Krummholz is typically found along treelines where persistent high winds, often carrying snow which acts as sand in a sandblaster, damage the windward (upwind) side of the trees, destroying buds on that side. Growth typically only occurs along the leeward (downwind) side of the trees. Clumps of krummholz can actually "migrate" downwind, as the leading edge of the clump dies while growth continues along the trailing edge.
In the case below, some topographic irregularity provided some shelter. In time, some individuals managed to poke above the pack and continue the growth vertically, but those stems have a "flag tree" appearance, with the foliage restricted to the leeward side.
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