Bonsai Forest style is one of the most exotic designs of bonsai. If you want to grow a bonsai in the forest style, then you must be prepared to spend a considerable amount of time in planning your bonsai design.
In the traditional forest planning approach, the tree with the maximum height and width is planted in the front and other trees are arranged in a retreating manner with a gradual ascendance in height. This gives a feeling of depth to the whole design.
A forest for bonsai is generally created with groups of odd number of trees, and is usually grown on shallow pots or slabs. More than the number of trees, it is the way they are planted which gives it an illusion of a forest. Having a triangular pattern to the whole design is the most common approach for forest style bonsai.
You should also take proper care of the space between the trees while growing in a group. If the inner growth is shadowed by the dominant trees in the front then it could lead them to die down. Also take care of the fact that the trees in the group should not have their foliage overlapping on each other.
Ideally you would want to have a rough sketch of the design for your bonsai forest, before starting. This will keep you on track for creating your masterpiece.
Master bonsai artists use various principles of spatial design to create an illusion of depth in the forest. The basic similarity in all these designs is that no tree in the group is aligned in the same line as the other, when seen from front or side view.
When choosing the trees, see that all the trees have a similar tapering ratio. Then arrange them on a slab or a shallow pot in decreasing order of height and width of the trunk, with the smallest one being placed at a maximum distance from the viewer. Also keep in mind that no tree should be blocked from the view of the viewer. Choosing a good soil mix, will go a long way in ensuring a healthy growth for the trees of your bonsai cluster.
Once planted on the slab, press the root balls of the trees in the group, into each other. This will promote interweaving of the root system and will give the bonsai forest a better stability. Adjust the angles of the trees before adding the final layer of soil. And in the end you could cover the soil with some sphagnum moss.
At times it becomes very difficult to keep the trees from blocking each other. In order to avoid this; you may grow trees at the back of your forest, in a slightly slanting style. This will make them visible when seen from the front, and will add to the impression of depth in your design.
You could choose many different species for your bonsai forest, the most common ones being Chinese elms, Maples, Tamarind, Ficus, Jade etc. As the years go by, you could prune the trees in the group to give a round shape to the crown of your bonsai forest.