Mechanistic models of seed dispersal by wind

Over the past century, various mechanistic models have been developed to estimate the magnitude of seed dispersal by wind, and to elucidate the relative importance of physical and biological factors affecting this passive transport process. The conceptual development has progressed from ballistic models, through models incorporating vertically variable mean horizontal windspeed and turbulent excursions, to models accounting for discrepancies between airflow and seed motion. Over hourly timescales, accounting for turbulent fluctuations in the vertical velocity component generally leads to a power-law dispersal kernel that is censored by an exponential cutoff far from the seed source. The parameters of this kernel vary with the flow field inside the canopy and the seed terminal velocity. Over the timescale of a dispersal season, with mean wind statistics derived from an “extreme-value” distribution, these distribution-tail effects are compounded by turbulent diffusion to yield seed dispersal distances that are two to three orders of magnitude longer than the corresponding ballistic models. These findings from analytic models engendered explicit simulations of the effects of turbulence on seed dispersal using computationally intensive fluid dynamics tools. This development marks a bifurcation in the approaches to wind dispersal, seeking either finer resolution of the dispersal mechanism at the scale of a single dispersal event, or mechanistically derived analytical dispersal kernels needed to resolve long-term and large-scale processes such as meta-population dynamics and range expansion. Because seed dispersal by wind is molded by processes operating over multiple scales, new insights will require novel theoretical tactics that blend these two approaches while preserving the key interactions across scales.     

Mechanistic analytical models for long-distance seed dispersal by wind

We introduce an analytical model, the Wald analytical long-distance dispersal (WALD) model, for estimating dispersal kernels of wind-dispersed seeds and their escape probability from the canopy. The model is based on simplifications to well-established three-dimensional Lagrangian stochastic approaches for turbulent scalar transport resulting in a two-parameter Wald (or inverse Gaussian) distribution. Unlike commonly used phenomenological models, WALD's parameters can be estimated from the key factors affecting wind dispersal--wind statistics, seed release height, and seed terminal velocity--determined independently of dispersal data. WALD's asymptotic power-law tail has an exponent of -3/2, a limiting value verified by a meta-analysis for a wide variety of measured dispersal kernels and larger than the exponent of the bivariate Student t-test (2Dt). We tested WALD using three dispersal data sets on forest trees, heathland shrubs, and grassland forbs and compared WALD's performance with that of other analytical mechanistic models (revised versions of the tilted Gaussian Plume model and the advection-diffusion equation), revealing fairest agreement between WALD predictions and measurements. Analytical mechanistic models, such as WALD, combine the advantages of simplicity and mechanistic understanding and are valuable tools for modeling large-scale, long-term plant population dynamics.     

Evidence for secondary seed dispersal by rodents in Panama

Summary The data presented show thatVirola nobilis (Myristicaceae), a bird/mammal-dispersed tree species in Panama, may also be dispersed by a terrestrial rodent, the agouti (Dasyprocta punctata). Using a thread-marking method, we observed that agoutis scatterhoardedV. nobilis seeds that they found both singly or in clumps. Seed removal and seed burial rates were strongly affected by features of forest habitats, such asV. nobilis tree richness (rich vs poor) and/or forest age (old vs young), but not by seed dispersal treatment (scattered vs clumped). Predation (mostly post-dispersal) of unburied seeds by weevils was independent of habitat and dispersal treatment. Seeds artificially buried in aVirola-rich area were more likely to escape predation and become established than unburied seeds under natural conditions. The food reward for agoutis is in the germinating seedlings. The seed dispersal syndrome ofV. nobilis involves long- and short-distance dispersers which both appear important for tree recruitment.     

Juvenile hormone mediates a trade-off between primary and secondary sexual traits in stalk-eyed flies

Trade-offs between developing body parts may contribute to variation in allometric scaling relationships in a variety of taxa. Experimental evidence indicates that both circulating levels of juvenile hormone (JH) and sensitivities of developing body parts to JH can influence morphology in polyphenic insects. However, the extent to which JH may regulate both the development of traits that scale continuously with body size and trade-offs between these traits is largely unknown. Here, I present evidence that the JH analog methoprene applied to final instar larvae of a stalk-eyed fly (Cyrtodiopsis dalmanni) can induce males to produce larger eye-stalks relative to their body size. Examination of testis growth, sperm transfer, and egg maturation indicates that JH induces a trade-off between eye-span and gonad development in adult males, but not females. Age at sexual maturity was unaffected by larval JH applications to either sex. Collectively, these results are consistent with JH-mediated allocation of resources to eye-span at the expense of testes, and indicate potential costs for the production of an exaggerated trait.     

Season of fire influences seed dispersal by wind in a serotinous obligate seeding tree

Plant Ecology - In temperate ecosystems, fire management involving prescribed burning and wildfire suppression often causes a shift in fire season from hot and dry summer conditions to cooler,...     

