A while back, on the ABC news, they had a segment on why leaves fall from trees in the fall; truth is, they don't fall, they are pushed off. The tree produces a certain chemical that makes it way to the leaves, in effect 'pushing' them off. It does this to keep the snow off the branches that would collapse the branches thus killing the tree. If you've ever seen trees in the winter that still had their leaves, after a snow fall the branches are bent way down to the ground almost. This in effect would kill the tree.
At my old place, I had a squirrel tree that did this, the tree wound up slowly dying. Best way I can explain this. Anyone want to add some info?

Steve

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Hmmm... thats interesting...

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Wow, I just thought they fell off because they dry up and die.
Thanks for sharing that with us.
Chris

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It's to do with the preservation of fluids and nutrients.

During the colder months in the temprate climate broad leaf trees will start to withdraw the fluid from their extremeties to prevent frosting and evaporation damaging the supply of nutrients. As the days close in and the temprature drops, tress will start to drain their sap down their trunks and into the base where it can be safely stored without risk of frost damage until the spring arrives and the sap can be returned to the branches to provide nutrients for new growth and new leaves.

A good example of this is to try and collect Maple sap for syrup during the colder months and you'll be hard puched to fill a thimble. Wait until spring/summer and you'll get the harvest you are after. On the Sub-Continent diciduous trees will shed their leaves in Summer, or the Dry Season, to minimise moisture loss.

The reason why more branches fall off in the winter is due to the branches not having the usual amount of moisture in than during the winter months so the branchges become more brittle. Add some heavy snow and ice to the weight on the branches and hey presto, the branch snaps.

If you pulled a leaf on a tree the branch wouldn't snap, the leaf would just come off, plus a leaf provides little support for a sufficient amount of snow and the leaves have a coating of wax to allow surface run off for moisture so snow would find it difficult to settle.

Once the leaves have been shed they begin to rot, usually around the base of the tree unless you sweep them up, which come spring and summer provides a natural boost in nutrients for the next growth spurt for the tree. If you swpet the leaves away from one tree but not the other, the one that had it's leaves left alone would grow quicker than the one that had it's leaves swept away.

Moisture retention is the reason why trees shed their leaves.

very interesting, and makes good sence.