Evaluating the Reproducibility of Tree Risk Assessment Ratings Across Commonly Used Methods

Abstract Background Tree risk assessment methods have been developed to assist arborists in conducting thorough and systematic inspections of trees and the threat they pose to people or property. While these methods have many similarities, they also have a few key differences which may impact the decisions of those employing them. Moreover, arborists specify the associated timeframe for their risk assessment, which can range from months to years. How this impacts risk assessment reproducibility is unknown. Methods To assess the impact of risk assessment methodology, we sent videos depicting trees in urban settings to arborists holding the International Society of Arboriculture (ISA) Tree Risk Assessment Qualification (TRAQ; n = 28) or Quantified Tree Risk Assessment (QTRA; n = 21) training. These assessments were compared to those prepared by North American arborists lacking the TRAQ credential (ISA BMP; n = 11). ISA BMP arborists were also asked to assess trees using both a 1-year and a 3-year timeframe. Results While a direct comparison between the QTRA and TRAQ assessments is not possible given differences in terminology, arborists with the latter training were less likely to rate trees as having “high” or “extreme” risk compared to their ISA BMP counterparts. Moreover, we found that switching to a longer timeframe did not increase the variability of risk assessments. Conclusions These results give further insights into how different risk assessment methods compare when assessing the same group of trees as well as the impact of training efforts and specified timeframe.