Krummholz or krumholtz formation (German: krumm, "crooked, bent, twisted" and Holz, "wood") — also called knieholz ("knee timber") — is a particular feature of subarctic and subalpine tree line landscapes. Continual exposure to fierce, freezing winds causes vegetation to become stunted and deformed. Under these conditions, trees can only survive where they are sheltered by rock formations or snow cover. As the lower portion of these trees continue to grow, the coverage becomes extremely dense near the ground.Common trees showing krumholtz formation include European spruce, mountain pine, balsam fir, red spruce, black spruce, subalpine fir, subalpine larch, Engelmann spruce, limber pine, and lodgepole pine. Instances of the krumholtz form of black spruce, Picea mariana, are found in the northern Canadian boreal forests. Krumholtz-form black spruce and balsam fir are abundant in the alpine transition zone of the White Mountains of New Hampshire.William Rogers Fisher introduced the English terms elfin-tree and elfin-wood to correspond to the German krummholz in his 1903 translation of Andreas Franz Wilhelm Schimper's Plant-geography upon a Physiological Basis (1898).