Issues of trees, health, and risks

Abstract

In this issue of the Arboricultural Journal the discussion of approaches and methods for tree valuation continues with particular but by no means exclusive relevance to amenity trees and trees in urban environments. There will be more on this important and controversial topic in future issues. Barrell (2021) considers how the complex process of tree risk management is applied to balance the benefits from trees and harm which may arise from proximity to people and property; a situation especially prevalent in urban settings. The level of management that the legal institutions, such as courts demand of duty holders, is continually subject to review and clarification. This process is driven by advances in technical knowledge and practical experience and through the societal insights of legal judgements and inquests. In this context, if they are to successfully avoid criticism and liability in the event of tree failure that leads to harm, then duty holders responsible for tree safety need to regularly review and adapt to the changing landscape of practice and litigation. This review considers implications from three recent developments which might have a bearing on how the courts assess tree risk management. Firstly, Barrell examines the civil case of Cavanagh v Witley Parish Council & Shepherd. In this example, the resultant High Court Appeal raised questions concerning inspection frequency and occupancy. Secondly, there have been two recent inquests which raised the importance of proactive management of highway trees (and hence a topic which today is never far from the headlines). Thirdly, there is the growing threat from millions of dying trees with ash dieback disease in particular and probably with globalisation and climate change, from other diseases too. Usefully the paper suggests a new model to incorporate such factors into strategic tree risk management with, for duty holders, two revised decisionmaking frameworks. Tree risks and failures are often connected to high winds and so our enhanced understanding of wind and its behaviour is always welcome. Goodwin (2021) addresses daily maximum high gust wind speeds at Dublin, Ireland. The study takes and analyses data from a recent 30-year period. In the research, the maximal and mean storm gusts were compared to those of the growing season. The work considers xylogenesis in a context of thigmomorphogenetic acclimation of temperate trees to the experienced wind environment. Considering notional minimal factors of biomechanical safety and major tree failures in storm winds, maximal annual gust probabilities were estimated. The correlation of the highest mean growing season gusts and extremum storm gusts is then reviewed. From the analysis, the successful acclimation to the mean highest 5% of growing season gusts was suggested to give a notional minimal factor of safety against major failure in extremum winter storm gusts of about four. From this study it is inferred that tree risk assessment might place more emphasis on factors of inadaptation which Arboricultural Journal 2021, VOL. 43, NO. 1, 1–2 https://doi.org/10.1080/03071375.2021.1898817