An old oak tree.

Instructions:

First, you build a kind of mesh with wooden logs:

You should take care to build as natural as possible, so it should not be too symmetrical or straight. However, even a thick branch should not be connected to the trunk only by a block.

Afterwards one circles first the branches again unequally and oddly with foliage. Then you connect the individual branches, now covered with leaves, with foliage, if they are not too far apart, so that now only the tree crown is missing.
For these, fill the area above the trunk with foliage until it connects all the branches surrounded by leaves in the middle until the whole thing looks something like this:


The last thing to do is to build roots, for which it is best to put a few tree trunks in the ground, moving away from the tree:


When you are done you can decorate the whole thing with mushrooms.