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.     

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.     

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.     

A trade-off between primary and secondary seed dispersal by wind

Plant Ecology - The seeds of most plant species are dispersed by multiple mechanisms. Whether functional traits mediate positive correlations or negative correlations (trade-offs) between different...     

Environmental variability and the initiation of dispersal: turbulence strongly increases seed release

Dispersal is a critical process in ecology. It is an important biological driver of, for example, invasions, metapopulation dynamics, spatial pattern formation and pathogen movement. Much is known about the effect of environmental variability, including turbulence, on dispersal of diaspores. Here, we document experimentally the strong but under-explored influence of turbulence on the initiation of dispersal. Flower heads of two thistle species (Carduus nutans and Carduus acanthoides) with ripe seeds were exposed to series of laminar and turbulent air flows of increasing velocity in a wind tunnel. Seed release increased with wind speeds for both laminar and turbulent flows for both species. However, far more seeds were released, at significantly lower wind speeds, during turbulent flows. These results strongly suggest a need for more quantitative studies of abscission in the field, as well as dispersal models that incorporate variability in the diaspore release phase.     

Seed dispersal by wind,birds, and bats between Philippine montane rainforest and successional vegetation

In the moist Neotropics, vertebrate frugivores have a much greater role in the dispersal of forest and successional woody plants than wind, and bats rather than birds play the dominant role in dispersing early successional species. I investigated whether these patterns also occurred in a Philippine montane rainforest and adjacent successional vegetation. I also asked whether seed mass was related to probability of dispersal between habitats. A greater number of woody species and stems in the forest produced vertebrate-dispersed seeds than wind-dispersed seeds. Although input of forest seeds into the successional area was dominated by vertebrate-dispersed seeds in terms of species richness, wind-dispersed seeds landed in densities 15 times higher. Frugivorous birds dispersed more forest seeds and species into the successional area than bats, and more successional seeds and species into the forest. As expected, seed input declined with distance from source habitat. Low input of forest seeds into the successional area at the farthest distance sampled, 40 m from forest edge, particularly for vertebrate-dispersed seeds, suggests very limited dispersal out of forest even into a habitat in which woody successional vegetation provides perches and fruit resources. For species of vertebrate-dispersed successional seeds, probability of dispersal into forest declined significantly with seed mass.     

Differences in life history traits of relatedEpilobium species: Clonality, seed size and seed number

Small changes in morphology can affect the performance and functions of organisms and hence their ecological success. In modular constructed plants, contrasting growth strategies may be realized by differences in the spatial arrangement and size of shoots. Such differences change the way in which meristems and resources are assigned to various functions during the lifespan of a plant. If such changes include the capacity to spread clonally, sexual reproduction may also be affected. I compare patterns in vegetative growth and sexual reproductive traits in four allopatric species ofEpilobium which are sometimes considered as subspecies of a single polymorphic taxon. The four species differ in the location of the buds which annually renew the aerial shoot system.E. dodonaei andE. steveni do not spread clonally and are characterized by a shrub-like habit.E. fleischeri, a species occurring only in the Alps, andE. colchicum, which occurs in the upper region of the Caucasus mountains, both produce buds on horizontal roots or plagiotropic shoots. Both alpine species exhibiting clonal growth have smaller shoots, fewer fruits and smaller seeds than the lowland species. An intraspecific trade-off between seed number per fruit and seed mass is realized. Both alpine species produce more seeds per fruit at the expense of seed mass. The morphological relationship between the four species and their geographical distribution suggest that clonal growth inE. fleischeri (restricted to the Alps) andE. colchicum (restricted to the Caucasus) is adaptively associated with the stressful conditions of alpine habitats. Our results suggest that clonal growth is not necessarily correlated with reduced reproduction by seeds. The success of plants which are already established may largely depend on clonal spread, but the colonization of new habitats depends on the production of a large number of small seeds with high dispersability.     

