Each spring and summer, wind gusts send whirling showers of maple seeds through the air and to the earth. They litter roadways and patios, and a few fortunate seeds germinate, take root and mature into maples. A delicately engineered marvel, the maple seed exemplifies how evolution has favored strategies that allow trees to widely disperse their seeds, rather than simply dropping them to the ground.
The Need to Disperse
Plants have evolved many different mechanisms that allow them to disperse their seeds away from themselves, commonly using the wind -- as maples do -- to carry seeds far from the parent plant. Seed dispersal prevents the parent plant from having to share resources -- water, nutrients and light -- with offspring growing up nearby. Particularly for trees, seeds need to disperse away from the parent tree's canopy, where they have a chance of receiving enough light for growth after germination.
Whirlybirds, helicopters, keys -- these names all describe the maple's distinctive seed, the "samara." Depending on the species, maple trees flower in spring to early summer, and female flowers give rise to the samara. Each samara contains a single seed attached to an asymmetrical wing-like projection that, through a complicated aerodynamic process, causes the seed to spin rapidly as it falls, resembling a helicopter propeller. The spiraling motion of the samara helps to keep the seed aloft for a longer period of time than if the seed was subject to gravity alone, causing samaras to appear as though they were floating or flying as the wind bears them away from their parent tree. Under the right conditions, this spinning action can carry the samara great distances from its parent, but even damaged samaras tend to rotate well.
The tendency of the samara to catch a ride on a strong breeze further aids in dispersal once winter arrives. Seeds that have not germinated carry easily on the wind across the smooth surface of snow.
Samaras typically grow in pairs, but typically, only one seed will germinate, so sibling trees don't need to compete for the same resources. Maples produce samaras at different times of the year, depending on species, with some starting to germinate as soon as they hit the ground and others overwintering and germinating the following spring. Maple samaras germinate with a high success rate, allowing stands of maples to quickly reproduce themselves through seed production. Seeds serve as a food source for wildlife and birds and, once germinated, maple seedlings are a favorite snack of deer.