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Spiral Trees on Windy Ridges

Jeff Mitton

Natural Selections (appeared September 23, 2005 in The Daily Camera)

A hike on Mount Audubon led me to develop several hypotheses about the different ways trees grow. As I climbed higher into the spruce-fir forest, I was struck by the contrasting patterns of growth form within each species. The bark had fallen from toppled trees, and the exposed grain revealed that some trees had grown straight, and some had spiraled as they grew.

            Trees can be assigned to three groups: sinistral trees spiraling to the left, straight trees, and dextral trees spiraling to the right. Spiral grain appears the same whether you look up or down the trunk, so            growth form can be identified even when decay has reduced a trunk to a short fragment.

Growth form is most apparent in dead trunks stripped of bark, but dead branches reveal the growth forms of live trees. This works best with large limbs, and does not work at all with twigs. Spiral grain is evident in trunks, limbs, and roots.

            Two types of observations suggest that tree growth form is genetically determined. First, growth forms are restricted within a species: limber pine grows straight or sinistrally; ponderosa pine, subalpine fir, and Engelmann spruce grow straight or dextrally. Second, the growth forms are intermixed in a stand. A sinistral limber pine can be surrounded by limber pines growing straight; the intermixing of growth forms in a stand suggests that the variation is not environmentally determined.

            I noticed that spiral trees were rare in the area around Long Lake, but they were common near tree line on Mount Audubon. This pattern is repeated in other areas, and appears to be common on the eastern slope of the Front Range. In general, spiral trees are rare in deep forests and valleys, and are most common on windy ridges and at tree line.

            Andrew Smith and Hans Kubler, both forest biologists, have identified characteristics of spiral trees that might explain why spiral trees are most common at high elevations and on windy ridges. Spiral growth reduces the absolute strength of the wood, but it makes the limbs and trunk more flexible. When heavy snow accumulates, flexible branches can dump the snow, rather than shattering. Enhanced flexibility also makes spiral trees less likely to be damaged by strong winds. 

            If a tree grew perfectly straight, needles would receive water from the roots directly below them, and roots would receive nutrients from the needles directly above them. If roots were damaged on one side of the tree, branches directly above would wither. Similarly, wind damage to branches would deprive roots directly below of nutrients. In contrast, spiral grain delivers water from a single root to all of the needles, and supplies nutrients from a single branch to all of the roots. This even distribution of water and nutrients is advantageous is where winds are severe, and during periods of water stress. Spiral trees are better adapted to the chronic water stress on steep, windy ridges.

            Lumber mills eschew trees with spiral grain, for the wood is weaker, and the lumber may twist as it dries. But spiral trees delight photographers who value esthetics and naturalists who appreciate adaptation.

            Several sites with easy access are convenient for viewing spiral trees. The knoll due north of Red Rock Lake, near Brainard Lake, has a stand of limber pine with sinistral and straight trees. The area around Rainbow Curve, on Trail Ridge Road in Rocky Mountain National Park, has spiral and straight limber pine, subalpine fir, and Engelmann spruce.  The ancient limber pine grove at 11,000 feet on Sheep Mountain, south and west of Fairplay, has straight and sinistral trees more than a thousand years old; park at Fourmile Campground and follow the signs to the grove.

 

Two adjacent limber pine trees, north of Red Rock Lake, exhibit sinistral spiral grain (left) and straight grain.

Photo by Jeff Mitton

Limber pine stump with sinistral spiral grain; the log in the background was cut from this stump, and it shows that the spiral grain continues up the trunk.

Photo by Jeff Mitton

Limber pine log showing sinistral spiral grain. Whether you stand at the roots or the crown of the log, the grain spirals to the left.

Photo by Jeff Mitton

A fallen Engelmann spruce, Deer Mountain, Rocky Mountain National Park, exhibits the dextral spiral grain

Photo by Jeff Mitton