The seed dimorphism of Spergularia marina in relation to dispersal by wind and water

Summary The seeds of the halophyte Spergularia marina differ both within and between individuals in that they either possess or lack a membranaceous border. This paper presents a morphological study of the length, weight and area of the seed types, and their dispersal characteristics under experimental conditions of wind and water dispersal. The winged seeds are shown to be larger both by length and by weight. Their rate of descent increases with wing loading. If the wing is lacking, however, the rate of descent increases with weight only. The distance of dispersal is equal for both seed types except at low wind speeds, when the winged seeds disperse farther. If the seed wing is removed, the excised seeds have shorter dispersal distances. When dispersed by water, a difference in the distance seeds are dispersed can only be detected in the presence of vegetation. The winged seeds are more frequently trapped in the vegetation as compared to the unwinged seeds. The hypothesis that the seed dimorphism is an adaptation for differential dispersal distances is discussed.     

Which factors have stronger explanatory power for primary wind dispersal distance of winged diaspores: the case of Zygophyllum xanthoxylon (Zygophyllaceae)?

Aims How seed dispersal distance is related to various factors is a major challenge for seed ecologists. However, there are different answers as to which factor is most important in determining wind dispersal distance. This study is to quantitatively describe the relationship between various factors and primary wind dispersal distance of winged diaspores.Methods The dispersal distances of five morphologies of winged diaspores in Zygophyllum xanthoxylum (Zygophyllaceae) were measured under controlled conditions in a wind tunnel. The explanatory power of environmental factor (i.e. wind speed), plant trait (i.e. release height) and diaspore attributes (i.e. wing loading (the ratio of diaspore mass to projected area), settlement-velocity, shape index (the variance of diaspore length, width and thickness)) to the variation in dispersal distance was assessed by releasing diaspores at varying wind speeds and release heights.Important findings Wind speed and seed release height were the strongest explanatory factors to dispersal distance, contributing 41.1% and 24.8% (P < 0.01) to total variation in dispersal distance, respectively. Wind speed accounted more for relatively light disc-shaped seeds than for relatively heavy spherical seeds. Wing loading, shape index and settlement-velocity explained 9.0% (P < 0.01), 1.4% (P < 0.01) and 0.9% (not significant) of the variation in dispersal distance, respectively. From disc-shaped to four-winged diaspores, relative contributions of wing loading and shape index decreased but contribution of settlement-velocity increased. The relative contributions of various factors to wind seed dispersal distance may change with the change in seed morphology.     

Seed dispersal in Erythronium grandiflorum (Liliaceae)

Primary and secondary seed dispersal was investigated for the glacier lily Erythronium grandiflorum in the Colorado Rocky Mountains. These heavy seeds have no obvious adaptations for biotic or abiotic dispersal, but can be thrown short distances when the dehiscent fruits are shaken by wind. We used sticky traps to measure primary transport of seeds up to 1 m away from individual plants. A seed cafeteria experiment examined the role of ants and rodents in secondary seed transport. Primary dispersal by wind was positively skewed and median transport distances were influenced by variation in plant height. Secondary dispersal was negligible compared to Viola nuttallii, an elaiosome-bearing species. Thus, seed dispersal was highly restricted in E. grandiflorum, and a 1 m radius encompassed the modal section of the seed dispersal curve. The seed dispersal component of gene flow was quantified and combined with previous measurements of pollen flow to yield a more complete estimate of Wright's neighborhood size, N _e, for E. grandiflorum. The lack of a special seed dispersal mechanism in E. grandiflorum is discussed in terms of a source-sink model for seedling establishment with respect to distance from the parental plants.     