Field experiments on seed dispersal by wind in ten umbelliferous species (Apiaceae)

This report presents data from experiments on seed dispersal by wind for ten species of the family Apiaceae. Seed shadows were obtained in the field under natural conditions, using wind speeds between four and ten m/s. The flight of individual seeds was followed by eye, and seed shadows were acquired, with median distances varying from 0.7 to 3.1 m between species. Multiple regression models of wind speed and seed weight on dispersal distance were significant for six out of ten species; wind speed had significant effects in seven cases, but seed weight only once. A good correlation between mean terminal falling velocity of the seeds of a species and median dispersal distance, indicates the promising explanatory power that individual terminal velocity data might have on dispersal distance, together with wind speed and turbulence. The theory that seeds that seem to be adapted to wind dispersal travel much longer distances than seeds that have no adaptation was tested. Flattened and winged seeds were indeed found to be transported further by wind, but not much further. Moreover, the species with wind-adapted seeds were also taller, being an alternative explanation since their seeds experienced higher wind speeds at these greater heights. Furthermore, flattened and winged seeds were disseminated from ripe umbels at lower wind speeds in the laboratory. This means that the observed difference in dispersal distance would have been smaller when species specific thresholds for wind speed were incorporated in the field experiments. We argue therefore, that seed morphology is not always the best predictor in classifying species in groups with distinctly different dispersal ability.     

Splash seed dispersal by raindrops

Splash seed dispersal by raindrops was investigated for plants in southern Japan. Nine families, 10 genera and 19 species were confirmed as raindrop-dispersed plants. The 10 genera were Gentiana, Gratiola, Chrysosplenium, Mazus, Mitella, Ophiorrhiza, Sagina, Sedum, Trigonotis and Veronica. The method of splash rain dispersal in these species was clarified. Raindrop-dispersed species were all small herbaceous plants with a vertical pedicel and an apically opening fresh capsule when the seeds mature. Open capsules were cup-shaped or boat-shaped and can accommodate raindrops easily. The raindrops splashed the seeds from the capsule. In general, the seeds weighed very little, but they were heavier than powder or dust seeds dispersed by wind. A strong negative correlation was found between seed weight and the number of seeds per capsule. In the case of Trigonotis brevipes (Maxim.) Maxim., raindrops were received into the cup-shaped calyx-tube and dispersed the fruitlets. Some species, such as Gentiana thunbergii (G. Don) Griseb., Gentiana zollingeri Fawcett and Ophiorrhiza japonica Blume, had hydroscopic movement capsules that opened widely only when wet. Raindrop-dispersed plants were found in various habitats. For example, some plants grew together on rocks along the mountain torrents where splash water could easily be caught. The results of the laboratory and field experiments indicated that the dispersal distance of seeds by raindrops was 1m or less. For small herbaceous plants, splash dispersal by rain might be an effective and advantageous method of seed dispersal because dispersal is not affected by plant height.     

Long-distance seed dispersal by wind: disentangling the effects of species traits,vegetation types,vertical turbulence and wind speed

Long-distance dispersal (LDD) of plant seeds by wind is affected by functional traits of the species, specifically seed terminal velocity and height of seed release above the vegetation cover (HAC), as well as by the meteorological parameters wind speed and vertical turbulence. The relative importance of these parameters is still under debate and the importance of their variability in vegetation types, sites and years has only rarely been quantified. To address these topics, we performed simulation studies for different vegetation types, sites, years and plant species with PAPPUS, a process based trajectory model. We found that LDD (measured in terms of migration rates) was higher in forests compared to open landscapes. Forests also showed greater between-year variability in LDD. Terminal velocity had an effect on LDD in both vegetation types, while the effect of HAC was significant only in the open landscape. We found considerable differences in how vertical turbulence and wind speed affect LDD between species and vegetation types: In the open landscape the strength of the positive relationship between vertical turbulence and LDD generally decreases with terminal velocity, whereas it increases in forests. The strength of the predominantly positive effect of wind speed on LDD increases with terminal velocity in both vegetation types, while in forests we found even negative relationships for species with low terminal velocity. Our results generally suggest that the effects of vertical turbulence and wind speed on LDD by wind diverge for species with different functional traits as well as in different vegetation types.     

