Those rules are made to ensure that some objects will not be automatically teared down and others will for sure when building other objects over them.
- All terrain objects (like mountains, streams, common resource deposits and such) which can not be build and are pre-definied in a map are not tearable at all.
- Buildings (in the meaning of real buildings and buildable resource deposits) can be build over trees and nothing else.
- Trees (due to buildable objects mentioned here) can be build over no other object - including trees themselves.
- Streets can be build over trees and nothing else.
- Walls can be build over trees and nothing else.
Basically you can say buildable objects can be placed over every natural object which can be teared down in no time in reality. This includes small rocks, ponds, bushes and such.
Trees are some kind of special object, because optically this is a terrain object, in function a buildable ressource deposit, but can be found already on the map in large amounts at game start like pre-defined deposits. Trees can be worked for resources, teared down manually and are affected by autotearing in every case except of building trees oder trees.
Should be implemented as soon as possible.