Changes of effective gene dispersal distances by pollen and seeds across successive life stages in a tropical tree

Pollen and seed dispersal are the two key processes in which plant genes move in space, mostly mediated by animal dispersal vectors in tropical forests. Due to the movement patterns of pollinators and seed dispersers and subsequent complex spatial patterns in the mortality of offspring, we have little knowledge of how pollinators and seed dispersers affect effective gene dispersal distances across successive recruitment stages. Using six highly polymorphic microsatellite loci and parentage analyses, we quantified pollen dispersal, seed dispersal, and effective paternal and maternal gene dispersal distances from pollen‐ and seed‐donors to offspring across four recruitment stages within a population of the monoecious tropical tree Prunus africana in western Kenya. In general, pollen‐dispersal and paternal gene dispersal distances were much longer than seed‐dispersal and maternal gene dispersal distances, with the long‐distance within‐population gene dispersal in P. africana being mostly mediated by pollinators. Seed dispersal, paternal and maternal gene dispersal distances increased significantly across recruitment stages, suggesting strong density‐ and distance‐dependent mortality near the parent trees. Pollen dispersal distances also varied significantly, but inconsistently across recruitment stages. The mean dispersal distance was initially much (23‐fold) farther for pollen than for seeds, yet the pollen‐to‐seed dispersal distance ratio diminished by an order of magnitude at later stages as maternal gene dispersal distances disproportionately increased. Our study elucidates the relative changes in the contribution of the two processes, pollen and seed dispersal, to effective gene dispersal across recruitment. Overall, complex sequential processes during recruitment contribute to the genetic make‐up of tree populations. This highlights the importance of a multistage perspective for a comprehensive understanding of the impact of animal‐mediated pollen and seed dispersal on small‐scale spatial genetic patterns of long‐lived tree species.     

Seed and microsite limitations of recruitment and the impacts of post-dispersal seed predation at the within population level

Seed predation may cause important seed losses in plant populations, but its impact on the dynamics of populations will depend on the degree of seed or microsite limitations for recruitment. Seed losses will only affect recruitment if it is seed limited. The nature of recruitment limitation (seeds or microsites) is usually ascribed to whole plant populations but it may vary within populations among microhabitats and habitats. Thus, the potential impact of seed predation will also vary within the population, being highest where recruitment is seed limited. The impact to the whole population will depend on the spatial concordance between the intensity of seed predation and that of seed limitation. Recruitment limitations (with seed addition experiments), seed predation (with seed removal experiments), and the dynamics of seed availability in the soil (with soil samples taken both after seed dispersal and before the following dispersal event) of the shrub Corema album (Empetraceae) were investigated in dunes in NW Spain, at microhabitats ‘open ground’, ‘underneath C. album ♀’, and ‘underneath C. album ♂’ at two habitats, sparse and dense scrub. The nature of recruitment limitation (seeds vs. microsites) varied within the population. It was seed limited in the microhabitat ‘open ground’ and microsite limited under shrub cover. The spatial patterns of seedling recruitment were unrelated to seed availability but strongly affected by germination requirements. The spatial discordance between seed availability and recruitment implies a crucial constraint for processes affecting seed availability (seed predation but also e.g., dispersal) to impact recruitment. They will not affect its spatial pattern but only its quantity as long as they act in those sites selected by seeds to germinate. Seed predation was highest underneath mother plants and lowest in open ground. Thus, its potential impact is low, as it is centred where recruitment is not seed limited. This study shows that the analysis of seed predation in relation to recruitment limitations at smaller spatial scales within the population provides more insight to understand its impact.     