Costs and benefits of non‐random seed release for long‐distance dispersal in wind‐dispersed plant species

The dispersal ability of plants is a major factor driving ecological responses to global change. In wind‐dispersed plant species, non‐random seed release in relation to wind speeds has been identified as a major determinant of dispersal distances. However, little information is available about the costs and benefits of non‐random abscission and the consequences of timing for dispersal distances. We asked: 1) to what extent is non‐random abscission able to promote long‐distance dispersal and what is the effect of potentially increased pre‐dispersal risk costs? 2) Which meteorological factors and respective timescales are important for maximizing dispersal? These questions were addressed by combining a mechanistic modelling approach and field data collection for herbaceous wind‐dispersed species. Model optimization with a dynamic dispersal approach using measured hourly wind speed showed that plants can increase long‐distance dispersal by developing a hard wind speed threshold below which no seeds are released. At the same time, increased risk costs limit the possibilities for dispersal distance gain and reduce the optimum level of the wind speed threshold, in our case (under representative Dutch meteorological conditions) to a threshold of 5–6 m s^–1. The frequency and predictability (auto‐correlation in time) of pre‐dispersal seed‐loss had a major impact on optimal non‐random abscission functions and resulting dispersal distances. We observed a similar, but more gradual, bias towards higher wind speeds in six out of seven wind‐dispersed species under natural conditions. This confirmed that non‐random abscission exists in many species and that, under local Dutch meteorological conditions, abscission was biased towards winds exceeding 5–6 m s^–1. We conclude that timing of seed release can vastly enhance dispersal distances in wind‐dispersed species, but increased risk costs may greatly limit the benefits of selecting wind conditions for long‐distance dispersal, leading to moderate seed abscission thresholds, depending on local meteorological conditions and disturbances.     

Seed dispersal characteristics of Brassavola nodosa (Orchidaceae)

We used mathematical models for wind-dispersed seeds and wind-tunnel experiments to predict modal seed dispersal distance of the Neotropical orchid Brassavola nodosa under conditions approximating those found in its natural habitat: mangrove islands in Belize, Central America. Key variables in a simple ballistic model for predicting modal dispersal distance (xm) of an individual seed include: height of release (h); free-stream velocity (Uc); and terminal velocity of the seed (Ut): xm = h Uc/Ut. Modal dispersal distance of dust-like orchid seeds were predicted adequately by this ballistic model at low wind velocities and low release heights, but it underestimated the increasing importance of turbulence at higher wind velocities and greater release heights. We estimated the magnitude and relative importance of one measure of turbulence, vertical mixing velocity (W*), on xm in wind tunnel experiments. Our estimates of W* were within the same order of magnitude as those found for other small dust-like seeds and pollen. In high turbulence conditions, incorporation of vertical mixing velocity effects into the ballistic model of seed dispersal overestimated modal seed dispersal distances.     

Effects of seed morphology and orientation on secondary seed dispersal by wind

传播体形态与方位对二次种子风力传播的影响     

Implications of nonrandom seed abscission and global stilling for migration of wind‐dispersed plant species

Migration of plant populations is a potential survival response to climate change that depends critically on seed dispersal. Biological and physical factors determine dispersal and migration of wind‐dispersed species. Recent field and wind tunnel studies demonstrate biological adaptations that bias seed release toward conditions of higher wind velocity, promoting longer dispersal distances and faster migration. However, another suite of international studies also recently highlighted a global decrease in near‐surface wind speeds, or ‘global stilling’. This study assessed the implications of both factors on potential plant population migration rates, using a mechanistic modeling framework. Nonrandom abscission was investigated using models of three seed release mechanisms: (i) a simple drag model; (ii) a seed deflection model; and (iii) a ‘wear and tear’ model. The models generated a single functional relationship between the frequency of seed release and statistics of the near‐surface wind environment, independent of the abscission mechanism. An Inertial‐Particle, Coupled Eulerian‐Lagrangian Closure model (IP‐CELC) was used to investigate abscission effects on seed dispersal kernels and plant population migration rates under contemporary and potential future wind conditions (based on reported global stilling trends). The results confirm that nonrandom seed abscission increased dispersal distances, particularly for light seeds. The increases were mitigated by two physical feedbacks: (i) although nonrandom abscission increased the initial acceleration of seeds from rest, the sensitivity of the seed dispersal to this initial condition declined as the wind speed increased; and (ii) while nonrandom abscission increased the mean dispersal length, it reduced the kurtosis of seasonal dispersal kernels, and thus the chance of long‐distance dispersal. Wind stilling greatly reduced the modeled migration rates under biased seed release conditions. Thus, species that require high wind velocities for seed abscission could experience threshold‐like reductions in dispersal and migration potential if near‐surface wind speeds continue to decline.     

