thanks @mrnightmare89 :D yes, the yellowing of the leaves happens in autumn when the temperatures begin to drop before winter. It will soon be freezing over night and the trees stop their metabolism in the leaves while returning the nutrition that's stored in there to endure winter. The process is called "plant senescence" and it's a beautiful spectacle every year around the forests. It doesn't last very long, though... all the leaves will soon drop to become mushroom food and create new soil.