Interactions between propagule pressure and seed traits shape human-mediated seed dispersal along roads

Human-mediated dispersal along the road network is a crucial process in the population dynamics of roadside vegetation and during plant invasions. The potential for a species to be dispersed by vehicles is, however, difficult to quantify. The predictive power of categorical classification schemes of human-mediated dispersal is limited as many species that are usually attributed to particular primary dispersal vectors may become subject to very different secondary dispersal vectors owing to human activity. Analysing seed traits that promote seed transport by human dispersal vectors could overcome these limitations. However, the analysis has to account for the divergent chance of seed transport that results from different propagule pressures along the transport corridor.To reveal the effects of seed traits and their interplay with propagule pressure on the chance and magnitude of human-mediated dispersal by vehicles, we compared traits and regional frequencies of a set of species that were dispersed by vehicles to a control set not dispersed but present in the same study area. We then used the same traits for a comparison of intentionally and unintentionally introduced species with the flora of Berlin.Different traits influenced the chance of vehicle dispersal and its magnitude. While propagule pressure was most important for determining the magnitude of seed transport, small seed mass and size best predicted the absolute chance of species dispersal by vehicles. The dispersal of nonnative species was least dependent on propagule pressure.Seed traits that were important in vehicle dispersal were similarly reflected in unintentionally introduced species in the Berlin flora. Mean seed size of these species was lower compared to the entire Berlin flora, whereas it was higher for intentionally introduced species. This suggests that unintentional introduction of nonnative plant species pre-selects for seed traits that promote further spread by human-mediated adhesive dispersal.Probability and magnitude of adhesive seed transport by vehicles can be predicted by dispersal-related plant traits. However, the effect size of plant traits on dispersal strongly depends on regional propagule pressure. This highlights the need to analyse interactions between species traits and propagule pressure.     

Long-distance dispersal of tree seeds by wind

Some mechanisms that promote long-distance dispersal of tree seeds by wind are explored. Winged seeds must be lifted above the canopy by updrafts to have a chance of further dispersal in high velocity horizontal winds aloft or in landscape-scale convection cells. Shear-induced turbulent eddies of a scale up to one-third of canopy height provide a lifting mechanism. Preliminary data suggest that all seeds of a given species may be viable candidates for uplift and long-distance dispersal, despite the evidence that slow-falling seeds are dispersed farther under any given wind conditions. Turbulence is argued more often and more extensively to advance long-distance dispersal than to retard it. Seeds may take advantage of Bernoulli sailing to move with faster than average winds. Elasticity of branches and trees may play a role in regulating the release of seeds into unusually favorable winds. Dispersal is at least biphasic, and the study of long-distance dispersal calls for mixed models and mixed methods of gathering data.     

A comparison between primary and secondary toe-to-hand transplantation

Although primary toe-to-hand transplantation is performed with increasing frequency, its use is still controversial because of the lack of any comparative studies documenting its safety and efficacy. Between August of 1990 and December of 1993, 175 consecutive toe-to-hand transplantations for crush and avulsion injuries were performed in 122 patients. The average interval between injury and primary reconstruction was 7 days, and the average interval between injury and secondary reconstruction was 10.7 months. Follow-up ranged from 18 to 91 months, with an average follow-up of 58 months. There were 31 primary transplantations and 144 secondary transplantations. The survival rate was 96.8 percent (30 of 31) for primary reconstruction and 96.5 percent (139 of 144) for secondary reconstruction. Intraoperative anastomotic revision was necessary in 3.2 percent (one of 31) of primary transplantations and 7.6 percent (11 of 144) of secondary transplantations. Three primary toe-to-hand transplantations (9.7 percent) and 17 secondary toe-to-hand transplantations (11.8 percent) were re-explored in the postoperative period. Each group had one superficial infection. The infection rate was 6.5 percent and 0.7 percent in the primary and secondary groups, respectively. Other complications included partial skin loss, which occurred in one patient (3.2 percent) in the primary group and six patients (4.2 percent of 144 transplantations) in the secondary group. Secondary procedures to improve function were necessary in six secondary transplantations (4.2 percent) and in none of the primary transplantations. There was no statistical difference between the two groups in terms of survival, intraoperative anastomotic revision, re-exploration, future secondary procedure, infection, and complications. This series demonstrates that primary toe-to-hand transplantation can be performed in the suitable candidate safely with as much success as secondary reconstruction. Primary toe transplantation can potentially reduce the overall period of recovery and rehabilitation, allowing the patient to return to work sooner. Further study to evaluate and compare the final functional outcome and return to work time between primary and secondary toe-to-hand transplantation is needed.     

A rain tower and wind tunnel for studying the dispersal of plant pathogens by rain and wind

The rain tower/wind tunnel complex at Rothamsted consists of a rain tower (height 11 m, cross-section 1 m²) linked to the upwind end of a wind tunnel (length 12 m, cross-section 1 m²), which may be operated in either an open or a closed configuration. At the top of the rain tower, water drops with diameters of 2.5 to 5 mm are produced by a drop generator, which can be fitted with different nozzles. Simulated rain with a drop diameter of 1 to 3 mm is produced at a rate of 8 to 12 mm h^-1 by a rain generator with an area of 52 × 67 cm. The rain tower may be operated in conjunction with the wind tunnel in an open configuration. The windspeed can be decreased from a maximum of 8 m s^-1 by decreasing the speed of the fan. The wind tunnel has its own internal lighting. When the wind tunnel is in a closed configuration, temperature and humidity can be controlled in the range 12 ^oC (62–80% r.h.) to 35 ^oC (22–50% r.h.). Data presented illustrate the use of this rain tower/wind tunnel complex to study dispersal of plant pathogen spores by rain-splash or wind.     