Conflicting selection on diaspore traits limits the evolutionary potential of seed dispersal by ants

Conflicts of selection on diaspore traits throughout the dispersal cycle can limit the evolutionary consequences of seed dispersal. However, these conflicts have never been investigated in directed dispersal systems. We explored conflicts of selection through life stages of dispersal in the myrmecochorous herb Helleborus foetidus. Seeds are subject to two contrasting partial selective scenarios. Undispersed seeds are subject to positive directional selection on seed size characters, whereas seeds dispersed are subject to stabilizing selection for size. In both scenarios, seedling establishment determined the magnitude and direction of selection. This does not reflect ant preferences for seed size. However, total selection still depends largely on ant activity, as ants control the relative importance of each selective scenario. We advocate the use of analytical approaches combining multiplicative fitness and microenvironment‐specific selection to more realistically estimate the realized selection on traits functional during several life stages. This approach may be extended to any organism dispersing offspring to different environments.     

果实类型和种子传播的进化:系统发育和生态学简论

Success of flowering plants is greatly dependent on effective seed dispersal. Specific fruit types aid different mechanisms of seed dispersal. However, little is known about what evolutionary forces have driven the diversification of fruit types and whether there were phylogenetic constraints on fruit evolution among angiosperm lineages. To address these questions, we first surveyed the orders and families of angiosperms for fruit types and found no clear association between fruit types and major angiosperm lineages, suggesting there was little phylogenetic constraint on fruit evolution at this level. We then surveyed fruit types found in two contrasting habitats an open habitat including the Indian desert and North American plains and prairies, and a closed forest habitat of Australian tropical forest. The majority of genera in the survey of tropical forests in Australia were fleshy fruit trees, whereas the majority of genera in the survey of prairies and plains in central North America were herbs with capsules and achenes. Both capsules and achenes are frequently dispersed by wind in the open, arid habitat, whereas fleshy fruits are generally dispersed by animals. Since desert and plains tend to provide continuous wind to aid dispersal and there are more abundant mammal and bird dispersers in the closed forest, this survey suggests that fruit evolution was driven at least in part by dispersal agents abundant in particular habitats.     

Growth strategies of tropical tree species: disentangling light and size effects

An understanding of the drivers of tree growth at the species level is required to predict likely changes of carbon stocks and biodiversity when environmental conditions change. Especially in species-rich tropical forests, it is largely unknown how species differ in their response of growth to resource availability and individual size. We use a hierarchical bayesian approach to quantify the impact of light availability and tree diameter on growth of 274 woody species in a 50-ha long-term forest census plot in Barro Colorado Island, Panama. Light reaching each individual tree was estimated from yearly vertical censuses of canopy density. The hierarchical bayesian approach allowed accounting for different sources of error, such as negative growth observations, and including rare species correctly weighted by their abundance. All species grew faster at higher light. Exponents of a power function relating growth to light were mostly between 0 and 1. This indicates that nearly all species exhibit a decelerating increase of growth with light. In contrast, estimated growth rates at standardized conditions (5 cm dbh, 5% light) varied over a 9-fold range and reflect strong growth-strategy differentiation between the species. As a consequence, growth rankings of the species at low (2%) and high light (20%) were highly correlated. Rare species tended to grow faster and showed a greater sensitivity to light than abundant species. Overall, tree size was less important for growth than light and about half the species were predicted to grow faster in diameter when bigger or smaller, respectively. Together light availability and tree diameter only explained on average 12% of the variation in growth rates. Thus, other factors such as soil characteristics, herbivory, or pathogens may contribute considerably to shaping tree growth in the tropics.     

Uncoupling the effects of seed predation and seed dispersal by granivorous ants on plant population dynamics

Secondary seed dispersal is an important plant-animal interaction, which is central to understanding plant population and community dynamics. Very little information is still available on the effects of dispersal on plant demography and, particularly, for ant-seed dispersal interactions. As many other interactions, seed dispersal by animals involves costs (seed predation) and benefits (seed dispersal), the balance of which determines the outcome of the interaction. Separate quantification of each of them is essential in order to understand the effects of this interaction. To address this issue, we have successfully separated and analyzed the costs and benefits of seed dispersal by seed-harvesting ants on the plant population dynamics of three shrub species with different traits. To that aim a stochastic, spatially-explicit individually-based simulation model has been implemented based on actual data sets. The results from our simulation model agree with theoretical models of plant response dependent on seed dispersal, for one plant species, and ant-mediated seed predation, for another one. In these cases, model predictions were close to the observed values at field. Nonetheless, these ecological processes did not affect in anyway a third species, for which the model predictions were far from the observed values. This indicates that the balance between costs and benefits associated to secondary seed dispersal is clearly related to specific traits. This study is one of the first works that analyze tradeoffs of secondary seed dispersal on plant population dynamics, by disentangling the effects of related costs and benefits. We suggest analyzing the effects of interactions on population dynamics as opposed to merely analyzing the partners and their interaction strength.     

