The Mammut Tree Wool Thermal FL Long-Sleeve pairs merino wool with lyocell in a crew neck base layer built for consecutive days of movement. A waffle knit structure holds in warmth while pulling sweat away from the skin, so the fabric stays dry from the first skin track to the last descent. Seamless side construction cuts down on friction under a pack or shell, and the same lightweight warmth carries over to a cool trail run when the snow melts out. What Our Gearhead(R) Experts Are Saying:"I packed just this one top for a five-day hut traverse and layered it under a shell every morning without ever needing a wash. The merino held up the same on day five as it did on day one, which is the only reason a single top can carry an entire trip."