Wing loading, not terminal velocity, is the best parameter to predict capacity of diaspores for secondary wind dispersal

Abstract

Lift-off velocity may be the most useful surrogate to measure the secondary dispersal capacity of diaspores. However, the most important diaspore attribute determining diaspore lift-off velocity is unclear. Furthermore, it is not known whether terminal velocity used to characterize the primary dispersal capacity of diaspores can also be used to predict their secondary wind dispersal capacity. Here, we investigate how diaspore attributes are related to lift-off velocity. Thirty-six species with diaspores differing in mass, shape index, projected area, wing loading, and terminal velocity were used in a wind tunnel to determine the relationship between diaspore attributes and lift-off velocity. We found that diaspore attributes largely explained the variation in lift-off velocity, and wing loading, not terminal velocity, was the best parameter for predicting lift-off velocity of diaspores during secondary wind dispersal. The relative importance of diaspore attributes in determining lift-off velocity was modified by both upwind and downwind slope directions and type of diaspore appendage. These findings allow us to predict diaspore dispersal behaviors using readily available diaspore functional attributes, and they indicate that wing loading is the best proxy for estimating the capacity for secondary dispersal by wind.

Keywords: Appendage type; diaspore mass; diaspore shape index; downwind slope; terminal velocity; upwind slope; wind tunnel; wing loading.

Fig. 1.

Diagram illustrating measurement of the diaspore lift-off velocity by wind tunnel. (1) Starting point section, (2) power section, (3 and 4) diversion section, (5) rectifying section, (6) transition section, (7) draft gear, (8) road wheel, (9) electric landing gear, (10) experimental section, (11) experimental section of wind tunnel, (12) pitot tube, (13) rubber tube, (14) differential pressure transmitter, (15) diaspore, (16) wire net. The wind tunnel is 2 m in height, 2 m in width, and 20m in length. The upwind slope is 6–8º with the power section of wind tunnel at the lower part of the slope (a). The downwind slope is 6–8º with the power section at the upper part of the slope (b). The pitot tube was located 10 m away from the power section. It was inserted through the roof via the pitot hole, and wind speed was measured 1 m above the underlying surface (c).

Fig. 2.

Relationship between lift-off velocities on upwind (a, c) and downwind (b, d) slopes, and wing loading and terminal velocity for diaspores with different types of appendage. Values shown were mean ±SE. Total, Thorn, None, Appendage, and Fly, diaspores of all 36 species, diaspores with thorns, diaspores without appendages, diaspores with any appendages (wings, thorns, or hairs), and diaspores with wings or hairs, respectively. Solid regression lines correspond to 36 species (squares), short-dashed lines to diaspores with thorns (triangles), long-dashed lines to diaspores without any appendages (circles), short-dotted lines to diaspores with appendages (rhombi), and dash–dot lines to diaspores with appendages (pentagrams).