I currently build a house. To get a permit for the building I have to plant and maintain(!) trees in the garden for the next 20 years - one large one (like beech or maple) or two smaller ones (like pear or hawthorn) for every 200 m2 of land. Only specific locally common trees are allowed. If I don't have the trees I will be fined.
I mean that's not much but it is a start. Trees should be mandatory.