The importance of willow thickets for ptarmigan and hares in shrub tundra : the more the better? - Ehrich, Dorothee; Henden, John-André; Ims, Rolf Anker; Doronina, Lilyia O; Killengreen, Siw Turid; Lecomte, Nicolas; Pokrovsky, Ivan; Skogstad, Gunhild; Sokolov, Alexander A.; Sokolov, Vasily A.; Yoccoz, Nigel (Oecologia, 2012)