Influence of Fruit Size and Weight on Wind Dispersal in Betula lenta, a Gap-colonizing Tree Species

Abstract

-The prevailing model of wind-borne seed dispersal assumes that variation in seed size and weight controls patterns of seed distribution by limiting exposure to horizontal airstreams. To test the applicability of this model in a natural dispersal situation, seeds of Betula lenta were collected downwind of a population along transects which trapped seeds (a) after primary dispersal through the air, and (b) after primary aerial dispersal plus secondary dispersal across a snow cover. In order to examine mechanisms of seed flight, weights of 296 seeds were experimentally modified and duration and distance of seed flight measured in laboratory trials. Rate of descent of modified seeds depended strongly on individual seed weight and wing loading, and lateral movement in a horizontal airstream was determined by rate of descent, as predicted. In the field, however, distance traveled by aerial movement was only weakly linked to seed size and weight, and the relationship did not hold during secondary dispersal near the ground. Thus, seed size and weight are not the principal factors controlling seed movement under all conditions. It may be necessary to invoke models based on airstream turbulence.