In the desert, plant diaspore spreading usually relies on both external vectors, such as wind, and internal factors, such as diaspore shape. Ammopiptanthus mongolicus is a heterocarpous shrub species in the cold desert in northwest China. Mature pods may have dehisced or not yet dehisced when abscising, and the dehiscent pods may be twisted or flattened. The propagation distances of diaspores might vary due to differences in their buoyancies in upward air and ground friction according to pod shape. The wind tunnel experiments were conducted to measure the horizontal displacement upon fruit dropping (D1) and the wind-blown distance traveled by a fallen pod (D2) of A. mongolicus. A generalized linear mixed model and generalized linear model were used to test the effects of pod shape, release height, wind speed and ground substrate type on the spread distance of pods. D1 is jointly determined by the effects of release height and pod shape. Wind speed, pod shape and ground substrate type together affect D2. A twisted pod has higher dispersibility than a flat pod, and dehiscent pods spread further than indehiscent ones. The positive correlation of D1 and D2 indicates that the difference in pod shape additively broadens the range of seed-spreading distance. Differential seed-spreading properties could be adaptively advantageous to disperse the risks associated with diasporic dissemination compared with the maintenance of a single optimal dissemination characteristic. Thus, heterodiaspory is an advantageous adaptive characteristic for seed spreading in the windy, arid and harsh desert environment.
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