How does a tree know when to drop its leaves and grow?

Question

"how does a tree know when to drop its leaves and grow?"

a question around the dinner table.

Answer 1

First, the easy one:

Growing is triggered by the sunlight.

How much red in the spectrum triggers when to start growng. (Although different trees have different periods)

Second, Leaves.

Ok, this is a little difficult to explain.'

Stay with me here.

Leaves are grown in spring from buds.

If this is a Deciduous tree(kind that drops leaves in fall) then there is something known as a Scission point where the Petiole (little stem that connects the leaf to the tree) connects to the tree.

This scission point is not actually "Connected" to the tree in a (what? flesh way? Physical way?)

Its basicly "glued" there by a hormone that is constantly released by the tree.

(Hormone may be the wrong word, I'd have to look it up, been a long time since Botany class)

The further into fall, the less "hormone" the tree makes, since it's concentrating on moving all the energy (sugar) it has from the leaves, processing it and converting it to starch, and putting it in the roots.

So, by the time it's finished moving all the energy from the leaves, the "Hormone is getting pretty thin.

No more "Hormone", the leaf falls off.

Thats why leaves look pretty dry by the time they fall off and not still green, everything has been sucked out of it and stored in the roots.

Answer 2

...when the time for it to conserve water...such as winter is coming or an extraordinary drought is coming...as leaves are the exit points of water from plants.

...when spring comes or when the drought is over...the plant will start to sprout leaves again...

...plants can respond to external factors such as temp and presence of water