Playing the Odds
In the spring—early April around Travelers' Rest—male trees ship pollen (fiber so fine that it's perceivable only by the sinuses of allergic humans) on the wind to any and all available female trees, to fertilize their flowers. The next stage is the emergence of the fruit.
Each female cottonwood tree produces thousands of fruits, and inside the husk of each are hundreds of seeds. The white, cottony filaments are the sails by which the seeds will be dispersed by other winds. Each female cottonwood tree, depending on its size, may produce millions of seeds.
The seeds are so small, as you can see on the following page, that they contain no food reserves, so those that germinate are the comparatively few that luck onto the ideal homesite. In the short term, that means a successful seed must alight on soil that is soft and moist, which means close to the edge of the stream. Furthermore, there must be no other vegetation nearby to compete for available water. If those conditions are met, the seed immediately begins to send down a root, which follows the water table as it recedes during the future tree's first summer. In the long run, it must be far enough away from the stream that a winter's ice will not scrape it away while it is still young.
At best, life is hard for a young cottonwood tree. To grow and thrive it needs as much sun as it can get, so if other trees, even other cottonwoods, are too close, it must spend extra energy to stay in the light. Besides, cottonwood trees contain a lot of sugar, and it's a fortunate seedling or sapling that escapes a wild ungulate's appetite.