nltk.tree.Tree uses the Tcl (tkinter) backend to draw trees. Unfortunately, that doesn't seem to work on a headless server, which is the setup I use: run Jupyter on the server and access it via the browser from another machine.

If python3-tk is not installed, I get

ImportError: No module named '_tkinter', please install the python3-tk package

If it is, I get

TclError: no display name and no $DISPLAY environment variable

However, dot works in such a setup, so it would be preferable if it was used as the backend instead of tk. Also, it could be used to implement _repr_svg_() on Tree, which would also be a very welcome addition, as svg is a much better format for these kinds of things than png.