Understanding the population genetic structure of Gleditsia triacanthos L.: seed dispersal and variation in female reproductive success

Fine-scale genetic structuring is influenced by a variety of ecological factors and can directly affect the evolutionary dynamics of plant populations by influencing effective population size and patterns of viability selection. In many plant species, genetic structuring within populations may result from highly localized patterns of seed dispersal around maternal plants or by the correlated dispersal and recruitment of siblings from the same fruit. This fine-scale genetic structuring may be enhanced if female parents vary significantly in their reproductive success. To test these hypotheses, we used genetic data from 17 allozyme loci and a maximum-likelihood, ‘maternity-analysis’ model to estimate individual female fertilities for maternal trees across a large number of naturally established seedlings and saplings in two populations of Gleditsia triacanthos L. (Leguminosae). Maximum-likelihood fertility estimates showed that the three highest fertility females accounted for 58% of the 313 progeny at the first site and 46% of the 651 progeny at the second site, whereas 18 of 35 and 16 of 34 females, respectively, had fertility estimates that did not exceed 1%. Additional analyses of the second site found individual female fertility to vary significantly both within and among juvenile age classes. Female fertility at the first site was weakly correlated with maternal tree size and spatial location relative to the open, old-field portions of the population, where the great majority of seedlings and saplings were growing, but no such correlations were found at the second site. Estimates of realized seed dispersal distances indicated that dispersal was highly localized at the first site, but was nearly random at the second site, possibly reflecting differences between the two sites in the behaviour of animal dispersers. The combined estimates of seed dispersal patterns and fertility variation are sufficient to explain previously described patterns of significant fine-scale spatial genetic structure in these two populations. In general, our results demonstrate that effective seed dispersal distributions may vary significantly from population to population of a species due to the unpredictable behaviour of secondary dispersers. Consequently, the effects of seed dispersal on realized fine-scale genetic structure may also be relatively unpredictable.     

Spatial genetic structure in populations of the terrestrial orchid Orchis cyclochila (Orchidaceae)

Orchid seeds are minute, dust-like, wind-borne and, thus, would seem to have the potential for long-distance dispersal. Based on this perception, one may predict near-random spatial genetic structure within orchid populations. In reality we do not know much about seed dispersal in orchids and the few empirical studies of fine-scale genetic structure have revealed significant genetic structure at short distances (< 5m), suggesting that most seeds of orchids fall close to the maternal plant. To obtain more empirical data on dispersal, Ripley’s L(d)-statistics, spatial autocorrelation analyses (coancestry, f_ij analyses) and Wright’s F statistics were used to examine the distribution of individuals and the genetic structure within two populations of the terrestrial orchid Orchis cyclochila in southern Korea. High levels of genetic diversity (H_e = 0.210) and low between-population variation were found (F_ST = 0.030). Ripley’s L(d)-statistics indicated significant aggregation of individuals, and patterns varied depending on populations. Spatial autocorrelation analysis revealed significant positive genetic correlations among individuals located <1 m, with mean f_ij values expected for half sibs. This genetic structure suggests that many seeds fall in the immediate vicinity of the maternal plant. The finding of significant fine-scale genetic structure, however, does not have to preclude the potential for the long distance dispersal of seeds. Both the existence of fine-scale genetic structure and low F_ST are consistent with a leptokurtic distribution of seed dispersal distances with a very flat tail.     

Seed dispersal and population structure in Vriesea gigantea,a bromeliad from the Brazilian Atlantic Rainforest

Seed dispersal, population structure and the mating system of plant species can have great consequences on the genetic structure of populations. Vriesea gigantea is a bromeliad from southeastern Brazil which is self‐compatible and pollinator dependent for fruit set. Its populations are fertile in terms of the production of flowers, pollen, fruits and seeds. To assess the importance of seed supply for gene flow, colonization and distribution of adult individuals, the seed dispersal and population structure of V. gigantea were studied. Seeds are dispersed over short distances; most seeds land close to the mother plant. This pattern coincides with the reported aggregate distribution of bromeliad seedlings. Population structure results showed high seedling recruitment, because 51.3% developed into adults, although few juveniles reached this stage. This result is different from that for other bromeliad species from different habitat conditions. Seed dispersal and population structural patterns are consistent with previous molecular studies, revealing that V. gigantea populations are genetically structured, with low gene flow and a moderate outcrossing rate. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 164 , 317–325.     

