Leaves don't actually turn green from top down in the spring. They turn green from the bottom up.

Here's why:

* New growth: In spring, new leaves emerge from the buds at the tips of branches and along the stems. These newly formed leaves are the youngest and most actively growing.

* Chlorophyll production: Chlorophyll, the green pigment that gives leaves their color, is produced in the chloroplasts within the leaves. New leaves produce chlorophyll more rapidly than older leaves.

* Sunlight: The lower leaves receive less direct sunlight, which is essential for chlorophyll production.

* Hormones: Hormones play a role in regulating growth and development, and they can affect chlorophyll production.

In summary: The new leaves at the bottom of the tree receive less sunlight and produce less chlorophyll than older leaves higher up. This is why they appear green first, while the older leaves at the top of the tree might still be brown or dormant.