Applications of particle image velocimetry for seed release studies.

Nonrandom seed release is an important determinant of how far seeds disperse, but the mechanisms that promote wind-related seed release under varying atmospheric conditions are poorly understood. We explored the use of particle image velocimetry (PIV) to gain a better mechanistic understanding of seed release by visualizing the flow velocities and vorticity in a two-dimensional slice of air around inflorescences. Pilot data taken in a wind tunnel show gradients in air velocity at the top of Carduus nutans capitula that may contribute to lift generation. Additionally, von Kármán vortex streets (vortices of opposite spin that are shed from the wake of an object) were observed shedding from capitula, which cause lateral forces on capitula and increase turbulence downwind at other locations. Avenues for further research include using PIV to examine the mechanisms of seed release and dispersal in wind tunnels and in the field. We found PIV to be a promising method to further explore the mechanisms behind seed release in wind dispersed plants, and a technique rich with opportunities for collaborations between plant dispersal ecologists and fluid dynamics specialists.

Seed release by invasive thistles: the impact of plant and environmental factors.

Dispersal is a key process in biological studies of spatial dynamics, but the initiation of dispersal has often been neglected, despite strong indications that differential timing of dispersal can significantly affect dispersal distances. To investigate which plant and environmental factors determine the release of plumed seeds by the invasive thistles Carduus acanthoides and Carduus nutans, we exposed 192 flower heads of each species to increasing wind speeds in a full-factorial wind tunnel experiment with four air flow turbulence, three flower head wetness and two flower head temperature levels. The number of seed releases was highest under dry and turbulent conditions and from heads that had already lost a considerable number of seeds, but was not affected by flower head size, head angle or temperature. Inspection of the trials on video showed that higher wind speeds were needed to meet the seed release threshold in laminar flows and for C. acanthoides heads that had been wet for a longer time. Species differences were minimal, although seed release was more sensitive to lower levels of turbulence in the larger-headed and more open C. nutans heads. Knowledge of seed release biases towards weather conditions favourable for long-distance dispersal improves our understanding of the spread of invaders and allows managers to increase the efficiency of their containment strategies by applying them at crucial times.

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.