Global warming has brought about both increased temperatures and an increased frequency of extreme weather events. These extreme weather events will have a significant impact on the stability of trees in the urban environment. This has initiated an increased interest in modelling predicting failure rates of trees under wind loading. Although tree stability is affected by root morphology and architecture, wind loading is the single largest force exerted on a trees root system and is the most common cause of failure (GARDINER et al. 2013)
Wind reaction measurements can be taken in real time during a wind event, or alternatively an artificial load can be exerted on the tree to elicit the same force placed by the wind (static pull test) to ascertain a trees stability, defined by its root anchoring force in the ground (ANGUS ELECTRIC 2018).
This is achieved through the placement of elastometers and inclinometers on the trunk of the tree which measure the strain created by windon the marginal wood fibres of the trunk and the resulting tilt of the root plate. It has been identified that the data collected from naturally occurring wind events is significantly more useful than the static pull test as it is hard to correlate artificial loads with environmental factors such as crown dampening and the effect of nearby vegetation and man-made structures on wind direction and speed (WESSOLLY & ERB,1998).
JAMES, HALLAM & SPENCER 2013 measured the root plate tilt of 250 trees in Victoria, Australia but no trees failed during the study. The maximum root plate tilt recorded during this experiment was 0.90o. Despite this other research has recorded the sound of breaking root fibres at 1o of tilt, therefore this is the recognised safety limit at which a tree should be considered for removal (WESSOLLY & ERB, 1998).
(Source: Angus Electric 2018)
(Source: USDA 2018)
Managing Tree Risk 101 