The role of seed dispersal and seedling traits in colonization and coexistence ofSalix species in a seasonally flooded habitat

Regeneration traits of six co-occurringSalix species were studied on a floodplain of the Sorachi River, central Hokkaido, Japan, and their colonization success and coexistence in a local habitat were discussed. MixedSalix communities contained sixSalix species; dominant:S. sachalinensis; four subordinates:S. rorida, S. pet-susu, S. miyabeana andS. subfragilis; rare species:S. jessoensis. Their phenology, falling velocity and longevity of seeds, and the effects of microtopography and soil texture on seedling establishment were studied. The sixSalix species had overlapped seed dispersal periods that coincided with the decrease of water level after a predictable spring flood. This coincidence was crucial for the colonization success because the seedlings were established on wet soils left by the decreasing water level. They showed two types of regeneration trait, specialization and generalization.S. rorida andS. subfragilis showed contrasting regeneration traits; early vs. late seed dispersal, large vs. small seeds, seedling distribution on coarse vs. fine soils, respectively. These two species rarely co-occurred. On the other hand, the dominantS. sachalinensis had an intermediate seed size and dispersal timing, and a wide range of seedling distribution from coarse to fine soils. These results revealed that flooding seasonality influenced the colonization success together with the regeneration traits ofSalix species, and that coexistence of theSalix species was facilitated primarily by regeneration niche separation related to flooding seasonality and soil heterogeneity.     

Aerial seed bank and dispersal traits in Anthemis chrysantha (Asteraceae), a critically endangered species

Mature plant density and fruit production were monitored in the main population of four successive cohorts of the endangered winter annual Anthemis chrysantha (Asteraceae) in southeastern Spain. Experiments were conducted with artificial rainfall and a wind tunnel to determine the temporal and spatial dispersal pattern of the species and the relationship with rain and wind.     

Theoretical habitat templets, species traits, and species richness: floodplain vegetation in the Upper Rhône River

• 1 The floodplain vegetation at approximately 100 sites located in nine different habitat types of the Upper Rhône River, France, was surveyed three times over the past 27 years. Information on species traits of the higher plants comprising the Rhône floodplain vegetation was based on studies conducted between Geneva, Switzerland, and Lyon, France.
• 2 These data were structured using a ‘fuzzy coding’ technique and then examined using ordination analyses to investigate: (i) relationships among species traits; (ii) habitat utilization; (iii) the relationship between species traits and habitat utilization; and (iv) trends of species traits and species richness in the framework of spatial–temporal habitat variability to test predictions of the river habitat templet and the patch dynamics concept.
• 3 Size, number of descendants per reproductive cycle, number of reproductive cycles per individual, and the regeneration potential of an individual were positively related with each other, whereas the degree of attachment to the soil decreased, and the reproductive period shifted from autumn/late summer towards early summer/spring, as size increased.
• 4 The habitat utilization by the higher plants of the floodplain revealed a double lateral gradient: the first was from the banks of the temporary waters to terrestrial flats; the second from aggrading pebble to aggrading silt habitats. These gradients were related to gradients in water saturation, oxygen conditions, nutrient loading, and nutrient retention of the soils.
• 5 A significant relationship between species traits and habitat utilization was observed for the floodplain vegetation, i.e. plant communities used particular habitat types with a particular set of species trait modalities (= categories).
• 6 Patterns of species trait modalities were significantly related to temporal and spatial habitat variability but only modalities of the trait ‘parental care’ conformed to trends predicted from theory.
• 7 No trends were observed when species richness of different habitat types was considered in the framework of spatial–temporal habitat variability.
• 8 Although the habitats of the Upper Rhône clearly act as a templet for the species traits of the floodplain vegetation, the lack of agreement between observations and predictions on trends in species traits and richness in terms of habitat variability suggest that important elements of theory should be rejected. However, human-induced changes in these habitats are too recent when compared with the longer time periods required for floodplain vegetation to respond to such changes.

     

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,...