Pollen dispersal from isolated field crops: boundary-layer scaling and effects of source field size

Pollen dispersion from isolated field crops is studied using a combination of simple theoretical models and large eddy simulation experiments. The pollen plume above the source field is assumed to behave as a pollen concentration boundary layer. Boundary-layer scaling and the assumption of self-preservation lead to a similarity function for vertical concentration profiles that has an additional dependence on the relative importance of gravitational settling and turbulent diffusion. Models used to predict growth rates of internal boundary layers are modified to account for gravitational settling and predict the growth of the pollen plume. In addition, the nature of the boundary layer equation allows one to solve the problem of dispersion downwind from the source field as an initial value problem in which the vertical concentration profile at the trailing edge of the field is the initial condition. As a consequence, the effects of changing the size of the source field on the deposition patterns are fully characterized by the changes in this profile.

A suite of high resolution numerical experiments, in which pollen terminal velocity and size of source field are systematically modified, is generated using large eddy simulation. Analysis of simulation results confirms the validity of the main assumptions used to construct the theoretical model. The simple expression to predict the growth of the pollen plume agrees with numerical results. In addition, the dispersion patterns from different field sources collapse remarkably well when scaled according to the proposed boundary-layer theory. In particular, the height of the pollen boundary layer plume at the trailing edge of the source field is shown to be the appropriate length scale to characterize the effects of field size on deposition patterns.

The present results suggest that isolation distances for genetically modified crops should not be independent of field size. Rather, the values should be proportional to the height of the pollen boundary layer at the trailing edge. In addition, measurements from small test plots can be used to make predictions of dispersion from fields of any size. However, the height of the pollen boundary layer plume at the trailing edge of the source fields must be known.