Acorn dispersal estimated by radio-tracking

Bird-dispersed seeds are difficult to track, especially in the case of long-distance dispersal events. To estimate the oak dispersal distance and the seed shadow generated by the European jay (Garrulus glandarius), we inserted radio-transmitters in 239 acorns, placed them in bird-feeders and then located them by radio-tracking. Using this methodology we located the exact caching site of 94 Quercus ilex and 54 Q. suber acorns and determined the caching habitat characteristics (vegetation type, distance, spatial distribution). The results show that: (1) there is no differences in the dispersal distance distribution between the different acorn species or sizes, (2) dispersal distances range from approximately 3 m up to approximately 550 m (mean = 68.6 m; median = 49.2 m), (3) recently abandoned fields and forest tracks were the sites preferred by jays to cache acorns, whereas fields and shrublands were avoided and (4) seed shadows showed acorn aggregation zones (i.e. clusters of caches) close to the feeder as well as isolated caches at longer distances. The results also suggest that radio-transmitters are a cheap and reliable way to determine seed shadows and quantify both seed dispersal and post-dispersal seed predation for medium to large seeds.     

Epizoochorous seed dispersal in relation to seed availability – an experiment with a red fox dummy

Questions: Is the red fox a potential vector for epizoochorous seed dispersal? Can seed attachment and retention be predicted from plant and seed traits? Location: Grasslands in southern Norway. Methods: Epizoochorous seed attachment on the red fox was studied by walking a dummy fox through the vegetation and comparing seeds found on the dummy with the estimated seed availability in the vegetation. Seed retention, i.e. the ability of different seeds to stay on the fox, was estimated in a separate experiment. Seed attachment and retention were related to plant and seed traits using statistical models that account for heteroscedasticity and zero‐inflated data. Results: The majority of seeds attached to the fox originated from a few species, but also species without specific seed traits that are supposed to enhance epizoochory attached at least some seeds to the fox. The probability of seed attachment was positively related to plant height, bristle and hooked seed appendages, and negatively related to winged appendages, seed mass, and seed sphericity. Seed retention was positively related to the seed traits bristles, hooks and pappus. For several species, the results indicate a high potential for dispersal over long distances. Conclusions: In modern agricultural landscapes, large herbivores are often restricted in their mobility or are found at low densities, and other animal vectors may therefore be important for seed dispersal. In our study, a range of plant species were able to disperse by attaching seeds to, and having their seeds retained in, the fox fur some distance. We suggest that the red fox may be an important vector for epizoochorous seed dispersal in the agricultural landscape.     

Wind dispersal in freshwater wetlands: Knowledge for conservation and restoration

Questions: For wetland plants, dispersal by wind is often overlooked because dispersal by water is generally assumed to be the key dispersal process. This literature review addresses the role of seed dispersal by wind in wetlands. Why is wind dispersal relevant in wetlands? Which seeds are dispersed by wind and how far? And how can our understanding of wind dispersal be applied to wetland conservation and restoration? Methods: Literature review. Results and conclusions: Wind is a widely available seed dispersal vector in wetlands and can transport many seeds over long distances. Unlike water, wind can transport seeds in all directions and is therefore important for dispersal to upstream wetlands and to wetlands not connected by surface water flows. Wind dispersal transports seeds to a wider range of sites than water, and therefore reaches more sites but with lower seed densities. Many wetland plant species have adaptations to facilitate wind dispersal. Dispersal distances increase with decreasing falling velocity of seeds, increasing seed release height and selective release mechanisms. Depending on the adaptations, seeds may be dispersed by wind over many km or only a few m. The frequency of long‐distance wind dispersal events depends on these adaptations, the number of produced seeds, the structure of the surrounding vegetation, and the frequency of occurrence of suitable weather conditions. Humans reduce the frequency of successful long‐distance wind dispersal events in wetlands through wetland loss and fragmentation (which reduce the number and quality of seeds) and eutrophication (which changes the structure of the vegetation so that seed release into the wind flow becomes more difficult). This is yet another reason to focus on wetland conservation and restoration measures at increased population sizes, prevention of eutrophication, and the restoration of sites at short distances from seed sources.     

