Seed Dispersal by Wind: Eastern Cottonwood - Populus deltoides - Cotonier

Populus deltoides (eastern cottonwood) is putting on a phenomenal show in Montreal. As one passerby commented on the Olmstead trail in the Mont Royal park, in Montreal it snows all winter, and then it snows in the summer:

Populus deltoides seeds flying in the wind in huge numbers

So what's the deal here?

I've mentioned seed dispersal by ants (myrmechory) a few times with respect to Sanguinaria canadensis, Trillium spp, Dicentra canadensis, and Dicentra cucullaria. But of course myrmechory is only one of many different strategies that plants have for dispersing their seeds. So today I'm going to talk a bit about anemochory (wind dispersal).

The general principle, of course, is that it is in the plant's best interest that its offspring are dispersed a reasonable distance away, so that it doesn't create offspring that compete with it for resources (generally speaking reproduction isn't adaptive if it involves shooting yourself in the foot).

So today I'm going to write a bit about wind seed dispersal.

But first, I'd like to introduce the star of the day, Populus deltoides (eastern poplar, eastern cottonwood, cottonwood). This tree is native eastern North America (US range map here, Canadian range map here) and has been introduced in British Columbia [1]. P. deltoides is listed as sensitive in Alberta [1]. It is indicated as a weedy species in some parts of the US [2].

This species is dioecious (any given individual is either male or female) and produces catkins (drooping cylinders of flowers) when it blooms [3].

Populus deltoides leaves

The lower bark is not smooth like the bark on the younger branches. Instead, it is deeply scored:

Populus deltoides bark (lower trunk)

The female catkins, once fertilized, produce a sort of egg-shaped fruit:

Populus deltoides fruit

And this brings us back to the whole matter of wind pollination. Inside these catkins there are tons of tiny little seeds, each of which is attached to a large number of lightweight filaments. These burst open in early summer to release the seeds:

Populus deltoides burst fruit

Wind can be a very effective means to ensure that seeds are dispersed far from the parent tree, particularly if they start high up and are in a windy place. Seeds that are wind-dispersed tend to be very small and lightweight. There are a variety of arrangements of filaments for the filamentous types. The one most familiar for most people is probably the umbrella sort of arrangement in the dandelion. P. deltoides seeds simply have a tuft of numerous filaments that attach to the seed. The seeds of Asclepias syriaca (milkweed) are also like this. There are also the samara types, which are also wind-dispersed but at shorter distances (they are heavier, but although they travel less distance due to increased weight, the larger seeds have more nutrients and so a greater chance of germination).

Populus deltoides burst fruit

It has been a great year for P. deltoides; the air is thick with seeds and the ground is just covered in places:

Populus deltoides seeds coating the ground

So if you get the chance, go take a stroll where you might find some poplars and look up. It's quite a lovely show right now!