Collaborative Research: Long Distance Dispersal of Tree Seeds by Wind: Underlying Mechanisms, Mathematical Prediction and Field Quantification

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Forests in our landscape are increasingly becoming isolated "islands" in a "sea" of urban, suburban, or agricultural land. For their populations to persist in such a landscape, trees must disperse their seeds from one island to the next. Many trees use the wind to disperse their seeds. In strong winds, especially over a rugged forest canopy or near the edge of a forest, three-dimensional "whirlpools" form with winds that sometimes blow upwards. If the upward velocity of the wind is greater than the rate of fall of the seed, the seed can go up rather than down. It may even get caught in a large updraft, such as happens where warm air rises, or where winds are deflected by a hill, a forest edge, or a large building. Such updrafts can take a seed high into the atmosphere, where it encounters faster winds. Or the updraft itself can move for miles, as it does in a thunderstorm, before letting the seed settle to ground. This project is a collaboration between biologists and atmospheric scientists to determine how often seeds go into these long-distance modes of dispersal. This will require measuring how strong the wind must be before seeds break loose from the tree, how slowly seeds fall in still air, and how the upward velocity of wind changes in different settings. Finally it will test experimentally whether winds can carry seeds as far as the distance between forest islands. The importance of updrafts for seed dispersal and the frequency of long-distance dispersal will be compared for 18 wind-dispersed tree species of the northeastern United States. Such information is vital for understanding the dynamics of forest species, and for maintaining them as humans increasingly fragment the landscape.