Tree Migration
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I'm Dave Thurlow for the Weather Notebook. Every Fall and every Spring, migrating birds fill the forests and fields of North America. In a reaction to one of the more readily observable cycles in nature, the seasons, birds will follow summer weather from north to south, year after year. But the birds aren't the only things migrating across the landscape, the very trees in which you may see migrating birds roosting, are also migrating.

Tree migration is really forest migration, an expanding or shrinking of whole forest zones over a huge area and a long, long time. The forests are responding to changes in the atmosphere -- just like the birds -- changes that take place over thousands of years, too long for us to notice unless we look really close. Right now in glacial time, over much of the east, the oak and hickory forests are slowly creeping northward from their position 15,000 years ago after the last ice age. But don't look for an oak tree to go tip-toeing past your window, the forest movement is only a matter of inches a year. And it takes place as each successive generation of trees grows from seeds spread to the north and are able to grow in an ever so slightly warmer climate zone.

Interestingly, it's a non-migrating bird -- the blue jay -- that helps oak tree migration by gathering acorns, flying to the north and burying them, forgetting where they are, and allowing a new oak tree to grow -- thus expanding the range of the forest. Forest migration is determined by climate, and, as scientists are finding, the climate can also be influenced by the forest.

The Weather Notebook, which is produced by the Mount Washington Observatory is funded by The National Science Foundation, and underwritten by Subaru -- the beauty of All-Wheel Drive.