Stem Fluting of Western Hemlock in Southeast Alaska

by Kent R. Julin and Wilbur A. Farr

A fluted western hemlock on the Old Hollis Town site, Prince of Wales Island, Alaska

Introduction

Western hemlock (Tsuga heterophylla (Raf.) Sarg.) trees with severely fluted trunks are often seen in the forests of southeast Alaska, but until recently, little was understood about the cause or distribution of fluted stands in the region. This pamphlet describes what is currently known about fluting and provides guidelines for reducing its severity in southeast Alaska.

 

Occurrence of Fluted Hemlock

Major disturbance, rapid growth, and coarse branching of young western hemlock are key factors in fluting. Fluted hemlock are found throughout southeast Alaska, most frequently in coastline stands that developed after catastrophic disturbances such as clearcutting or windthrow. Severely fluted hemlock also occur in abandoned Native and contemporary villages where land was cleared and in stands where pilings were cut for fish traps and canneries.

The degree to which western hemlock trees are fluted varies with tree age. Flutes often begin to form atthe root collar when a tree is about 10 years old. With time, flutes progressively deepen and eventually trap bark within the stem. After several centuries, bark becomes completely engulfed and flutes may no longer be outwardly visible. Thus, old-growth trees that were once severely fluted may not appear fluted today.

What Causes Fluting?

It is evident that western hemlock growing in coastal stands of southeast Alaska are genetically predisposed to form fluted trunks. Fluted western hemlock growing in England are the progeny of trees in coastal British Columbia and southeast Alaska. The result of this genetic predisposition for fluting is a confined type of transport system in the stem that limits the availability of carbohydrates below an obstruction such as a dead branch. Annual rings develop asymmetrically as a consequence of an unequal distribution of carbohydrates around the stem.

Branch Location and Activity

Branch location and activity determ ine the occurrence of flutes on the trunk. Flutes generally start where branches grow out from the stem and after a decline of branch activity from shading slows radial growth beneath the branch. Flutes from vertically aligned branches coalesce and produce long grooves that spiral downward. As branch stubs decay, flutes become overgrown.

Locations of documented fluted stands in southeast Alaska.

Locations of documented fluted stands in southeast Alaska.

 

Soil Conditions and Tree Stability

The prevalence of fluted trees near the coast may result from soil conditions affecting tree growth and stability. Fluted stands frequently occupy uplifted beach soils that charac. teristically have a thin soil mantle over gravel. Trees situated on these well-drained soils grow rapidly. and this may promote fluting. Limited anchorage provided by these soils may also cause increased mechanical stress in the stem.

Flutes become more pronounced with tree age. After several centuries flutes are no longer outwardly visible because they have been engulfed within the stem.

Flutes become more pronounced with tree age. After several centuries flutes are no longer outwardly visible because they have been engulfed within the stem.

Fluted trees may be more stable in a windy environment than are cylindrical trees. With a broader base and pronounced buttressing, fluted trees are probably adapted to the shallow soil and gusty wind conditions of southeast Alaska. The largest buttresses on leaning trees occur in the direction of the lean, indicating that buttresses are a gravity response to mechanical stresses. Mechanical stress has been linked to buttress formation in other tree species such as Lombardi poplar (Populus nigra L. CV italica). More information is needed about the relation between stem fluting and soil characteristics (depth, drainage, and stability).

Guidelines for Reducing Fluting

Although stem fluting may have an adaptive value in areas where trees are prone to windthrow. the monetary value of fluted logs is low. Fluted logs are difficult to debark for pulping; moreover, saw mills report a reduction in usable volume from fluted logs because flutes and bark seams can extend nearly to the center of the log. Even though stands with fluted hemlock trees exist throughout southeast Alaska, they generally occur on uplifted coastal beaches and alluvial flood plains. Application of specific silvicultural treatments in problem areas will reduce the prevalence of western hemlock and severity of fluting in managed stands.

Branches disrupt the vertical flow of carbohydrates in the stem causing annual rings to become asymmetrical. Flutes originate beneath decadent branches and extend downward, forming long grooves where

Branches disrupt the vertical flow of carbohydrates in the stem causing annual rings to become asymmetrical. Flutes originate beneath decadent branches and extend downward, forming long grooves where other branches are intersected.

 

Control of Stand Composition and Structure

Mechanical scarification or prescribed fire will reduce advanced western hemlock regeneration that often dominates after disturbance. Stand composition can be controlled further by planting Sitka spruce (Picea sitchensis (Bong.) Carr.).

Planting of Sitka spruce is recommended for areas where fluting was a serious problem in the previous stand. Manipulation of stand structure by thinning will also reduce the severity of fluting.

Western hemlock trees in suppressed or intermediate positions in the canopy exhibit mild degrees of fluting. Thinning to remove fluted hemlock from the overstory will create a stand with Sitka spruce as dominates and relatively cylindrical western hemlock as subdominants.

Western hemlock progeny from coastal southeast Alaska and British Columbia, such as this specimen growing at Alice Holt, England, often exhibit fluted stems.

Western hemlock progeny from coastal southeast Alaska and British Columbia, such as this specimen growing at Alice Holt, England, often exhibit fluted stems.

 
Six factors that probably affect the degree of fluting in western hemlock.
 

Six factors that probably affect the degree of fluting in western hemlock.

Pruning

Removal of decadent and dead branches may reduce severity of fluting. A strong relation exists between branch location and the occurrence of flutes. Flutes begin following a decline in branch activity and persist after the branch is lost through self-pruning. The flutes will likely persist for a shorter time if the loss of nonproductive branches is accelerated. Production of clear wood is an added benefit of pruning.

Plantations of Nonfluted Hemlock

Genetic selection and cultivation of nonfluted hemlock is another possible silvicultural option. More cylindrical western hemlock might be grown if progeny are selected from areas where the incidence of fluting is low.

Summary

Stem fluting of western hemlock is controlled by genetic and environmental factors. The fluted trait is most strongly expressed in coastal stands where a catastrophic disturbance has created an even-aged stand structure. Soil characteristics as they influence tree growth and stability seem to affect flute expression. Site preparation, prescribed fire, planting of Sitka spruce, thinning, pruning, and genetic selection are management strategies that can be used to reducethe occurrence and severity of stem fluting.

Authors

KENT R. JULIN was a Ph.D. candidate at the University of Washington, Seattle, WA, when the research was done. WILBUR A. FARR is a research mensurationist at the Pacific Northwest Research Station, Forestry Sciences Laboratory, P.0. Box 20909, Juneau, AK 99802.

Cover photo: A fluted western hemlock on the Old Hollis Town site, Prince of Wales Island, Alaska.