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.