Remember:
A reversed tree happen when the distance between top and ground (or structure you are on) is nearly equal to tree size.
In atcs you can do a reversed tree on bunker by building two bamboos or two bamboos and a brick on the highest part of the bunker.
you can see that two bamboos and a brick is the max size if you want to build a tree on them.
Once you build these structures you can build the tree like this:
or this:
Then, wallwalk under the tree you just built and select a tree again.
you can see it indicates green as the picture under.
Now you only need to click with mouse to build it
