A new article analyses results of the first longitudinal study of sonic tomography and electrical resisitivity tomography repeated across several years.

Background

Tomography is widely used in tree risk assessment and forest ecology. Tomograms should be accurate, repeatable and comparable between consecutive measurements. This is the first longitudinal study addressing reproducibility over a period of several years and the key factors influencing it, from raw data to final tomograms.

Methods

Trees were either repeatedly measured by the same researcher over periods of hours to months, by different researchers using either the same or a new installation of the same equipment, or with different tomographs.

Results

Overall, differences between tomograms were small and without impact on the assessment of the safety of the tree. Between initial and follow-up measurements after 2 to 5 years, the coefficients of correlation of stress wave velocity ranged from 0.75 to 0.96, those of loss in section modulus estimated from tomograms ranged from 0.82 to 0.93. The coefficient of correlation of repeated sapwood area estimates with electrical resistivity tomography was 0.97. The major cause of deviations was variation in the sensor positions between repeated tomographies, and, for electrical resistivity tomography, variation of temperature.

Conclusion

Provided that sensor positions are accurately recorded and documented, tomograms can reliably be repeated by different assessors over periods of years. Assessments based on complex calculations like loss of section modulus should be treated with caution, because they accumulate and amplify all sources of variation.

Further reading

Rust, Steffen. 2022. “Reproducibility of Stress Wave and Electrical Resistivity Tomography for Tree Assessment”. Forests 13 (2): 295.