They know census data from tree rings? Cool!!
Ha, no, the census data they garner from historical records, mostly. That and piecing together information from the archeaological record.
But, actually, tree rings do identify sunlight and moisture and that then identifies where to focus for animal/human life changes accordingly.
If you'd like to read a good book explaining how tree rings have been used to unravel historical mysteries, try "Exodus to Arthur". Great read.