As I understand it, the problem is that the chemicals are killing the trees, so whether they're old or newly planted, they will die. No living thing is likely to survive against barium, strontium, and aluminum -- even the people in charge of this worldwide poisoning will not survive it. Even if they have a escape hatch in a mountain or under the sea or on Mars, they will never be able to survive on this planet without trees. They must think they have found another way to produce oxygen than plant life, but they will still have to deal with all of those chemicals, which will kill them. (Of course, they think they will replace all their parts with machine parts and be able to coexist with such chemicals and live forever. Just looking at the fake trees, you have to ask who would want to live in a fake Truman Show world like that.)