A novel trap for quantifying the dispersal of seeds by wind

Question: Understanding the aerial movement of seed is of great significance to the management of native and invasive plant species, but has proven difficult to measure. Here we examine how a more quantitative approach to measuring the aerial movement of seed can be achieved. Location: SE Australia. Methods: We describe a novel seed trap (the ‘Melbourne trap’), for which the proportion of free‐stream airflow through the trap can be measured, allowing a more quantitative approach to measuring aerial seed movement. We assessed airflow through the Melbourne trap in a wind tunnel and describe how this information, along with measurements of wind speed and direction, can now be used to derive seed density per volume of airflow. We compare the seed capture and retention efficiency of the Melbourne trap with two simpler and cheaper trap designs, bucket traps and sticky traps. Results: Melbourne and bucket traps captured significantly more species than sticky traps. Seed catch was dominated numerically by Lachnagrostis filiformis (G. Forst.) Trin. Melbourne traps proved more effective than sticky traps, but not bucket traps, in capturing L. filiformis, based on intake area. For all other seeds, Melbourne traps were more effective than both bucket and sticky traps. Conclusion: The Melbourne trap design is a significant advance in quantifying seed dispersal by wind. Melbourne traps will improve the capacity and accuracy of studies that seek to: (i) quantify seed fluxes across landscapes boundaries; (ii) assess directionality of dispersal; (iii) understand processes controlling seed release; and (iv) compare dispersal in wind and water.     

Tamarind tree seed dispersal by ring-tailed lemurs

In Madagascar, the gallery forests of the south are among the most endangered. Tamarind trees (Tamarindus indica) dominate these riverine forests and are a keystone food resource for ring-tailed lemurs (Lemur catta). At Berenty Reserve, the presence of tamarind trees is declining, and there is little recruitment of young trees. Because mature tamarinds inhibit growth under their crowns, seeds must be dispersed away from adult trees if tree recruitment is to occur. Ring-tailed lemurs are likely seed dispersers; however, because they spend much of their feeding, siesta, and sleeping time in tamarinds, they may defecate a majority of the tamarind seeds under tamarind trees. To determine whether they disperse tamarind seeds away from overhanging tamarind tree crowns, we observed two troops for 10 days each, noted the locations of feeding and defecation, and collected seeds from feces and fruit for germination. We also collected additional data on tamarind seedling recruitment under natural conditions, in which seedling germination was abundant after extensive rain, including under the canopy. However, seedling survival to 1 year was lower when growing under mature tamarind tree crowns than when growing away from an overhanging crown. Despite low fruit abundance averaging two fruits/m³ in tamarind crowns, lemurs fed on tamarind fruit for 32% of their feeding samples. Daily path lengths averaged 1,266 m, and lemurs deposited seeds throughout their ranges. Fifty-eight percent of the 417 recorded lemur defecations were on the ground away from overhanging tamarind tree crowns. Tamarind seeds collected from both fruit and feces germinated. Because lemurs deposited viable seeds on the ground away from overhanging mature tamarind tree crowns, we conclude that ring-tailed lemurs provide tamarind tree seed dispersal services.     

A telemetric thread tag for tracking seed dispersal by scatter-hoarding rodents

The seeds of many tree species are dispersed more than once, and this secondary seed dispersal is believed to enhance seedling recruitment. However, the effectiveness of secondary seed dispersal has rarely been assessed because it is difficult to track seeds until they die or germinate. We describe a new technique that uses thread tags attached to radio transmitters (telemetric thread tags) to track long-distance multistep seed dispersal by scatter-hoarding rodents. These telemetric thread tags can be turned off with a magnet and are reactivated when the seed moves. This method allows for seed tracking with minimal cache disturbance or distance bias, over long time spans, multiple seed movements, and with few effects on animal behavior. We used telemetric thread tags to track seed dispersal of the palm tree Astrocaryum standleyanum in a Neotropical forest, and achieved near-complete recovery of dispersed seeds tracked over distances as far as 241?m. We were also able to record the recovery time and fate of cached seeds without disturbing caches. Neither the removal rate nor the dispersal distance differed between seeds with telemetric thread tags and thread-tagged seeds. We conclude that telemetric thread tags can be used to document secondary seed dispersal by scatter-hoarding animals with unprecedented efficacy and precision. Given the size of these tags relative to the size of seeds and their dispersers, this method is applicable to the majority of tree species that are secondarily dispersed by scatter-hoarding mammals.