system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 how trees know when autumn starts

How do plants differentiate between chilly, dark and wet summer days and actual autumn? Why do they only start to loose their leaves when the appropriate season comes and not earlier?
gpt
It's how many hours of sunlight it receives.  Summer is when daylight starts getting shorter.  By the time you reach autumn, that's the equinox.

Basically the trees know how much energy it takes to maintain their leaves and when it's not worth the time because there isn't enough sun.