Plants which cope with cold climates must tolerate deep freezing temperatures. Otherwise they have to "seek" secure places during critical periods: below snow or below ground. Control over their phenology is critical, and includes not getting tricked by short warm spells and becoming active at the wrong time. The thick snow cover in the sub-arctic mountains beyond the polar circle in N-Sweden represents a "warm blanket", often even preventing soils from freezing. Lemmings are most active under this protective shield and die if snow is missing. Photograph: Deep winter in March in the northern Scandes, 800 m, 68° N. See also Morphological or phenological escape.