An important question, Fulgour, couched in important unstated questions.
For example, are the verticals actually living (though clipped or lopped) trees, or are they inserted into the ground as large posts?
Certainly the latter would also account for the 'mounds' obviously apparent generally in Marseille style decks.
Of course, it may be that the 'material reason' is no more than the woodcarver maximising the image within the constraints of the frame. Symbolically, however, this is not sufficient.
In the thread on XII - Le Pendu (in the Marseille section), mention is also made that as a consequence Le Pendu's head is placed, effectively, within the Earth. Could it also be, then, that the 'mounds' are apparent due to a hollow dug between vertical scaffold?
I have personally preferred to usually see the trunks as precisely that: living trunks - that also then show the distinct difference and opposition between plant and human form. Our 'roots' are within our heads, connected to the spiritual realm, and 'free' of the Earth. A plant's is firmly planted within the Earth's bosom (of course, there is much more inversion between the two than simply this, as some of Olive Whicher work explains).
By placing the human being in this un-natural state, a closer connection with the plant realm is made apparent - yet, to what purpose?
Symbolically, and for me at least, as pointing towards initiation.
You have entered, in the northern hemisphere, spring. We have entered autumn, and pruning of trunks is quite apt - and reminds me I'll have to cut a trailor load in the next week or so!!!