The effect of collisions with vegetation elements on the dispersal of winged and plumed seeds
Journal of Ecology
1. The role of collisions with vegetation elements in seed dispersal by wind has been examined only anecdotally. In particular, the idea that the effect of collisions in dispersal may depend on the aerodynamic class of the diaspore has not been broached. 2. We adapted a collision model for small particles to predict the probability of a winged or plumed seed colliding with a vegetation element as a function of the Stokes number (a dimensionless parameter which quantifies inertial tendency and includes diaspore terminal velocity, wind speed and diameter of the target element). 3. We performed experimental releases of seeds upwind of tree boles in two deciduous forest types (temperate and dry tropical) for 10 mid-latitude and tropical species to test the collision model. The model was a reasonable expression of collision efficiency. At higher Stokes numbers, collisions were far more likely for seeds than for water droplets or other small particles. 4. Experimental releases were used in both forests to determine the effect of collisions with boles on distance travelled for several species. Seeds of the three species with asymmetric wings had significantly reduced dispersal following collisions; the single species with plumed seeds did not. 5. In a leafless tropical forest, we experimentally determined the frequency distribution of collisions per metre of travel for seeds released from a canopy. Approximately one collision occurred for every 2 m of flight through the volume occupied by branches and lianas. Distance achieved by asymmetric samaras was relatively unaffected by collisions with small vegetation elements, because the samaras quickly readopted stable autorotation. Conversely, bilaterally symmetric samaras had their dispersal greatly reduced. 6. Synthesis. The effect of collisions on dispersal depends on aerodynamic type. Collisions with small vegetation elements in forests should be more common for samaras than for plumed seeds, because plumed...