Complex sentence: The people involved did not know of its world-record age before the cutting (see below), but the circumstances and decision-making process remain controversial; not all the facts are agreed upon by all involved.
Split into simpler sentences: The people involved did not know of its world-record age before the cutting (see below). However, the circumstances and decision-making process leading to the felling of the tree remain controversial; not all of the basic facts are agreed upon by all involved.