Each tree has a maximum of four wood it can generate (pre-doubling) based on it's age. This does not mean you will get four if it's old. It means that the base number you can get is:
New tree: (1)
Young tree: (1-2)
Old tree: (1-3)
Ancient tree: (1-4)
Wait you say! I got 6 and 8 per tree! That's right. Because each time you kill a tree, there is a chance of doubling the amount based on skill level. So, its:
New tree: (1) (2)
Young tree: (1-2) (2,4)
Old tree: (1-3) (2,4,6)
Ancient tree: (1-4) (2,4,6,8)
Younger cutters do not take good advantage of the tree age because of their poor skills. They can take advantage of their poor skill by cutting in Kija's grove which is overcut by far, but yields slightly faster kills (younger trees have less vita)
and a quick trip to the inn. (both kija's grove and the inn to deposit are near south gate nagnang).
As you get older, your chance of getting the higher base numbers goes up, your average hit strength goes up, your average *crack* damage goes up, your chance of doubling goes up.... in short, it's time tomove gradually to more isolated groves that are not
as overcut and provide a greater chance of older trees.