THE AERODYNAMICS OF POLLEN CAPTURE IN TWO SYMPATRIC EPHEDRA SPECIES

Wind-tunnel analyses of the behavior of airborne pollen around ovules of two Ephedra species (E. trifurca and E. nevadensis) indicate that at certain airflow speeds (0.5 m/sec and 1.0 m/sec) each species is capable of biasing pollination in favor of conspecific pollen. A computer procedure was designed to evaluate the physical basis for this aerodynamic discrimination. This procedure indicates that differences in size and density confer significantly different inertial properties to the two pollen species. Operating within the specific aerodynamic environments generated around ovules from each species, these differences are sufficient to account for the biases observed in the probability of pollination. Within natural populations, there exists significant variation in pollen size (and possibly in density). Accordingly, it is possible that, under certain ambient wind conditions, ovules from each species can select subsets of the entire airborne population of Ephedra pollen.