Improving measurements of the falling trajectory and terminal velocity of wind‐dispersed seeds

Abstract

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”, V t ), the maximum falling speed of a seed in still air. Accurate estimates of V t are crucial for predicting intra‐ and interspecific variation in seed dispersal ability. However, existing methods produce biased estimates of V t 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 V t 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 V t . We validated this method by comparing the estimated V t of spheres of different diameters and materials to theoretical expectations and by comparing the estimated V t of seeds to measurements in a vertical wind tunnel. V t estimates closely match theoretical expectations for spheres and vertical wind tunnel measurements for seeds. However, our V t 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.

Density dependence of seed dispersal and fecundity profoundly alters the spread dynamics of plant populations

“…The paper bags were then transported to the laboratory and air‐dried for at least 2 weeks before the measurement of seed terminal velocity. Three seeds were haphazardly selected from each capitulum, and their terminal velocity was measured following Zhu et al (2022).…”

“…The paper bags were then transported to the laboratory and air-dried for at least 2 weeks before the measurement of seed terminal velocity. Three seeds were haphazardly selected from each capitulum, and their terminal velocity was measured followingZhu et al (2022).Except for C. album, the fecundity of each target plant was measured by multiplying the total number of capitula produced and the mean seed number per capitulum, which was estimated by counting the seed number in each of the first 10 matured capitula. For C. album, the fecundity was measured by dividing the total seed mass of the target plant by the mass of haphazardly selected 50 seeds(Kleyer et al, 2008).At the end of the experiment, the above-ground biomass in each pot was harvested and dried in a laboratory drying oven at 70°C for 72 h. The total above-ground biomass was determined by summing up the weight of the oven-dried plant material and the weight of airdried capitula or branches, as well as the weight of seeds dispersed during the experiment, which was estimated by multiplying the number of empty capitula and the average weight of a capitulum.The mean horizontal wind speed, aerodynamic roughness length, and friction velocity were acquired from the Land-Atmosphere Feedback Observatory (https://lafo.uni-hohen heim.de/en/lafostart page;Späth et al, 2022) at the experimental site, covering the dispersal season from 1 July to 31 October 2019.…”

The role of maternal environment and dispersal ability in plants' transgenerational plasticity

“…This procedure was repeated, and H r values and seed heads were collected from the upper and lower parts of the target plant. Seed terminal velocity ( V t ) was estimated using the method described by Zhu et al (2022) 2022…”