Consequences of frugivore-mediated seed dispersal for the spatial and genetic structures of a neotropical palm

The idiosyncratic behaviours of seed dispersers are important contributors to plant spatial associations and genetic structures. In this study, we used a combination of field, molecular and spatial studies to examine the connections between seed dispersal and the spatial and genetic structures of a dominant neotropical palm Attalea phalerata. Field observation and genetic parentage analysis both indicated that the majority of A. phalerata seeds were dispersed locally over short distances (<30 m from the maternal tree). Spatial and genetic structures between adults and seedlings were consistent with localized and short-distance seed dispersal. Dispersal contributed to spatial associations among maternal sibling seedlings and strong spatial and genetic structures in both seedlings dispersed near (<10 m) and away (>10 m) from maternal palms. Seedlings were also spatially aggregated with juveniles. These patterns are probably associated with the dispersal of seeds by rodents and the survival of recruits at specific microsites or neighbourhoods over successive fruiting periods. Our cross-cohort analyses found palms in older cohorts and cohort pairs were associated with a lower proportion of offspring and sibling neighbours and exhibited weaker spatial and genetic structures. Such patterns are consistent with increased distance- and density-dependent mortality over time among palms dispersed near maternal palms or siblings. The integrative approaches used for this study allowed us to infer the importance of seed dispersal activities in maintaining the aggregated distribution and significant genetic structures among A. phalerata palms. We further conclude that distance- and density-dependent mortality is a key postdispersal process regulating this palm population.     

Spatial genetic structure of the southeastern North American endemic, Ceratiola ericoides (Empetraceae)

Understanding the spatial distribution of genetic diversity (i.e., spatial genetic structure [SGS]) within plant populations can elucidate mechanisms of seed dispersal and patterns of recruitment that may play an important role in shaping the demography and spatial distribution of individuals in subsequent generations. Here we investigate the SGS of allozyme diversity in 2 populations of the southeastern North American endemic shrub, Ceratiola ericoides. The data suggest that the 2 populations have similar patterns of SGS at distances of 0-45 m that likely reflect the isolation by distance (IBD) model of seed dispersal. However, at distances >or=50 m, the pattern of SGS differs substantially between the 2 populations. Whereas one population continues to reflect the classical IBD pattern, the second population shows a marked increase in autocorrelation coefficient (r) values at 50-75 m. Furthermore, r values at these distances are as much as 33% higher than at 0-5 m where the highest r value would be predicted by IBD. A likely explanation is the differing frequencies of 2 fruit morphologies in these populations and the greater role that birds play in seed dispersal in the second population.     

Plant Parentage,Pollination, and Dispersal: How DNA Microsatellites Have Altered the Landscape

DNA microsatellites provide plant ecologists with molecular markers precise enough to assign parentage to seeds and seedlings. This allows the exact distance and trajectory of successful pollen to be traced to characterize pollination patterns. Parentage assignment of established seedlings also allows researchers to accurately determine how far new recruits have traveled from their seed parent. This paper reviews the history and development of molecular parentage assignment in studies of native plants, as well as the limitations and constraints to this approach. This paper also reviews 53 articles published in the past 15 years that use parentage assignment to study pollination and seed dispersal in native plants. These parentage studies have overturned many common assumptions regarding pollen and seed dispersal patterns. They show that long-distance dispersal of pollen is common in both wind and animal dispersed systems, with average pollination distances commonly being hundreds of meters. The pollination neighborhood is often extremely large, and simple dispersal functions based on distance alone fail to make accurate predictions of pollination. Rather than hindering gene flow, fragmentation and isolation sometimes, and perhaps even commonly, results in increased pollination distances. Studies of seed dispersal using parentage assignment have also yielded some surprises. We now know that it may be erroneous to assume that seeds growing under the crown of a conspecific adult are growing beneath their mother, or that seed dispersal distances are more limited than pollen dispersal distances. Taken together, the studies to date demonstrate that both seed and pollen dispersal are quite complex phenomena influenced by many ecological processes.     

