By Wendy Langhans
Oh look, there goes one....and there goes another...the air was full of them. I was sitting out on my back porch, enjoying a cup of tea, literally watching the world pass by. The sky was full of tiny little white pieces of fluff, swirling past me on the ever-changing air currents. What was going on?
The fluff was Cottonwood seeds, casting their fate to the warm spring winds. When we think of seeds, we often picture maple seeds, with their sturdy samara (wings) or dandelion seeds, with their elegant pappus (gossamer-like umbrellas). There's nothing aerodynamically elegant about Cottonwood seeds. They are borne aloft on tufts of plant fibers known as trichomes. But trichomes are quick and easy to produce, and the Cottonwood trees produce mass quantities of seeds, which are carried for miles by air and water currents.
Cottonwood seed-pods opening to disperse seeds
As Annie Dillard wrote in Pilgrim at Tinker Creek, "Nature is, above all, profligate." Seed production carries a metabolic cost. For example, acorns are costly to produce. But acorns can rely on their ample endosperm for nourishment and can be stored  for long periods of time (up to 6 mo for white oaks and 3 years for red oaks). In contrast, Cottonwood seeds, which the tree produces in seemingly wild and reckless abundance, are much easier to produce. They're tiny, about 1 mm in diameter, and therefore have a brief "shelf life". According to the U.S. Forest Service , these "seeds may remain viable for 1 to 5 weeks after dispersal" and "Viability is lost if a suitable microsite is not found within 2 or 3 days of seed becoming wet."
Cottonwood seeds on the trail at East & Rice Canyon
With such a limited shelf life, cottonwood seeds need to land at a favorable spot to germinate and thrive - one that is not too wet and not too dry - a riparian habitat near a stream bed is ideal. Since riparian habitats are few and far between in Southern California, cottonwood seeds must be light enough to travel for miles. Launch time must also be favorable - at a time when the heaviest winter runoff in the streambed is beginning to recede but before the ground dries up from the heat - say - sometime in April. Which is why the air is full of cottonwood fluff right now.
Is Annie Dillard correct: is nature, above all, profligate? I'm not so sure. Given the constraints of the environment, which seed strategy would consume more of the Cottonwood tree's limited resources - a few elegant seeds or a ginormous number of "just good enough" seeds? Should the tree produce a few Rolls-Royce "Silver Ghosts" or a larger number of "Model T Fords"? Perhaps Cottonwood trees aren't so profligate, after all. And perhaps this story isn't just a "fluff piece", either.
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