The core of each tree is made from a branching section of twig. On the left, you see a tree whose foliage is made from a mass of moss, found on the ground in a park. Amazingly enough, this is still green after several years. I store it in a dark box.

On the right, you see a tree the foliage of which is rubberised lichen, sold in most model shops (though not tremendously cheaply). Stick the lichen on the tree and hey presto!