Long‐term consistency in spatial patterns of primate seed dispersal

Seed dispersal is a key ecological process in tropical forests, with effects on various levels ranging from plant reproductive success to the carbon storage potential of tropical rainforests. On a local and landscape scale, spatial patterns of seed dispersal create the template for the recruitment process and thus influence the population dynamics of plant species. The strength of this influence will depend on the long‐term consistency of spatial patterns of seed dispersal. We examined the long‐term consistency of spatial patterns of seed dispersal with spatially explicit data on seed dispersal by two neotropical primate species, Leontocebus nigrifrons and Saguinus mystax (Callitrichidae), collected during four independent studies between 1994 and 2013. Using distributions of dispersal probability over distances independent of plant species, cumulative dispersal distances, and kernel density estimates, we show that spatial patterns of seed dispersal are highly consistent over time. For a specific plant species, the legume Parkia panurensis, the convergence of cumulative distributions at a distance of 300 m, and the high probability of dispersal within 100 m from source trees coincide with the dimension of the spatial–genetic structure on the embryo/juvenile (300 m) and adult stage (100 m), respectively, of this plant species. Our results are the first demonstration of long‐term consistency of spatial patterns of seed dispersal created by tropical frugivores. Such consistency may translate into idiosyncratic patterns of regeneration.     

Proximity is not a proxy for parentage in an animal-dispersed Neotropical canopy palm

We used parentage analysis to estimate seedling recruitment distances and genetic composition of seedling patches centred around reproductive trees of the animal-dispersed Neotropical canopy palm Iriartea deltoidea in two 0.5 ha plots within second-growth forest and one 0.5 ha plot in adjacent old-growth forest at La Selva Biological Field Station in north-eastern Costa Rica. Seedlings were significantly spatially aggregated in all plots, but this pattern was not due to dispersal limitation. More than 70 per cent of seedlings were dispersed at least 50 m from parent trees. Few seedlings were offspring of the closest reproductive trees. Seedling patches observed beneath reproductive trees originate from dozens of parental trees. Observed patterns of seedling distribution and spatial genetic structure are largely determined by the behaviour of vertebrate seed dispersers rather than by spatial proximity to parental trees.     

Spatial patterns of acorn dispersal by rodents: do acorn crop size and ungulate presence matter?

Seed dispersal is qualitatively effective when it increases recruitment probability. A poorly studied factor likely affecting recruitment is the spatial distribution of dispersed seeds. Seed-caching animals are thought to disperse seeds in a way that reduces clumping and density to impede cache pilfering. Furthermore, dispersal might differ depending on whether the seed is immediately consumed versus cached for later consumption, and might differ depending on the ecological context. The main objectives of this study were to determine: 1) the spatial pattern of seed dispersal by rodents in a heterogeneous environment; 2) whether the patterns differ among years and among acorn competitor exclosure treatments, and 3) whether rodents create different spatial patterns of dispersal for acorns that are cached versus consumed immediately following dispersal. We studied the degree of spatial aggregation of acorn dispersal by rodents using two different estimators derived from the Ripley K and the Diggle G functions. We also analyzed various metrics of dispersal distances. For both analyses we used observed acorn dispersal patterns in two years differing in crop size and inside versus outside exclosures restricting access to acorn-consuming ungulates. During 2003, a year with a larger crop size, maximum seed dispersal distances were less, and the pattern of dispersed seeds was more clumped, than in 2004, a year with a smaller crop size. Median dispersal distances did not differ between years. In the presence of ungulates, seed dispersal was marginally sparser than in their absence. Cached acorns were dispersed more sparsely than acorns eaten immediately. These results have important implications for the quality of seed dispersal for oak recruitment that are likely relevant to other systems as well.     

