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 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).
Flag tree 
A variation of this formation is a flag tree or banner tree. Branches on the windward side are killed or deformed by the almost constant strong winds, giving the tree a characteristic flag-like appearance. Where the lower portion of the tree is protected by snow cover or rocks, only the exposed upper portion may have this appearance. This is a rather common occurrence in Red Spruce trees of the highest peaks of the central, even southern Appalachian Mountains, and is most commonly seen in the wind-swept high peaks and plateaus of the Allegheny Mountains. This formation most notably occurs with high frequency in the Dolly Sods and Roaring Plains West Wilderness areas along the Allegheny Front in eastern West Virginia, typically occurring at elevations of 3,800 feet (1,100 m) and higher. Trade winds in tropical regions near the equator can also deform trees in a similar manner.
See also 
- "The Subalpine Ecosystem". National Park Service. Archived from the original on 2007-08-09. Retrieved 2009-01-29.
- C. Michael Hogan. 2008. Black Spruce: Picea mariana, GlobalTwitcher.com, ed. N. Stromberg
- Oxford English Dictionary, entry for 'elfin'
- Krummholz formation on Snowy Range Pass, Wyoming[dead link]
- Krumholtz formation in Hudson Bay subarctic landscape
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