Last week, I was heading into the woods, when a strong gust of wind swept across a white pine. A ghost form billowed out from its branches, an ethereal yellow in a mirage-like visage of the pine. The apparition was a wave of pollen, on the order of billions of pollen grains, the pollen grains that are are mostly responsible for the yellow patina coating New England (link). Each of those pollen grains is technically a tree, albeit an alternate life stage (the male gametophyte) from the more familiar sporophyte stage we typically think of when we imagine a white pine tree. I wrote last week that spring is a time of great dying, and true to that tune, virtually every one of those billions of tiny trees, or pollen grains, is destined for death. For those lucky few that find their way onto and fertilize the female gametophyte, the chances don’t improve much in the next generation of their life cycle, as most seeds never make it to the canopy. I’ve followed this thread of thinking into this week as it continues to “snow” cottonwood seeds.

A green frog covered in pine pollen (Centennial Woods, Burlington)

Estimating seed production in cottonwoods

The cottonwood “snow storms” are one of those big annual events that seems to cut through the noise and demand your attention. Looking at one of the cottonwood (Populus tremuloides) snowdrifts, I was curious how many seeds a single tree could produce. I made some initial observations about one of the bigger cottonwoods near me, then did some simple math, and came up with a nearly unfathomable number of seeds (though one that is near what other, reputable sources report). Read on for the math (yawn, am I right?) or just skim to the end to get the number.

A profusion of cottonwood fruits gathered on the side of the road through the Intervale (Intervale, Burlington)
A pair of cottonwoods, female is the bigger one on the right (Centennial Woods, Burlington)
Seeds per fruit

Cottonwoods are dioecious, which means a tree is either male (pollen-producing) or female (fruit-bearing). My large cottonwood, a female, is about 3′ in diameter and absolutely, positively laden with what my son calls “pillberries.” The pillberries, technically fruits called capsules, are small green pea-sized balls that hang in a pendulous cluster, like a tiny sleigh bell strap. Each capsule has 4 bracts that split along their seams as they dry to release the white fluffy seeds. I cracked about a dozen fruits open and found about 25-50 seeds per fruit (7-12 seeds per locule”). In the image below I pried the capsule apart and removed all of the seeds. You can see the small knobs on what’s called the placenta where each seed attaches

A cottonwood capsule split open to reveal the "placenta" or attachment sites for each seeds funiculus (Centennial Woods, Burlington)
Spent cottonwood capsules with seeds (Centennial Woods, Burlington)
Seeds per twig

Each stalk contains about 25-35 fruits, so at the minimum, each stalk would hold around 625 seeds. As each twig contains 2-4 (usually 3) of these stalks, that would come to about 25 * 25 * 3 = 1,875 seeds on a twig. I should mention here, that nearly 100% of cottonwood seeds that drift out from the capsule are viable.

A stalk of cottonwood fruits, or capsules (Centennial Woods, Burlington)
A mature fruit stalk where the capsules have all cracked open and revealed the fluffy seeds (Centennial Woods, Burlington)
Seeds per secondary branch

Cottonwoods are dioecious, which means a tree is either male (pollen-producing) or female (fruit-bearing). My large cottonwood, a female, is about 3′ in diameter and absolutely, positively laden with what my son calls “pillberries.” The pillberries, technically fruits called capsules, are small green pea-sized balls that hang in a pendulous cluster, like a tiny sleigh bell strap. Each capsule has 4 bracts that split along their seams as they dry to release the white fluffy seeds. I cracked about a dozen fruits open and found about 25-50 seeds per fruit (7-12 seeds per locule”). In the image below I pried the capsule apart and removed all of the seeds. You can see the small knobs on what’s called the placenta where each seed attaches

Eastern cottonwood branch laden with capsules about to release their seeds (Centennial Woods, Burlington)
Seeds per primary branch + limb

Each of these smaller branches is borne on a larger, which is borne on a one of several large limbs that fork off from the main trunk. There was some variability here, but it seemed like each of the 6 limbs had about 10-15 primary branch points. So the primary branches, which hold those secondary branches, have 12.5 * 84,375 = 1,054,688 seeds. Limbs had 10 primary branches each, so 10 * 1,054,688 = 10,546,880 seeds.

Assuming each limb on the tree has around the same number of seeds, that brings us to a grand total of about 63,281280 seeds on the tree!
The silky moths have been most unkind, eating all of the leaves on these branches and revealing the shear number of fruit stalks (Centennial Woods, Burlington)
Part of tree Approx. # of seeds it holds
Single fruit 25-50
Fruit stalk 625
Twig 84,375
Secondary branch 1,054,688
Limb 10,546,880
Tree 63,281,250

 

Chance of a seed becoming a tree

This estimate isn’t all that far off of what other sources list on the upper end of cottonwood seed production (The Sylvics Manual lists an upper limit on open grown trees as 48 million). There are a half dozen of these mammoth cottonwoods in the immediate area. Since half are female, that means only three trees actually produce seeds, for a total of around 180,000,000 (by my count) hopeful little trees floating aimlessly out on the wind each year. Hopeful doesn’t even really begin to reflect just how misguided their optimism is. Centennial Woods is not great cottonwood habitat. There’s only one suitable area, a swath under the powerlines where a small handful of young cottonwoods grow, maybe 200 or so, ranging in size from <1″ to about 4″ in diameter.

All of these are less than 15 years old (so about 13 germinate per year). Lets say all of these small trees come from the 3 cottonwoods near the baseball field. That would mean that an average of 4.25 cottonwood seeds from each tree germinate, root, and survive out each year. That’s a 0.000001% chance of any of those seeds surviving. The rest. Gonzo.

Oh, and VELCO brush hogs under the powerlines about once a decade, so those cottonwoods will never reach maturity.

The limitation with this method is that the needles can live so long that the annual ring becomes nearly indiscernible. You can always cut a twig just behind the oldest needle and count the growth rings. Similar problem here for slender branches where the small rings can be exceptionally difficult to make out.

More on the topic

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