Can the Variation in Samara Mass and Terminal Velocity on an Individual Plant Affect the Distribution of Dispersal Distances?

We examined the contribution of seed mass variation to (1) variation in terminal velocity and (2) variation in dispersal distances for species with winged seeds. Coefficients of variation of terminal velocity were usually on the order of 0.13. These values were too low to have a significant effect on distance variation because of the much larger magnitude of the turbulent intensities of the wind in the vertical and horizontal directions. This assertion was tested with a micrometeorological model of dispersal from an elevated point source within a forest canopy. The coefficient of variation of terminal velocity was necessarily low because of (1) the aerodynamic specification that terminal velocity be proportional to the square root of mass and (2) the design problems inherent in a negative correlation between mass and planform area. We conclude that there is unlikely to be selection for mass variation to reduce sibling densities and therefore intersib competition.