Low plant density enhances gene dispersal in the Amazonian understory herb Heliconia acuminata

In theory, conservation genetics predicts that forest fragmentation will reduce gene dispersal, but in practice, genetic and ecological processes are also dependent on other population characteristics. We used Bayesian genetic analyses to characterize parentage and propagule dispersal in Heliconia acuminata L. C. Richard (Heliconiaceae), a common Amazonian understory plant that is pollinated and dispersed by birds. We studied these processes in two continuous forest sites and three 1‐ha fragments in Brazil's Biological Dynamics of Forest Fragments Project. These sites showed variation in the density of H. acuminata. Ten microsatellite markers were used to genotype flowering adults and seedling recruits and to quantify realized pollen and seed dispersal distances, immigration of propagules from outside populations, and reproductive dominance among parents. We tested whether gene dispersal is more dependent on fragmentation or density of reproductive plants. Low plant densities were associated with elevated immigration rates and greater propagule dispersal distances. Reproductive dominance among inside‐plot parents was higher for low‐density than for high‐density populations. Elevated local flower and fruit availability is probably leading to spatially more proximal bird foraging and propagule dispersal in areas with high density of reproductive plants. Nevertheless, genetic diversity, inbreeding coefficients and fine‐scale spatial genetic structure were similar across populations, despite differences in gene dispersal. This result may indicate that the opposing processes of longer dispersal events in low‐density populations vs. higher diversity of contributing parents in high‐density populations balance the resulting genetic outcomes and prevent genetic erosion in small populations and fragments.     

植物种群更新的补充限制

补充限制基于生态位理论, 从种子萌发、幼苗存活和生长、繁殖体扩散等生活史阶段的种群统计特征及环境因素(土壤水分、养分、凋落物等)着手, 探讨种群的更新问题。种源限制和微生境限制是补充限制理论研究的核心内容, 但是哪个更为重要并没有统一的结论。种源限制与种子生产、土壤种子库和地下芽库中的繁殖体数量不足有关。其中, 气候的年际波动、土壤种子库寿命和动物的捕食都会影响种子生产在种群更新中的作用; 土壤种子库常被视为种群更新的保险库, 与地上种子雨共同促进种群更新, 但是, 如果土壤里种子具有较高的死亡率和休眠率, 将会降低种子库的作用; 地下芽库及其产生的无性分株对于种群更新的意义更多地体现在干扰后种群更强的恢复能力上。扩散限制是种群更新中的普遍现象, 与种子产量、散布能力、传播媒介、幼苗密度等因素有关。微生境限制主要表现为水分、养分、凋落物等非生物因素以及竞争、捕食等生物因素对种子的活力、萌发性、幼苗的存活力、物质分配等过程的影响, 其重要性随着植物生活史阶段而发生变化。未来需要进行综合的、长期的实验, 并应着重加强种源限制及相关生态过程的进化与生态相结合的机理性研究, 从而更深刻地认识和理解种群更新问题, 建立更为综合、系统的种群更新理论体系。     

Assessment of spatial discordance of primary and effective seed dispersal of European beech (Fagus sylvatica L.) by ecological and genetic methods

Spatial discordance between primary and effective dispersal in plant populations indicates that postdispersal processes erase the seed rain signal in recruitment patterns. Five different models were used to test the spatial concordance of the primary and effective dispersal patterns in a European beech (Fagus sylvatica) population from central Spain. An ecological method was based on classical inverse modelling (SSS), using the number of seed/seedlings as input data. Genetic models were based on direct kernel fitting of mother‐to‐offspring distances estimated by a parentage analysis or were spatially explicit models based on the genotype frequencies of offspring (competing sources model and Moran‐Clark's Model). A fully integrated mixed model was based on inverse modelling, but used the number of genotypes as input data (gene shadow model). The potential sources of error and limitations of each seed dispersal estimation method are discussed. The mean dispersal distances for seeds and saplings estimated with these five methods were higher than those obtained by previous estimations for European beech forests. All the methods show strong discordance between primary and effective dispersal kernel parameters, and for dispersal directionality. While seed rain was released mostly under the canopy, saplings were established far from mother trees. This discordant pattern may be the result of the action of secondary dispersal by animals or density‐dependent effects; that is, the Janzen‐Connell effect.     

