K2 uses mostly fir in their twintips, fir is also pretty common in other skis. Stiffer skis often use hardwoods (usually maple or ash). Bamboo is used by Liberty and some other companies. It has similar properties to ash, but is an easier resource to renew, is a little lighter and is more lively. Poplar is pretty common, especially in women's models of some companies. Spruce is used too, it is similar to fir but flexier. whether it is white, red or black spruce is another question. I would hazard a guess at red spruce, since it tends to be the highest quality lumber of the three, with the most consistent grain, and other good properties. I think for ash, white ash would be used, since it is more common than black ash, which tends to grow in wetter, swampy areas. I know out of the species of maple (sugar, red, rock, or striped), rock maple makes good handles for tools (axes, etc) but I don't know if that's what they would use in skis or not.