I am assuming that the purpose of this idea is to make the plant's trunks easy to convert to lumber by making them grow into straight forms. If that is the purpose of the metal form then it should be possible to create a similar effect by controlling the plant's lighting.
If a plant is placed in a room where it gets a low amount of sunlight, the plant will usually grow towards the place that the sunlight enters the room. Instead of using metal forms that prevent the plant from growing out of the desired shape. The controlled lighting will make the plant grow into the desired shape or pattern. This will make the plant easier to machine into lumber.
The controlled lighting can be achieved by wrapping cellophane materials around some parts of the plants. If this approach is used in a plantation, the cellophane material used should not only be opaque but very reflective. That way the light that a plant is shielded from is reflected to other plants at places they are needed more.
Some of the advantages that cellophane has over a metal form, in this case, is that cellophane is much cheaper. It is also easier to remove or adjust than a metal form. There is less cost and more flexibility in growing the plants into planks.
This system of reflections can also be used to grow more dense forests since the larger trees will not shade smaller ones as in uncontrolled plantations. Even when the smaller plants are shaded from sunlight they can still get some of the light by a series of reflections from the parts that are covered by cellophane.