A practitioner's guide to stem girdling roots of trees
Impacts on Trees, Symptomology, and Prevention
If a tree falls in a forest and no one is there to hear it fall, does it make a sound? A similar riddle can be applied to trees with stem-girdling roots: If a dead, declining, or fallen tree has roots encircling and compressing the stem, were those roots responsible for the damage? It's a controversial question. Opinions, anecdotal information, and some research argue yes, no, and sometimes.
Stem girdling roots (SGRs) do affect trees, as any disorder would. SGRs, as opposed to root-girdling roots, encircle or run tangential to a tree's stem, eventually compressing the woody and nonwoody tissues of the stem. The degree to which trees are impacted varies with severity of encirclement, site (growing) conditions, weather, age, size, and, very likely, genetics. Urban trees are subjected to a continual barrage of natural and unnatural stresses-conditions that deviate from optimal. SGRs add another layer of stress, sometimes significantly.
Trees may decline and prematurely die as a result of the stresses induced by stem girdling roots. Or, they may appear healthy and normal until they suddenly fail during a windstorm, breaking at or near the point where girdling roots have compressed and weakened the stems.
Trees commonly fail for structural and physiological reasons. Physiologically, trees might slowly decline and die as a result of SGRs. Or, a tree might suddenly fail during a windstorm when stresses accumulate to the point of acute strain on the tree's structural system from stem compression and decay due to SGRs. This translates to economic and environmental losses: labor and materials to maintain trees; labor to remove and replace trees; cost of new trees; damage to personal and public property; and loss of carbon sequestration, shade, wildlife habitat, noise attenuation, and other benefits of trees. The degree to which SGRs affect urban forest health and condition is not known due to insufficient research and, very likely, to inaccurate diagnoses of tree disorders and losses.
The purpose of this publication is to present an objective perspective of SGRs. Most importantly, this publication reviews the symptomology, potential causes, treatments, and prevention of decline associated with SGRs. It is intended for field and diagnostic applications by arborists, landscape managers, growers, and urban forest health specialists. Research on SGRs is limited, but more practitioners are becoming aware of the problem, and new information on the formation and effects of SGRs has been recently collected. The lack of information might be partly due to the fact that roots are not always clearly visible because they are buried under soil or mulch. Another reason for the lack of research on this subject might be incorrect or incomplete diagnoses of tree problems. Few practitioners investigate below ground during tree disease/disorder diagnostic efforts.
Perhaps the most important sections of this publication are those on the symptomology and prevention of SGRs. Although the extent is relatively uncertain, SGRs do cause damage and premature loss of trees or tree health. As with any other natural or unnatural stress, recognizing the problem and preventing future damage to urban trees are the practitioner's responsibilities and goals.
About the authors
Gary R. Johnson is an associate professor of urban and community forestry at the University of Minnesota, College of Natural Resources, Department of Forest Resources.
Richard J. Hauer is a tree health specialist with the Minnesota Department of Agriculture, Agronomy and Plant Protection Division.
Funding for this publication was provided by the technology transfer program of the USDA Forest Service Urban Forestry Center for the Midwestern States.
The authors gratefully thank Jana Albers (Minnesota Department of Natural Resources), Dr. Jeffrey Dawson (University of Illinois at Urbana/Champaign), Dr. Robert Miller (University of Wisconsin–Stevens Point), Dr. Rex Bastian (Hendricksen The Care of Trees), Dr. E. Thomas Smiley (Bartlett Tree Research Laboratories), Mark Stennes (Top Notch Treecare), and Dr. Pat Weicherding (University of Minnesota Extension Service) for their constructive reviews and comments.
Product Manager--Gail M. Tischler
Editor--Mary K. Hoff
Graphic Designer--Jim Kiehne
Photo credits: the authors, Angie Hauer, Dr. George Hudler, Dr. Cynthia Ash, Douglas Courneya, Don Breneman, Jim McGannon