If they are part of the border, they can not be cut down. The one partly obscuring your horse there couldn't be cut down no matter what as it isn't one of the three "wild trees", and the same for the other ones to its left. To the right, though, that one appears to be one that could be cut down... but is it up on the cliff? If so, since you can not get to it, no, it also can not be cut down.
About them blocking, though... they should only be blocking sight. It is the cliffface/border that is actually blocking placement.