Krummholz
Krummholz — also called knieholz — is a type of stunted, deformed vegetation encountered in the subarctic and subalpine tree line landscapes, shaped by continual exposure to fierce, freezing winds. Under these conditions, trees can survive only where they are sheltered by rock formations or snow cover. As the lower portion of these trees continues to grow, the coverage becomes extremely dense near the ground. In Newfoundland and Labrador, the formation is known as tuckamore. Krummholz trees are also found on beaches, such as the Oregon coast, where trees can become much taller than their subalpine cousins.
The labeling of diverse sets of tree species in different ecological contexts may be problematic. The ecological requirements of krummholz trees in the Alps, for example, are different from those in the Rockies. The terms scrub or shrubland may be more appropriate for some communities with krummholz trees.
Krummholz trees can cover nearly all of the area in which they inhabit, with only patches of moss and flowers in between. Frequent fog and cloudy conditions, along with cool weather, create a rather moist microclimate around the shrubs. Krummholz might depend on less acidic soil to survive. This means that they are threatened by acid rain. The thin soils that cover mountaintops have low buffering capacity, that is the capacity to resist changes in acidity. These trees are also endangered by the use of them as timber for fires, and other human activity.
Species
Common trees showing krummholz formation include European spruce, mountain pine, balsam fir, red spruce, black spruce, subalpine fir, subalpine larch, Engelmann spruce, whitebark pine, limber pine, bristlecone pine, and lodgepole pine. Instances of the krummholz form of black spruce, Picea mariana, are found in the northern Canadian boreal forests. Krummholz-form black spruce and balsam fir are abundant in the alpine transition zone of the White Mountains of Maine and New Hampshire and of the same zone in the Green Mountains of Vermont.Subalpine fir is the most common associate of spruce in krummholz vegetation. Other associated coniferous species include alpine larch, whitebark pine, limber pine, and western white pine in southern British Columbia, and mountain hemlock on the eastern slopes of the Coast Range and in the Revelstoke area of British Columbia. Lodgepole pine is a minor associate in most of the British Columbia interior, except in dry alpine areas of the southwest Cariboo/Chilcotin district where it is abundant. Ericaceous species are common in the snow accumulation zone around the base of krummholz colonies. In the coastal Pacific Northwest, douglas fir stands at higher elevation also have many examples of krummholz.
In the Alps in Europe, a scrubland of Pinus mugo is described as occupying the area above the tree line. This is formed by variants of the spruce, beech, and rarely the green alder. These European species were first labeled as a "krummholz belt" by scientists. In the Rocky Mountains, several tree species appear in a similar stunted form, such as specific North American variants of spruce, fir, and pine. These formations were sometimes called "elfin-wood" or "wind-timber". However, English-speaking scientists began to refer to these formations as krummholz as well.