Raised beds for any plants is work. The trees will always be attracted to the nutrients and water. Many trees like oak shed tannins that restrict younger growths thereby protecting the parent tree. Can't say if that would work for raised beds, but I find I need to renew soils and nutrients every so many years in my raised beds anyway. SOME trees have compartmentalized root systems- if you chop this root, these branches die. There are trees like salix that can find water and send a root for miles to access that water.
If your raised beds are perhaps 3' plus high this may be enough to keep roots from finding your beds as the water won't seep all the way to the ground. Drawback to this raised bed is drainage in high rainfall areas I would think. Barriers at any depth can be a time limited endeavor.
No salt. Salt would be a Trojan Horse.
