The plan to plant trees as outlined in the article is wrong. That's not to say planting trees at all is wrong. As you say for flood defence it would seem like a good idea.

Re wildlife, yes they would need to be controlled if trees are to take root. Sheep and trees aren't natural bed fellows. I'm not a making a man vs nature argument here; support is necessary in all manner of ways, in some it means taking a hands-off approach.

As for lack of trees in some areas, that might be for a good reason. Not every bit of land is suitable for woodland. Where it doesn't take root, it's a hint. And where it takes root, I would ask how you come up with a figure of hundreds of years.