It sounds like a good idea; I make sure the floor of my patch of forest is clear. But there are also vast tracks of almost inaccessible forest in Portugal; the cost of clearing the undergrowth would be far beyond the value of the property.
What will the stressed owners do? Harvest all the trees and leave it barren, or if that’s not possible, just torch it now before the new rules come in.
A rule requiring cutting diseased infectious trees would be a good idea. But again, who will be the judge, how would the borders be defined? It’s just not workable in practice.