So how does a deciduous tree know when to shed it's leaves and when to allow winter buds to grow in the springtime. The truth is that botanists aren't exactly sure. It obviously has to do with day length, as well as temperatures both in the air and soil. The process is not the same in all plants. There is still a lot to be learned.

It was discovered in the 1960's that some plants produce a hormone known as Abscisic Acid (ABA) which tells the plant to produce "terminal buds" for winter. This hormonal signal slows leaf growth and instructs the leaf primordia to produce scales to protect dormant leaf buds during cold weather. ABA also instructs the leaves to stop producing chlorophyll allowing the other pigments in the leaves to show through (fall color) and eventually allowing the leaves to part from (abscise) the tree.  ABA signals to the the vascular tissue in the cambium layer to stop growing. What does that mean you ask? Those annual rings that you see inside of a log or tree trunk stop growing during the cold weather.

Thank goodness for leaves. They cool the Earth from the hot summer sun, turn CO​2 into oxygen, prevent erosion by slowing rainfall, provide habitat and create amazingly cozy places to hike along the forest floor. 

Next time you're under a tree...look up. There is beauty in those branches.