扩散的潜力, 对丹江口库区内4条支流及其入库滩涂的地表植被和土壤种子库进行了调查。结果显示, 支流地表植被和种子库的物种多样性高于库区滩涂, 汉江库区滩涂和支流的地表植被与种子库均有较高的物种相似性。DCA排序和TWINSPAN聚类可以明显地将支流的植被和种子库样方与滩涂的植被和种子库样方分到不同的样方组, 结合物种排序图可以清楚地发现, 流入汉江库区支流的植被和种子库的物种组成与滩涂的植被和种子库的物种组成有密切联系。汉江库区滩涂0-15 cm的土层内都有种子萌发, 而丹江库区种子库中只有狗牙根(Cynodon dactylon)一种植物, 且仅存在于0-5 cm土层。研究结果表明, 繁殖体通过支流向滩涂的水传播作用对植被建立有积极贡献。     

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.

     

Assessing the relative importance of dispersal in plant communities using an ecoinformatics approach

Increased insight into the factors that determine the importance of dispersal limitation on species richness and species composition is of paramount importance for conservation and restoration ecology. One way to explore the importance of dispersal limitation is to use seed-sowing experiments, but these do not enable the screening of large sets of species and habitats. In the present paper we present a complementary approach based on comparing small plots with larger regions with regard to species composition and distribution of functional traits. We developed a GIS tool based on ecological and geographical criteria to quantify species pools at various spatial scales. In this GIS tool, containing floristic, large databases, phytosociological and functional information are exploited. Our premise is that differences in the nature of the species in local and regional species pools with regard to functional traits can give important clues to the processes at work in the assembly of communities. We illustrate the approach with a case study for mesotrophic hay meadows (Calthion palustris). We tested the effects of differences in frequency in the local Habitat Species Pool and differences in dispersal and persistence traits of species on local species composition. Our results show that both species pool effects and functional traits affect the probability of occurrence in small plots. Species with a high propagule weight have, given the frequency in the Local Habitat Species Pool, a lower probability of occurrence in small plots. The probability of local occurrence, however, is increased by the ability to form a persistent soil seed bank and by adult longevity. This provides support for the view that the degree of dispersal limitation is dependent on the degree of spatial isolation of the focal site relative to source populations and moreover that species inherently differ in the degree to which dispersal limitation is a limiting factor for local occurrence.     

Go with the flow: Fragment retention patterns shape the vegetative dispersal of aquatic plants in lowland streams

1. The dispersal of aquatic plant propagules is highly facilitated in streams due to flow. As many aquatic plants predominantly spread through vegetative propagules, the specific retention and thus drift distance of dispersed plant fragments largely contribute to the rapid spread along the course of a stream.
2. We determined fragment retention for four aquatic plant species (Elodea canadensis, Myriophyllum spicatum, Ceratophyllum demersum, Salvinia natans; representing four different common morpho-structural groups) in sections of small to medium-sized German streams with different levels of stream sinuosity.
3. The number of fragments showed a logistic decline over drift distance. In two small streams, 90% of drifting fragments were retained at distances (D₉₀) of only 5–9 m and 19–70 m, while higher D₉₀ values of 116–903 m and 153–2,367 m were determined for sections of a medium-sized stream. The likelihood of retention thereby decreased significantly with increasing stream size and was reduced in straightened stream sections.
4. Differences in retention were more strongly related to fragment buoyancy rather than fragment size and morphology. Increasing buoyancy significantly lowered the likelihood of fragment retention over drift distance by a factor of 3–8, whereas contrasting effects were documented for size and morphology of fragments.
5. The relevance of different obstacles was highly stream section-specific and depended on obstacle abundance, distribution, and the degree of submergence/emergence.
6. Our findings elucidate the dynamic retention patterns of plant fragments and highlight the strong interplay between extrinsic (stream) and intrinsic (fragment) properties. We conclude that straightened lowland streams of intermediate size promote the rapid dispersal of invasive aquatic plants and are particularly prone to invaders producing large amounts of small and highly buoyant plant fragments. Information on the species-specific fragment colonisation dynamics in the field is further required to improve our understanding of the vegetative dispersal capacity of invasive aquatic plants in stream ecosystems.

     

Nested communities of alpine plants on isolated mountains: relative importance of colonization and extinction

Aim This paper seeks to investigate whether alpine floras on isolated mountains in boreal forest show nestedness, and, if that is the case, to determine whether selective extinction or colonization is the likely cause of the observed patterns. Location Isolated mountains in the boreal coniferous forests of northern Sweden (province of Norrbotten, c. 66°N; 18°E). The timberline in the region probably has been 300–400 m above the present some thousands of years before present, potentially covering these mountains. Methods A data matrix of twenty‐seven alpine plant species on twenty‐seven isolated mountains was subjected to nested subsets analysis. Extinction probability was assumed to increase with decreasing area, and colonization probability was assumed to decrease with increasing isolation. By sorting the data matrix by these factors and sequentially computing the degree of nestedness, we were able to determine whether the alpine floras were structured mainly by selective extinction or mainly by differential colonization. Results When ordered by decreasing area the data matrix was significantly more nested than random, but that was not the case when ordered by decreasing isolation. Ordering by maximum altitude also produced significant nestedness. Main conclusions Contrary to the conventional view that isolated mountains were completely covered with boreal forest some thousands of years ago, the nestedness patterns of alpine plants indicate that many of them survived the forest period on the isolated mountains, probably on cliffs and slopes too steep for the formation of closed forest.     

Experimental studies of diaspore attachment to animal coats: predicting epizoochorous dispersal potential

The transport of diaspores on animal hairs depends on the ability of a diaspore to attach to the hair and to be retained in it over longer periods of time. Whereas several studies of diaspore retention on animal hairs have been conducted recently, the process of diaspore attachment to the hair has not yet been studied systematically. We describe a new method to quantify the attachment potential (AtP) of plant diaspores. Attachment potential was measured as the proportion of diaspores of a given species that attached to pieces of an animal coat in a standardised experiment. The experiment was conducted for 58 plant species (herbs and grasses) and three different coat types: sheep wool, cattle and roe deer hair. Attachment potentials differed widely between the three coat types, but also between plant species. We found diaspore surface structure (a quantitative measure of diaspore morphology) and diaspore exposition (describing the morphology of the infructescence) to be the most important plant traits regulating AtP. An influence of seed mass on attachment potential could not be detected. For sheep wool, a general linear model (with diaspore exposure as a factor and diaspore surface structure as covariate) explained 77% of the variation in AtPs. To validate this model, we predicted AtPs for 27 additional species and compared these to the measured Atps; the predicted and measured AtPscorrelated significantly with r _s = 0.68. A comparison of attachment and retention potentials to sheep wool for 127 randomly selected plant species showed that attachment and retention are only very weakly correlated, indicating that both processes act rather independently of each other. Since many diaspores seem to perform well in only one of these processes, attachment can be considered to be as equally as decisive as retention in terms of epizoochorous dispersal. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.