绒毛白蜡二态型果实的扩散特征

多态型果实或种子的出现对植物种群的扩散具有重要的意义。绒毛白蜡(Fraxinus velutina)的果实具有二态型特征,主要表现在果翅数量上不同,分别定义为二翅型和三翅型果实,为了比较两类果实在风传扩散时的差异,研究了两类果实的形态、果翅结构和扩散距离及扩散时长。在大型封闭地下室内,以电扇在不同速度档位产生的气流作为风源,分别从2、1.5、1m处手动释放果实,对风速为0、4.6、6.5、7.3m/s时的果实扩散距离及扩散时长进行了比较;并在此基础上对果实的形态特征与扩散特征进行了线性相关分析。结果表明:在同一高度及相同风速下,三翅型果实的水平扩散距离都极显著的大于二翅型,但其相应的扩散时长都小于二翅型。在相同情况下,三翅型的果实沉降速度显著高于二翅型。两类果实随着释放高度的增加,其扩散距离和扩散时长都相应的增加;随着风速的升高,其扩散距离及扩散时长都相应的增加。三翅型果实质量显著高于二翅型,相反,三翅型果翅长与宽都显著小于二翅型。两种翅型的果翅细胞结构都一样,细胞内部都呈现气囊状,果翅表面沿纵轴方向有流线型的纵棱。通过直线相关分析发现,翅型是对扩散距离和扩散时长影响最显著的形态特征;与果实释放高度相比,风速是影响绒毛白蜡果实扩散距离与扩散时长最显著的环境因素。绒毛白蜡三翅型果实比二翅型果实传播的远,关键在于其具有三翅,三翅对阵风瞬间响应,使得沉降速度较二翅型高,可以在瞬时风的作用下,快速传播到较远的距离。三翅型与二翅型在扩散方式上的结合增强了绒毛白蜡的生存与定殖机会。     

Dispersal mode,seed shadows,and colonization patterns

This review assesses the state of our knowledge about comparative seed shadows. Using data presently available in the literature, I compare the slopes (on a log-linear scale) of seed shadows for plants with different morphologically characterized modes of dispersal. The seeds of many species have no evident morphological adaptation for dispersal and seem to achieve only short-distance dispersal. Seed shadows for herbaceous species with devices for wind have flatter slopes and more distant modes and maxima than those of ballists, which in turn exceed those with no special devices. Seed shadows for wind-dispersed trees and shrubs had similar or steeper slopes than those for vertebrate-dispersed species in this sample. Species with poor mechanisms for dispersal in space only sometimes had the capacity for better dispersal in time (dormancy). Although some species exhibited seed shadows sufficiently steep to be predicted to colonize new-habitat in a front or phalanx pattern, actual colonization patterns must reflect many other factors.     

Ficus seed shadows in a Bornean rain forest

Due to their copious seed production and numerous dispersers, rain forest fig trees have been assumed to produce extensive and dense seed shadows. To test this idea, patterns of seed dispersal of two species of large hemiepiphytic fig tree were measured in a Bornean rain forest. The sample included four Ficus stupenda and three F. subtecta trees with crop sizes ranging from 2,000 to 40,000 figs (400,000 to 13,000,000 seeds). Seed rain out to a distance of 60 m from each study tree was quantified using arrays of seed traps deployed in the understory. These trees showed a strongly leptokurtic pattern of dispersal, as expected, but all individuals had measurable seed rain at 60 m, ranging from 0.2 to 5.0 seeds/m². A regression of In-transformed seed rain density against distance gave a significant fit to all seven trees' dispersal patterns, indicating that the data could be fitted to the negative exponential distribution most commonly fitted to seed shadows. However, for six of seven trees, an improved fit was obtained for regressions in which distance was also In-transformed. This transformation corresponds to an inverse power distribution, indicating that for vertebrate-dispersed Ficus seeds, the tail of the seed rain distribution does not drop off as rapidly as in the exponential distribution typically associated with wind dispersed seed shadows. Over 50% of the seed crop was estimated to fall below each fig tree's crown. Up to 22% of the seed crop was dispersed beyond the crown edge, but within 60 m of the tree. Estimates of the maximum numbers of seeds which could have been transported beyond 60 m were 45% for the two largest crops of figs, but were under 24% for the trees with smaller crops. Seed traps positioned where they had an upper canopy layer above them were associated with higher probabilities of being hit by seeds, suggesting that vertebrate dispersal agents are likely to perch or travel through forest layers at the same level as the fig crown and could concentrate seeds in such areas to some degree. The probability of a safe site at 60 m from the fig tree being hit by seeds is calculated to be on the order of 0.01 per fruiting episode. Fig trees do not appear to saturate safe sites with seeds despite their large seed crops. If we in addition consider the rarity of quality establishment sites and post-dispersal factors reducing successful seedling establishment, hemiepiphytic fig trees appear to face severe obstacles to seedling recruitment.     

