Pollen dispersal models in Quaternary plant ecology: Assumptions, parameters, and prescriptions

Models of atmospheric dispersal of anemophilous pollen are important tools in Quaternary plant ecology for determining pollen-source areas and for applying distance-weightings to vegetation data in formal pollen-vegetation calibrations. The most widely applied model is Prentice's model, which uses a modified form of Sutton's equation for atmospheric diffusion to predict pollen-source areas from size of the depositional basin and a set of depositional parameters (deposition velocity of the pollen grains and mean windspeed) and atmospheric parameters (turbulence parameter, vertical diffusion coefficient). We review the physical theory underlying Sutton's equation and Prentice's model, explore the effects of different values of the depositional and atmospheric parameters on model predictions, and provide prescriptions for model application, parameter specification, and further research on pollen dispersal. Most applications of the models to pollen dispersal have assumed neutral atmospheric conditions. We argue that most pollen dispersal takes place in unstable atmospheric conditions, and prescribe appropriate values for the atmospheric parameters for unstable conditions. Our simulations using these parameters indicate more widespread pollen dispersal from a source than under neutral conditions. We review available data sets for sedimentation velocity of pollen grains, and compare the measured estimates with sedimentation velocities predicted from Stokes's Law to assess validity of the data. Substantial variability exists among data sets, but several are suitable for application to pollen-dispersal models. Finally, we discuss aspects of release, dispersal, and deposition of anemophilous pollen that are in need of further theoretical and empirical study. Such studies will contribute not only to Quaternary plant ecology but also to understanding of pollination biology, population genetics, and functional morphology of pollen grains and pollen-bearing organs.