What causes conspecific plant aggregation? Disentangling the role of dispersal,habitat heterogeneity and plant–plant interactions

Spatial patterns of plant species are determined by an array of ecologica factors including biotic and abiotic environmental constraints and intrinsic species traits. Thus, an observed aggregated pattern may be the result of short‐distance dispersal, the presence of habitat heterogeneity, plant–plant interactions or a combination of the above. Here, we studied the spatial pattern of Mediterranean alpine plant Silene ciliata (Caryophyllaceae) in five populations and assessed the contribution of dispersal, habitat heterogeneity and conspecific plant interactions to observed patterns. For this purpose, we used spatial point pattern analysis combined with specific a priori hypotheses linked to spatial pattern creation. The spatial pattern of S. ciliata recruits was not homogeneous and showed small‐scale aggregation. This is consistent with the species’ short‐distance seed dispersal and the heterogeneous distribution of suitable sites for germination and establishment. Furthermore, the spatial pattern of recruits was independent of the spatial pattern of adults. This suggests a low relevance of adult‐recruits interactions in the spatial pattern creation. The difference in aggregation between recruits and adults suggests that once established, recruits are subjected to self‐thinning. However, seedling mortality did not erase the spatial pattern generated by seed dispersal, as S. ciliata adults were still aggregated. Thus, the spatial aggregation of adults is probably due to seed dispersal limitation and the heterogeneous distribution of suitable sites at seedling establishment rather than the presence of positive plant–plant interactions at the adult stage. In fact, a negative density‐dependent effect of the conspecific neighbourhood was found on adult reproductive performance. Overall, results provide empirical evidence of the lack of a simple and direct relationship between the spatial structure of plant populations and the sign of plant–plant interactions and outline the importance of considering dispersal and habitat heterogeneity when performing spatial analysis assessments.     

Reliability of macaques as seed dispersers

Seed dispersal is an ecological process crucial for forest regeneration and recruitment. To date, most studies on frugivore seed dispersal have used the seed dispersal effectiveness framework and have documented seed-handling mechanisms, dispersal distances and the effect of seed handling on germination. In contrast, there has been no exploration of “disperser reliability” which is essential to determine if a frugivore is an effective disperser only in particular regions/years/seasons or across a range of spatio-temporal scales. In this paper, we propose a practical framework to assess the spatial reliability of frugivores as seed dispersers. We suggest that a frugivore genus would be a reliable disperser of certain plant families/genera if: (a) fruits of these plant families/genera are represented in the diets of most of the species of that frugivore, (b) these are consumed by the frugivore genus across different kinds of habitats, and (c) these fruits feature among the yearly staples and preferred fruits in the diets of the frugivore genus. Using this framework, we reviewed frugivory by the genus Macaca across Asia to assess its spatial reliability as seed dispersers. We found that the macaques dispersed the seeds of 11 plant families and five plant genera including at least 82 species across habitats. Differences in fruit consumption/preference between different groups of macaques were driven by variation in plant community composition across habitats. We posit that it is essential to maintain viable populations of macaques across their range and keep human interventions at a minimum to ensure that they continue to reliably disperse the seeds of a broad range of plant species in the Anthropocene. We further suggest that this framework be used for assessing the spatial reliability of other taxonomic groups as seed dispersers.     

森林种子雨研究进展与展望

种子雨阶段是植物更新的关键环节,它连接着繁殖生产与植物后续生活史阶段,对群落结构有着重要的影响.虽然早在19世纪中叶达尔文就认识到种子扩散的重要性,然而对种子雨的广泛研究于20世纪80年代初才开始.本文聚焦于森林木本植物种子雨研究,首先介绍了种子雨监测方法,包括收集器的布置,种子雨的收集、分离和鉴定.然后综述了种子雨的4个主要研究方向:种子产量的时间和空间变化格局(包括季节变化、年际变化和空间变化)、增补限制及其在物种多样性维持中的作用、验证负密度效应假说、种子雨与其他生活史阶段(土壤种子库、幼苗、幼树及母树)的比较.未来还需要加强对种子雨的长期监测,开展增补限制的跨纬度比较研究,探讨植物早期更新阶段负密度效应沿纬度梯度的变化规律,加强数学模型以及分子标记和稳定同位素技术等新手段的运用.     

