Green leaves make carbohydrates by photosynthesis. This means that green leaves are abundant source of energy. Can humans eat enough green leaves off the trees and get necessary daily energy intake?
Something can be called a food when it is edible, and most tree leaves are not.
Carbohydrates or other nutrients can be a source of energy when they are digestible, but cellulose and (most) other carbs in leaves are not (Elmhurst.edu).
Finally, to get "necessary daily energy," you need to consume, for example, 2,000 Kcal. So, even if you find edible tree leaves with digestible carbs, they will still not likely provide enough calories. Think of lettuce: 360 grams of lettuce = 54 Kcal.