Giant sequoia trees.
Comes in six parts:
Lower trunk, middle trunk, upper trunk, and lower foliage, middle foliage, and upper foliage. Each set is 1024 tall.
There is also a skybox version of a full tree plus skybox sized foliage.
Stack one upper trunk/foliage set for a "short" sequoia (~1024 hu tall).
Stack one upper trunk/foliage set + one lower trunk/foliage set for a "medium" sequoia (~2048 hu tall).
Stack one one upper trunk/foliage set + one middle trunk/foliage set + one lower trunk/foliage set for a full sized sequoia (~3072 hu tall).
Stack the above plus many middle trunk/foliage sets and go up forever if you want, who am I to tell you how to map.
Set the foliage pieces to NON-SOLID with vertex lighting DISABLED. The trunks can be set however you like.