Published accounts of SGRs first appeared in the 1930s. Van Wormer (1937, 1940) observed that many trees that were declining during droughts had roots that encircled the stems, while many healthy trees did not have girdling roots. He surmised that girdling roots "strangled" the trees. Van Wormer’s account, however, was anecdotal. Scientific studies that quantified the impact and frequency of SGRs were first conducted approximately 40 years later by Tate (1980).
Interestingly, during the time span between Van Wormer’s and Tate’s published work, common tree care and tree pathology texts described SGR causes, tree health impacts, symptoms, and treatments (Pirone 1941 and all subsequent editions; Marshall 1942; Hallar 1959; Tattar 1978). It is not known whether these are first- or second-hand accounts. The authors presented no citations or original data to support their statements. It is plausible that Van Wormer (1937, 1940) formed at least a partial basis for the textbook statements, based on similarities in SGR descriptions. Pirone (1941) listed Van Wormer’s papers in his suggested reading section, indicating he was familiar with them. In sum, many reasons for SGR formation and the subsequent impact on trees were given prior to Tate’s work, but these were probably speculative and lacked scientific scrutiny.
Tate (1980, 1981) studied morphological characteristics of Norway maple trees with SGR in Ann Arbor, Michigan. He observed that trees with SGRs often lacked a normal root flare, had flattened stems, and had smaller leaves. However, he did not detect differences in survival, diameter, or crown density between trees with and without SGRs.
Hudler and Beale (1981) reported on the anatomical effects of girdling roots on stem wood and bark tissue. Roots that encircled the stem tissue caused deformation of xylem tissues, including fewer vessels with a smaller diameter; skewed rays; and compressed bark tissues. However, no research to date has been conducted that has investigated how or if these anatomical changes affect tree physiology.
|Fig. 1 - During a windstorm, this littleleaf linden failed below ground at a compression point created by stem girdling roots.|
The frequency of SGRs in various tree species was reported by d’Ambrosio (1990) and Watson et al. (1990). SGRs were commonly found on maple species (i.e., red, silver, sugar, and Norway), green ash, and honeylocust. (See Appendix for scientific names of trees referred to in this publication.) Tree care practitioners throughout the United States and Canada also reported in a 1997 survey their observations of 56 tree species that had SGRs (Hauer and Johnson 1997). And in a 1997 randomized study of 100, 3- to 9-inch diameter breast height (d.b.h.) sugar maples, 41% of the trees had stems that were compressed from girdling roots (Johnson and Johnson 1997).
Removal of SGRs has been recommended to prevent and reduce the impact of stem compression on tree health, but little scientific research has been conducted that either refutes or supports this recommendation. Watson and Clark (1993) found that after SGRs were removed, roots often grew in directions that placed them in conflict with the tree stems, creating new and potential SGRs. However, their research was limited to Norway maple.
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