Extension > Garden > Yard and Garden > Trees and shrubs > A practioner's guide to stem girdling roots of trees > Occurrence of SGRs
Occurrence of SGRs
SGRs are not always at the soil surface and easily noticed. A respected pathologist was called upon to diagnose several Norway and sugar maple trees in Northfield, Minnesota, a few years ago. It was suspected that they were suffering from general decline and/or Verticillium wilt. Field symptoms for Verticillium wilt proved to be negative, and searches for above-ground signs and damage were fruitless. Root collar examinations revealed that the trees were severely affected by SGRs several inches beneath the ground level. Without root collar examinations, the primary cause of the trees' decline would not have been discovered.
Fig. 2 - This Norway maple had two SGRs that compressed almost 100% of the stem's circumference.
Fewer than 50% of the practitioners who responded to a 1997 survey (Hauer and Johnson 1997) performed root collar examinations as part of their diagnostic procedures. In that same survey, girdling roots were observed 52% of the time root crown examinations were conducted. Most practitioners reasoned that these examinations were too time consuming and/or their clients were not willing to pay for them. When below-ground examinations are performed as part of the diagnostic process, however, the frequency of SGRs associated with tree decline and/or sudden failure is noteworthy. In a recent five-year study, more than 80% of declining sugar maples had SGRs, which were presumably associated with the decline in health or death of those trees (Johnson 1999).
In storm damage research conducted since 1997 by the University of Minnesota (n=600), 73% of linden species that failed completely in the storms broke at SGR compression points (Figure 1). For all species, 30% of trees that failed completely and were not located in storm centers but at the edge, broke at SGR compression points (Johnson et al. 1999). ("Edges of storms" are areas outside the direct paths of straight-line windstorms or tornadoes.)
SGRs have been observed on a wide variety of tree species. In a practitioners' survey (Hauer and Johnson 1997), 56 tree species and genera, ranging from Acer to Zelkova, were identified as having been observed with SGRs (Appendix). The most commonly observed species were Norway, red, silver, and sugar maples and littleleaf lindens. SGRs also were observed frequently on Norway, red, and sugar maples by Watson et al. (1990), but not on littleleaf linden. D' Ambrosio (1990) observed SGRs frequently on Norway and sugar maples.
A note of caution: In a scientific study addressing SGR frequency, a population of trees would be sampled and a mean incidence of girdling within the population could be derived. However, root crown examinations are often conducted on trees that are exhibiting symptoms of decline or have failed during storms. Thus, frequency calculations derived only from a pool of symptomatic trees could underestimate or overestimate the incidence of trees with SGRs in any given population.