Macaques as Seed Dispersal Agents in Asian Forests: A Review

The role of primates in seed dispersal is well recognized. Macaques (Macaca spp.) are major primate seed dispersers in Asia, and recent studies have revealed their role as seed dispersal agents in this region. Here, we review present knowledge of the traits that define the role of macaques as seed dispersers. The size of seeds in fruit influences whether macaques swallow (0.5–17.1 mm; median: 3.0), spit (1–37 mm; median: 7.6), or drop (8.2–57.7 mm; median: 20.5) them. Dispersal distances via defecation are several hundreds of meters (median: 259 m, range: 0–1300 m), shorter than those achieved by some mammals and birds in tropical and temperate regions. However, macaques disperse seeds by defecation at comparable distances to omnivorous carnivores, and further than passerines. Seed dispersal distance by spitting is much shorter (median: 20 m, range: 0–405 m) than by defecation. Among Asian primates, seed dispersal distances resulting from macaque defecation are shorter than those for gibbons and longer than those for langurs. The effects of seed ingestion on the percentage and speed of germination vary among both plant and macaque species. The degree of frugivory, fruit/seed handling methods, seed dispersal distance, microhabitats of dispersed seeds, and effects of dispersal on seed germination vary seasonally and interannually, and long-term studies of the ecological role of macaques are needed. Researchers have begun to assess the effectiveness of seed dispersal by macaques, secondary dispersal of seeds originally dispersed by macaques, and the effects of provisioning on seed dispersal. Future studies should also test the effects of social factors (such as age and rank), which have received little attention in studies of seed dispersal.     

Aerial dispersal of phytoseiid mites (Acari: Phytoseiidae): estimating falling speed and dispersal distance of adult females

Aerial dispersal is important to immigration and redistribution of phytoseiid mites that often can provide biological control of spider mite pests. Falling speed of a mite and wind largely determine dispersal distance of such a passively blown organism. A diffusion model of wind-blown phytoseiids could provide insight into their dispersal. To this end, we measured body weights and falling speeds of adult females of 13 phytoseiid and one tetranychid mite species. These data were then incorporated into seed dispersal models (Greene and Johnson, Okubo and Levin) and results were compared to mite dispersal distances in wind tunnel, greenhouse and field. Weights of phytoseiid species ranged from 5.25 to 2l.7 μg; starved mites weighed less than fed mites. Geometric diameters ( d _g ) of idiosomas were correlated to weights. Falling speeds for phytoseiids were 0.39–0.73 m/s, and less than for T. urticae (0.79 m/s) in still air. In some species, active mites had slower falling speeds than inactive (anesthetized) mites indicating that behavior may influence falling. Starved mites had significantly slower falling speeds than fed mites and dispersed farther. Equation-based estimates of falling speed were close to measured ones (2–8% deviation) for some species. There were significant relationships between falling speed and body weight and morphological traits. Greene and Johnson's seed dispersal model provided better fits to dispersal of mites in the wind tunnel, greenhouse and field studies than Okubo and Levin's model. Limits of models in describing mite dispersal distance and applications to IPM are discussed.     

