Simulating the effects of local wind reduction on shrub seed dispersal

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The dynamics of vegetation communities are highly dependent on seed dispersal kernels (the probability distribution functions of dispersed seeds). Wind-driven dispersal is important in both microscale and macroscale ecological systems. As such, many field, theoretical derivation and numerical simulation studies have aimed to obtain the distribution functions of dispersed seeds. On the one hand, owing to the complexity of real cases, theoretical derivations cannot always reproduce field observations because many of the parameters have been simplified. On the other hand, field measurements cannot both gather all of the dispersed seeds from the plants and provide sufficient wind information, which suggests that empirical functions are usually case dependent.

Numerical simulations are a more economical and flexible method of studying wind dispersal of seeds in both ideal cases, such as those commonly found in theoretical modelling studies, and complicated field cases. The reliability of numerical simulations depends on having a comprehensive and accurate description of the factors that can affect dispersal of seeds. The influences of many factors, including the physical properties of seeds, the time-averaged wind speed and the wind turbulence, on seed dispersal have been studied. However, in previous studies, wind speed has usually been assumed to be only height-dependent, whilst variation in wind speed in the streamwise direction has generally not been considered.

Illustrations of the local wind reduction in the lee of a single shrub element (A) and the distribution pattern of the wind friction velocity near the surface both within and in the lee of the shrub element (B). H and D denote the shrub height and diameter, respectively, and L_x denotes the maximum streamwise length of the wind reduction region. (A) The region enclosed by dashed lines is the wind reduction region. (B) The colour depth along the streamwise direction suggests the change of the wind friction velocity (light colour denotes low velocity); the dotted lines suggest the identical friction velocities. Image credit: L.T. Fu.

In their new study published in AoBP, Lin-Tao Fu numerically simulated the trajectories of seeds released from a single shrub element. In the study, they considered the spatial distribution of wind intensity around the shrub element, with an emphasis on variation in the streamwise direction rather than just the vertical. Shrub communities are particularly important to arid regions and the edges of deserts, as they provide a buffer to reduce wind erosion. Therefore, it is critical that we are able to accurately predict the development of shrub communities and how this is influenced by wind.

The simulation results revealed that localised reduction in wind increased seed deposition in nearby regions and decreased seed deposition in the regions farther away. Wind intensity and release height weaken the effect of local wind reduction. Shrub porosity promoted seed dispersal by wind. The results of this numerical simulation may help to explain common disagreements between theoretical mechanistic models and fitting curves to real field data. The results of this study may improve currently used mechanistic seed dispersal models by either increasing their flexibility in case studies or by helping explain the variations in observed distributions.