Jamis,
If you hit ctrl-D in the game, and then hit (F3), you should be able to see the graphs that govern the values we use for things like tree hit points. You can change these during play using the variable directory (F2). I don't recommend loading/saving variable directory stuff, it's buggy, but it should give you a way of examining how the variables affect each other, and then you can manually edit your xml to reflect this stuff.
HTH.
Alex
