Which factors have stronger explanatory power for primary wind dispersal distance of winged diaspores: the case of Zygophyllum xanthoxylon (Zygophyllaceae)?

Aims How seed dispersal distance is related to various factors is a major challenge for seed ecologists. However, there are different answers as to which factor is most important in determining wind dispersal distance. This study is to quantitatively describe the relationship between various factors and primary wind dispersal distance of winged diaspores. Methods The dispersal distances of five morphologies of winged diaspores in Zygophyllum xanthoxylum (Zygophyllaceae) were measured under controlled conditions in a wind tunnel. The explanatory power of environmental factor (i.e. wind speed), plant trait (i.e. release height) and diaspore attributes (i.e. wing loading (the ratio of diaspore mass to projected area), settlement-velocity, shape index (the variance of diaspore length, width and thickness)) to the variation in dispersal distance was assessed by releasing diaspores at varying wind speeds and release heights. Important Findings Wind speed and seed release height were the strongest explanatory factors to dispersal distance, contributing 41.1% and 24.8% (P < 0.01) to total variation in dispersal distance, respectively. Wind speed accounted more for relatively light disc-shaped seeds than for relatively heavy spherical seeds. Wing loading, shape index and settlement-velocity explained 9.0% (P < 0.01), 1.4% (P < 0.01) and 0.9% (not significant) of the variation in dispersal distance, respectively. From disc-shaped to four-winged diaspores, relative contributions of wing loading and shape index decreased but contribution of settlement-velocity increased. The relative contributions of various factors to wind seed dispersal distance may change with the change in seed morphology.