Krummholtz

Krummholz, the crooked trees that are battered and stunted by harsh microclimates, exist at the upper edge of tree growth in mountains. Their name is derived from the German term for crooked or twisted wood. These krummholz, or elfin wood, suffer strong winds, low temperatures, intense radiation, and short growing seasons, and thus they become deformed. The geography of krummholz provides clues about microclimates, species’ distributions, water supplies, and climate change.

Most krummholz are evergreen trees, such as the Engelmann spruce (Picea engelmannii); however, occasionally, deciduous trees, such as the quaking aspen (Populus tremuloides), will have a stunted form. In some areas (e.g., the European Alps), the dwarfed and crooked krummholz forms are genetically induced. In other areas (e.g., North America), the krummholz forms are considered [Page 1670]to be caused by microclimatic factors that have acted on the tree during its lifetime.

An ecotone of krummholz

Source: Author.

The windward growth of the flag krummholz becomes battered.

Source: Author.

Krummholz extend from the upper edge of a noncrooked, dense forest, through a transitional zone or ecotone of increasingly crooked trees, to the treeless tundra (see first photo). Often, the ecotone appears as a belt of krummholz, paralleling the contours of a slope. The belt might be quite wide on a leeward slope, where strong, downslope winds, moving first across the tundra and carrying ice and rock particles, blast into the trees. In contrast, winds moving upslope through a forest will cause little deformation because they tend to move slower and be less abrasive.

A variety of factors affect the type and degree of stunting that occur in krummholz. Short and cool growing seasons may prevent maturation and acclimatization of tree tissues to be prepared for winter extremes. Immature tissues may not limit water loss to the dry atmosphere through the waxy coverings or cuticles of their needles. If krummholz are not protected by snow cover during winter, they are exposed to wind-driven ice and rock crystals that can pit the tissues and accelerate evaporation. Without insulating snow cover, soil water can freeze, preventing roots from uptaking water. Intense ultraviolet radiation, especially at high elevations, can damage exposed tissues. In all these cases, the growth tissues can be killed and may fall off the tree, leaving it weakened and in a crooked and stunted form.