The wood alone will not likely do much to the GH and pH. Diluting with some form of "pure" water is the best method. Then the wood and leaves will help keep things stable.
Natural habitat water conditions are far removed from what many of us assume. In all the test numbers that I have seen for South American streams and creeks, the GH and KH is zero or at most 1 dGH and the TDS are extremely low. The pH does vary, depending upon the water course; some run around 4, others higher.
I've no idea on my TDS, but I asssume it is fairly low. The tap water is probably close to the water in Amazonia to begin with (chlorine will add TDS obviously), except my pH is around 7; they add soda ash (sodium carbonate) for corrosion control as the natural pH is around 5 (this used to be what came out of the taps until 2000 when the ash began) for corrosion control, but this either dissipates out or with the biological balance in the tanks it quickly lowers. Even in my tanks with the pH at 5 or below, doing a 50% water change only raises the pH by about 0.2 or 0.3 and this does not last more than a few minutes. It certainly makes water changes easier when I can use the tap water without any adjustments other than dechlorinator as there is a lot of chlorine.
Byron.