Dispersal Patterns of some Phyteuma Species (Campanulaceae)

Abstract: In the genus Phyteurna spadix-shaped spikes or capitula are composed of xerochasic poricidal capsules, holding unspecialized seeds. Phyteurna betonicifoliurn VILL., Phyteuma hemisphaericum L, Phyteurna scheuchzeri ALL., and Phyteurna spica-turn L. were examined in wind tunnel experiments to determine the minimum wind speed necessary for seed release, and the relationship between wind speeds and dispersal distances. In a simplified practical simulation the dispersal strategies of these species were described with a leptokurtic curve. The short-distance seed dispersal of Phyteuma hernisphaericurn allows a limited enlargement of the occupied area, while the seeds of P. scheuchzeri are dispersed more remotely from the mother plant. P. spicaturn and P. betonicifoliurn mainly disperse closely around the mother plant but provide a certain percentage of seeds for colonization of more distant areas. It is demonstrated that the dispersal modes are determined by the characteristics of fruits, infructescences, and seeds. It is also shown that plants with similar morphological organization show different dispersal patterns, which must be interpreted as a fine-tuned adaptation to the habitat with all its biotic and abiotic factors.     

Effects of seed size on dispersal distance in five rodent-dispersed fagaceous species

We studied the effect of seed size on dispersal by comparing dispersal distances in five rodent-dispersed fagaceous species (Lithocarpus harlandii, Quercus variabilis, Q. serrata, Cyclobalanopsis glauca, Castanopsis fargesii) with different seed size. We tracked individual seeds with coded tin-tags in two stands over 3 years in a subtropical evergreen broadleaved forest in the Dujiangyan Region of Sichuan Province, Southwest China. Our seed tracking data indicate that dispersal distances (including mean, maximum and distribution range) of seeds in primary caches and of seeds eaten after dispersal significantly increased with seed size, for both stands and all years. In addition, larger seeds (L. harlandii and Q. variabilis) were re-cached more often than smaller ones, which further reduced the relative density among caches and extended dispersal distances. Our findings indicate that greater dispersal distances for larger seeds might benefit the evolution of differences in seed size, and that scatter-hoarding might be advantageous for rodent-dispersed tree species.     

Human-Mediated Dispersal of Seeds by the Airflow of Vehicles

Human-mediated dispersal is known as an important driver of long-distance dispersal for plants but underlying mechanisms have rarely been assessed. Road corridors function as routes of secondary dispersal for many plant species but the extent to which vehicles support this process remains unclear. In this paper we quantify dispersal distances and seed deposition of plant species moved over the ground by the slipstream of passing cars. We exposed marked seeds of four species on a section of road and drove a car along the road at a speed of 48 km/h. By tracking seeds we quantified movement parallel as well as lateral to the road, resulting dispersal kernels, and the effect of repeated vehicle passes. Median distances travelled by seeds along the road were about eight meters for species with wind dispersal morphologies and one meter for species without such adaptations. Airflow created by the car lifted seeds and resulted in longitudinal dispersal. Single seeds reached our maximum measuring distance of 45 m and for some species exceeded distances under primary dispersal. Mathematical models were fit to dispersal kernels. The incremental effect of passing vehicles on longitudinal dispersal decreased with increasing number of passes as seeds accumulated at road verges. We conclude that dispersal by vehicle airflow facilitates seed movement along roads and accumulation of seeds in roadside habitats. Dispersal by vehicle airflow can aid the spread of plant species and thus has wide implications for roadside ecology, invasion biology and nature conservation.     

Secondary dispersal mechanisms of winged seeds: a review

Winged seeds, or samaras, are believed to promote the long‐distance dispersal and invasive potential of wind‐dispersed trees, but the full dispersive potential of these seeds has not been well characterised. Previous research on the ecology of winged seeds has largely focussed on the initial abscission and primary dispersal of the samara, despite it being known that the primary wind dispersal of samaras is often over short distances, with only rare escapes to longer distance dispersal. Secondary dispersal, or the movement of the seeds from the initial dispersal area to the site of germination, has been largely ignored despite offering a likely important mechanism for the dispersal of samaras to microhabitats suitable for establishment. Herein, we synthesise what is known on the predation and secondary dispersal of winged seeds by multiple dispersive vectors, highlighting gaps in knowledge and offering suggestions for future research. Both hydrochory and zoochory offer the chance for samaroid seeds to disperse over longer distances than anemochory alone, but the effects of the wing structure on these dispersal mechanisms have not been well characterised. Furthermore, although some studies have investigated secondary dispersal in samaroid species, such studies are scarce and only rarely track seeds from source to seedling. Future research must be directed to studying the secondary dispersal of samaras by various vectors, in order to elucidate fully the invasive and colonisation potential of samaroid trees.