The influence of vegetation structure on lift-off velocity of diaspores during secondary wind dispersal

Lift-off velocity, the wind speed at which a diaspore starts moving during secondary wind dispersal, representing diaspore mobile capacity, may greatly depend on vegetation where dispersal takes place. However, how vegetation structure influences lift-off velocity and which is the most important vegetation attribute remain unknown. The vegetation with various attributes (four coverages-0, 10 %, 20 % and 30 %, three life-form compositions-herb, shrub and herb + shrub, two vertical patterns-one and two layers, and two horizontal patterns-uniform and aggregated) were set up for the wind tunnel experiment. Diaspores of 30 species varied in wing loading, terminal velocity, mass, projected area, length, width, height, and shape index were selected to measure their lift-off velocity within various vegetation structures. Lift-off velocity significantly increased with the increasing vegetation coverage, in the case of any life-form composition, vertical pattern and horizontal pattern of vegetation. For the vertical pattern of vegetation, the lift-off velocity in one-layer vegetation was significantly higher than that in two-layer vegetation, but only for 10 % vegetation coverage. Both the life-form composition and the horizontal pattern of vegetation had no influences on lift-off velocity. The correlation between lift-off velocity and diaspore attributes (length, width, height, projected area, mass and wing loading) increased with vegetation coverage and the correlation in two-layer vegetation was larger than that in one-layer vegetation. Our results indicate that vegetation coverage is the key vegetation attribute determining lift-off velocity during secondary wind dispersal. Lift-off velocity is also influenced by diaspore traits, but their relationships tend to be regulated by vegetation attributes, especially the coverage and vertical pattern of vegetation. Our study demonstrates the important role of vegetation coverage in determining the second dispersal capacity, which is helpful to understand the dispersal strategies of plants in response to vegetation structure and succession.