I understand that it's used to buy buildings, but for some reason my stockpile of wood is constantly emptying. Was not able to find any building that should be using it. (Missing 1 Act 3, if that helps at all)
For people who are interested: The ascension of Tier 2 eco houses to Tier 3 costs Wood. If you have it set to "Automatic Ascension" (Top right corner of the stat bar when selecting a city center) then you will constantly lose wood to this.
Hope that helps, and thank you!