Pollen and seed flow patterns of Carapa guianensis Aublet. (Meliaceae) in two types of Amazonian forest

Various factors affect spatial genetic structure in plant populations, including adult density and primary and secondary seed dispersal mechanisms. We evaluated pollen and seed dispersal distances and spatial genetic structure of Carapa guianensis Aublet. (Meliaceae) in occasionally inundated and terra firme forest environments that differed in tree densities and secondary seed dispersal agents. We used parentage analysis to obtain contemporary gene flow estimates and assessed the spatial genetic structure of adults and juveniles. Despite the higher density of adults (diameter at breast height ≥ 25 cm) and spatial aggregation in occasionally inundated forest, the average pollen dispersal distance was similar in both types of forest (195 ± 106 m in terra firme and 175 ± 87 m in occasionally inundated plots). Higher seed flow rates (36.7% of juveniles were from outside the plot) and distances (155 ± 84 m) were found in terra firme compared to the occasionally inundated plot (25.4% and 114 ± 69 m). There was a weak spatial genetic structure in juveniles and in terra firme adults. These results indicate that inundation may not have had a significant role in seed dispersal in the occasionally inundated plot, probably because of the higher levels of seedling mortality.     

Hydrochory as a determinant of genetic distribution of seeds within Hibiscus moscheutos (Malvaceae) populations

Seed dispersal is a major determinant of the spatial genetic structure of plant populations. In this study, we evaluated the role of distinct hydrologic regimes in determining the spatial genetic structure of the seed bank of the wetland plant Hibiscus moscheutos. We analyzed seeds in surface soil samples collected in the autumn and the following spring by determining their allozyme genotypes and estimated the pattern in seed movements during flooding. We selected study sites in nontidal and tidal wetlands with different flooding regimes. One nontidal site had no flooding, while the second nontidal site was inundated for most of the year. One tidal wetland site flooded with almost every tide, and a second tidal site was inundated at moderate frequency. Genetic makeup of the seed bank at the nonflooded site changed little between seasons. Secondary seed dispersal altered absolute allele frequencies at the other three sites, with the greatest change occurring at the two tidally influenced sites. This study demonstrates that secondary hydrochory influences the genetic composition of the seed bank and that hydrologic conditions play an important role in determining the local patterns in seed movements.     

Seed dispersal and seedling recruitment of trees at different successional stages in a temperate forest in northeastern China

Aims Spatial distribution of adult trees in a forest community is determined by patterns of both seed dispersal and seedling recruitment. The objectives of our study were to understand the processes of seed dispersal and seedling recruitment of dominant tree species in a temperate forest of northeastern China and to identify the factors constraining seed dispersal and seedling establishment at different stages of forest succession.Methods During three summer and autumn sessions between 2006 and 2008, altogether 113080 seeds from 22 different tree species were collected in three large field plots representing different forest types in the Changbai Mountain region of northeastern China. The spatial distribution of seed abundance was analyzed using a Syrjala test. Regeneration success of nine major tree species was assessed using variables defining 'limitations' in 'seeds' and 'seedling establishment'.Important findings We found that seed production fluctuated between years and varied greatly with forest types. Four tree species, Acer spp., Fraxinus mandshurica, Tilia amurensis and Betula spp., had the greatest seed production and the widest range of seed dispersal, whereas Quercus mongolica showed the most sustained seed production pattern. The spatial patterns of seed abundance differed significantly among forest types and years. The tree species investigated in this study differed in the degree of seed limitation, as well as in limitation of seedling establishment. There were both negative and positive correlations between seed density and seedling density, depending on site and parental tree density. Seeds of 16 tree species were found in the Populus davidiana–Betula platyphylla forest (PBF) plot, 11 in the conifer and broad-leaved mixed forest (CBF) plot but only 8 in the broad-leaved-Korean pine mixed forest (BKF) plot. The number of seed-contributing species was not only greater in the secondary forests (CBF and PBF plots) than in the primary forest (BKF plot) but was also more variable during the 3 years of assessment. Results from the correlations between seed density and seedling occurrence and that between parental tree density or seed weight and dispersal limitation confirm our intuitive expectations, i.e. heavy seeds had greater dispersal limitation but higher establishment success than light seeds.