The “stratum statement” is generally shown on the page of the Strata Plan where the Plan of the Lots (floor plan) is shown, and it’s typically (but not always) 2.5m above and 1m below some fixed / permanent feature of the development; like a concrete driveway or the floor of a couryard.
You could contact Land & Property Information (in NSW) and ask them to check the Strata Plan for you; their website is https://www.lpi.nsw.gov.au/
The fact that the tree was on the exclusive-use area before Registration of the Special By-Law is of no legal consequence, but as I said in my earlier posts the height of the tree is.
As you say, the tree is “large”, so let’s assume (without clarification 
) that it’s above the stratum of the Lot, where in those circumstances removal of it is the responsibility of the Owners Corporation after it has checked the requirements of Council under its Tree Preservation Orders.
I’m big on compromise in circumstances such as these, so why not have your Owners Corporation (O/C) approach the Lot Owner/s who benefit from the exclusive use of the area for a contribution?