Browsing by Subject "Seed dispersal by wind"

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    Density dependence of seed dispersal and fecundity profoundly alters the spread dynamics of plant populations
    (2023) Zhu, Jinlei; Lukić, Nataša; Pagel, Jörn; Schurr, Frank M.
    Plant population spread has fundamental ecological and evolutionary importance. Both determinants of plant population spread, fecundity and dispersal, can be density‐dependent, which should cause feedback between population densities and spread dynamics. Yet it is poorly understood how density‐dependence affects key characteristics of spread: spread rate at which the location of the furthest forward individual moves, edge depth (the geographical area over which individuals contribute to spread) and population continuity (occupancy of the spreading population). We present a general modelling framework for analysing the effects of density‐dependent fecundity and dispersal on population spread and parameterize this framework with experimental data from a common‐garden experiment using five wind‐dispersed plant species grown at different densities. Our model shows that density‐dependent fecundity and dispersal strongly affect all three population spread characteristics for both exponential and lognormal dispersal kernels. Spread rate and edge depth are strongly correlated but show weaker correlations with population continuity. Positive density‐dependence of fecundity increases all three spread characteristics. Increasingly positive density‐dependence of dispersal increases spread rate and edge depth but generally decreases population continuity. Density‐dependent fecundity and dispersal are largely additive in their effect on spread characteristics. For population continuity, the joint effects of density‐dependent fecundity and dispersal are somewhat contingent on the dispersal kernel. The common‐garden experiment and the experimentally parameterized mechanistic dispersal model revealed density‐dependent fecundity and dispersal across study species. All study species exhibited negatively density‐dependent fecundity, but they differed qualitatively in the density‐dependence of dispersal distance and probability of long‐distance dispersal. The negative density‐dependence of fecundity and dispersal found for three species reinforced each other in reducing spread rate and edge depth. The positively density‐dependent dispersal found for two species markedly increased spread rate and edge depth. Population continuity was hardly affected by population density in all study species except Crepis sancta in which it was strongly reduced by negatively density‐dependent fecundity. Synthesis. Density‐dependent fecundity and seed dispersal profoundly alter population spread. In particular, positively density‐dependent dispersal should promote the spread and genetic diversity of plant populations migrating under climate change but also complicate the control of invasive species.
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    Improving measurements of the falling trajectory and terminal velocity of wind‐dispersed seeds
    (2022) Zhu, Jinlei; Buchmann, Carsten M.; Schurr, Frank M.
    Seed dispersal by wind is one of the most important dispersal mechanisms in plants. The key seed trait affecting seed dispersal by wind is the effective terminal velocity (hereafter “terminal velocity”, Vt), the maximum falling speed of a seed in still air. Accurate estimates of Vt are crucial for predicting intra‐ and interspecific variation in seed dispersal ability. However, existing methods produce biased estimates of Vt for slow‐ or fast‐falling seeds, fragile seeds, and seeds with complex falling trajectories. We present a new video‐based method that estimates the falling trajectory and Vt of wind‐dispersed seeds. The design involves a mirror that enables a camera to simultaneously record a falling seed from two perspectives. Automated image analysis then determines three‐dimensional seed trajectories at high temporal resolution. To these trajectories, we fit a physical model of free fall with air resistance to estimate Vt. We validated this method by comparing the estimated Vt of spheres of different diameters and materials to theoretical expectations and by comparing the estimated Vt of seeds to measurements in a vertical wind tunnel. Vt estimates closely match theoretical expectations for spheres and vertical wind tunnel measurements for seeds. However, our Vt estimates for fast‐falling seeds are markedly higher than those in an existing trait database. This discrepancy seems to arise because previous estimates inadequately accounted for seed acceleration. The presented method yields accurate, efficient, and affordable estimates of the three‐dimensional falling trajectory and terminal velocity for a wide range of seed types. The method should thus advance the understanding and prediction of wind‐driven seed dispersal.