Trees species such as alder, birch, poplar, and willow, will disperse their seeds via wind. However, just because the seeds are dispersed by wind it does not necessarily mean that they will travel huge distances. In fact, a large percentage of the tree’s seed crop will fall within a short distance from the seed tree – perhaps only as far as a few times its height.
Despite this, the distance a seed can potentially travel will vary by several orders of magnitude, and therefore seeds do have the capacity to travel huge distances. Storms have been found to enable seeds of poplar to travel up to 30km, maple 4km, scots pine 2km, birch 1.6km, and ash 0.5km. Of course, this really does depend upon the direction and the strength of the wind, and for how long the wind gusts durate for.
The ‘fall velocity’ of a seed (as in, the time it takes the seed to fall from the parent tree to the floor in still air) may also be a determining factor in how far a seed may potentially travel. If a seed is heavy (such as an acorn), it will fall to the ground is a few seconds, though for trees that rely on wind dispersing their seed they will usually have much lighter and less aerodynamic seeds. This increases the time it takes for the seed to fall to the floor and thus increases the likelihood of the seed being ‘caught’ by wind gusts. To give an example, the average ‘fall velocity’ of Acer platanoides seeds is 107cm per second, whilst Ailanthus altissima seeds fall at 122cm per second. Comparing this to the ‘fall velocity’ of an acorn, which unfortunately I cannot find (one article seems to be behind a paywall), there would very likely be a marked difference (with the acorn’s ‘fall velocity’ being much greater).
In fact, seed dispersal distance increases disproportionately as greater fall distances are reached. In one study, seeds dropped from a height of 61m in wind speeds of 3.1m per second were recorded from 61-975m from the point of fall, whilst seeds dropped from a height of 30.5m in identical wind speeds were recorded no further than 244m away.
Now, as even the authors recognise, such distances don’t explain how poplar seeds were found 30km away (or even maple at 4km). To answer this, it is suggested that seeds may, in times of very marked wind storms, ‘rise’ above their release height due to air turbulence and then steadily glide downwards from this higher point (assuming they are not picked up by further turbulence further along the way, which would increase distance travelled yet further and / or change the direction of travel).
Source: Johnson, W., Sharpe, D., DeAngelis, D., Fields, D., & Olson, R. (1981) Modelling Seed Dispersal and Forest Island Dynamics. In Burgess, R. & Sharpe, D. (eds.) Forest Island Dynamics in Man-Dominated Landscapes. Germany: Springer-Verlag.
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