Here's the Resource Model that I'd use for Timber:
Visible on the map at the Start (It's pretty hard to hide a 150 foot tall tree!)
Timber Resource can appear in any Forest tile, including those on Tundra base tiles.
(SIDE NOTE: We could even include a Luxury Resource of Exotic Woods, a timber resource found only in Jungle Tiles, representing hardwoods like mahogany or teak, which provide material for luxury furniture and interiors. I happen to think we have enough Luxury resources already, but it's certainly a reasonable addition to the game)
Timber is Required for construction of all ships until Ironclads. Including Cargo Ships.
Construction of a Lumber Mill is required to access the Timber Resource. Consequently, Lumber Mill is one of the Improvements available at the start of the game, BUT the Lumber Mill does not provide +1 Production to the tile it is on until after the discovery of Bronze or Iron Working (without metal tools, felling timber is 'way too labor intensive to provide any benefit to ordinary construction projects.)
The Timber Resource, being a Strategic Resource, is Depleted after all the Timber is harvested (typically, a single Timber Resource would have 2 - 6 Timber Resource Points in it). Once depleted, the Timber will regrow ONLY if the Lumber Mill is removed by Razing or Selling it off. Renewal of the resource takes 200 years, which will be a variable number of Game Turns.
What this means is, a Naval Power or major Sea Trade Civilization (Venice, Portugal, England, probably Polynesia to name a few) will need to actively search out and access Timber Resources early on. In fact, any civilization with an early Naval Unique Unit or Trade-oriented Unique Attribute - like Venice, Carthage, Byzantium, would have to have Timber Resource programmed as a starting or near-starting position Resource.
The alternative to this, and I find that the Starting Position Bias is one of the worst-programmed aspects of the game, would be to always provide an alternative to any Resource Requirement that is a Game Breaker for the civilization, as Timber would be for the civilizations mentioned above.
For Timber, for instance, the alternative could be to chop down the entire tile of forest, representing using smaller timbers spliced together to build up larger pieces, or chopping down everything to get at the few large trees isolated in the midst of the forest. Either way, replenishing the forest tile would require leaving it unused for 100 years or so for it to regrow. It would also be a distinctly Lesser Alternative, since a large fleet would require you to chop an awful lot of forest tiles, taking time and Worker Resources in game turns to do.