Some context:
As stated in the Release Notes, trees cannot be placed next to roads and buildings anymore (i.e. the places where they would disappear from after loading).
The reason for this automatic removal is, that the game automatically removes trees etc. close to buildings to make sure they’re not going inside the building model itself. You can already see that when you place a building or a road somewhere.
Upon loading, the game does this check again and removes vegetation that is too close to a building.
So, to avoid that you can place trees in these spots from which they would be removed anyway, these spots are now blocked automatically.
edit: We will take a look at providing more ornamental trees in future updates.