I think the situation at Ma On Shan (upland shrubland/woodland) would be very different from Mai Po. As Bena pointed out, Mai Po is managed primarily for the benefit of wetland wildlife which benefits from the presence of fewer trees. The tree species present are inevitably less diverse and support less fauna than in hillside shrubland.
At the moment, I think the policy at MP is just right - maintaining some trees on the landward side to enhance biodiversity but keeping the seaward side largely clear of trees for the benefit of waterbirds. I also agree with the policy of replacing exotic species with native shrubs.
