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How Well do Arborists Assess Likelihood of Failure Due to Stem Decay?
Dr. Brian Kane
Massachusetts Arborists Association Professor
University of Massachusetts – Amherst
Arborists often investigate the extent of stem decay to assess the likelihood of stem failure when conducting tree risk assessments. We asked 18 experienced tree risk assessors who held the ISA’s Tree Risk Assessment Qualification to assess the likelihood of stem failure due to decay (in a three-year timeframe) on 30 trees in two genera (Quercus and Pinus). On each tree, assessors rated likelihood of failure five times, after (1) visual assessment, (2) sounding the trunk with a mallet, (3) viewing a diagram of the cross-section showing sound and decayed wood determined from resistance drilling, (4) viewing sonic and electrical resistance tomograms, and (5) consulting with another assessor.
Of nearly 2,300 individual assessments, 6% were for a probable likelihood of failure, 40% were for a possible likelihood of failure, and 54% were for an improbable likelihood of failure. Despite the small number of probable ratings, assessors rarely agreed on the likelihood of failure rating. On average, of 30 trees, only 3 had a single likelihood of failure rating that all assessors agreed on. In contrast, 19 trees had 2 different likelihood of failure ratings, and 8 trees had 3 different likelihood of failure ratings. Compared to the initial visual assessment, assessors more frequently changed their likelihood of failure rating after seeing the resistance drilling diagram and tomograms rather than sounding with a mallet or consulting with another assessor. Tree risk assessors should recognize that variation among likelihood of failure ratings is common, but may be reduced with training and experience.
Dr. Kane would like to acknowledge his co-authors, A. Okun, N. Brazee, M. Cunningham-Minnick, J. Clark, and D. Burcham and the study’s participants.
This research study was funded in-part by TREE Fund's 2019 John Z. Duling Grant Program.
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