system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do the deciduous trees know when autumn begins, and when it's just a cold front?
gpt
Deciduous trees (trees that lose their leaves in winter) aren't actually responding to the cold, they're responding to changes in the quantity of daylight.

These trees have chemicals inside them all the time that break down the chlorophyll in their leaves, and then break down the stems of the leaves so that the leaves fall off the tree.

Light breaks these chemicals down, and the tree has to keep producing more of them.

In each hemisphere of the Earth, summer is marked by a shift of the Earth's axis so that that hemisphere gets more sunlight (days are longer), and the other hemisphere gets less sunlight (days are shorter). As the amount of daylight hours decreases, the amount of these chemicals being destroyed by the light is reduced. When too much of the chemical remains in the tree, it starts to break down the leaves.

EDIT: Fun fact, this is also related to why maple syrup is harvested in the winter and not the summer.

Trees have vein/artery like structures called xylem and phloem that help transport different nutrients, waste, and water through the trunk (this is how nutrients get from the roots to the leaves, for example). Xylem transports mostly water.

Because there isn't a lot more in the xylem other than water, the water would normally freeze in the winter, and the freezing would cause the xylem to explode as the water expands (in fact, you may hear or see this on trees in forests with cold winters, it may appear as a bulge in a tree, or the be source of a loud pop from a tree during cold winter days or nights).

To help prevent this, trees pump other nutrients into their xylem during the winter - primarily sucrose, which helps to lower the freezing temperature of water, meaning it has to get colder before the water can freeze enough to destroy the xylem.

That means the contents of the xylem during winter are mostly a water-sugar mixture which can be boiled down into a syrup (this can be done on more than just maples, but maples tend to put a lot of sugar in their xylem, meaning it requires less boiling to make the syrup - hence, maple syrup).