Work-related deaths associated with trees occur most often with tree felling and lopping, logging, sawmilling and arborist activities [8]. Logging and forestry work is inherently dangerous with significant rates of serious injury and death. More than 100 deaths per 100,000 person years are reported, which is 2–3 times greater than other high-risk occupations such as mining and construction [9,10,11]. Tree lopping may cause injury and deaths amongst arborists and also amongst amateur gardeners, who tend to be in an older age bracket (40 vs. 60 years on average). Electrocution and chainsaw injuries are also possible [12].

Falls from trees may be either work-related or recreational. Work fatalities from falls include not only descent from trees but also from ladders and boom lift buckets [12]. Trees may be involved in lethal vehicle crashes and it may be difficult in certain cases to determine whether the impact was inadvertent or intentional. In the absence of documented suicidal ideation and/or a note, careful assessment of the crash scene looking specifically at weather and road conditions and for the presence or absence of skid marks or attempts to avoid the impact may provide important information to evaluate possible intent [13].

Trees may provide convenient attachment points for ligatures in suicidal hangings and have the convenience sometimes of being located in secluded places to facilitate the activity [14]. In such cases it must be remembered that hangings may also be used to disguise homicides [15, 16] as occurred in one of the victims of the infamous Snowtown killings in South Australia who was found hanging from a tree at Humbug scrub [17].

Deaths involving trees may also occur with bush/brush fires, during hunting using tree stands, and also from animal or insects resident in trees or from tree derived toxins or poisons [8, 18, 19]. Impact may occur from falling seeds such as coconuts which may hit the head with considerable force [20].

Finally deaths may be associated with so-called tree failures, or summer branch drop, where large branches or whole trees may unexpectedly descend [21]. As can be seen from the reported cases, lethal episodes may occur when individuals are merely standing or sitting underneath tress, or are driving along roads with overhanging branches. The size and weights of the branches or trunks are such that the episode often occurs extremely quickly usually precluding escape.

Windy conditions and storms, with or without ice or snow accumulation, may predispose to tree falls (reported in 90% of fatalities [22]). This includes thunderstorms, cyclones and tornadoes [23, 24]. This is not always the situation as fire, drought and insect activity also predispose to failure. It is sometimes the case, however, that an apparent spontaneous dropping of a branch may actually represent a branch that was broken off during a previous storm that was trapped in the canopy, only to subsequently fall. These have been called ‘widow makers’ [8]. Predicting the likelihood of tree failure before and during storms has proven challenging [1, 25, 26].

A wide variety of trees may suffer from failure, the causes of which include auto-amputation where trees preferentially drop branches to conserve moisture in periods of hot weather, and collapse from rotting at the base (Figs. 1 and 2). Branch drop may occur following heavy rain with internal degradation of trees often being difficult to detect. In urban situations soil compaction and reduced light may accelerate tree decline [21]. In one study half of deaths reported during outdoor recreational activities occurred when a branch or tree fell onto a tent [27].

A street tree in nearby North Adelaide which suddenly collapsed shortly after the incident described in case 3 narrowly missing parked cars. The tree is a hackberry (Celtis occidentalis) and has also rotted through at the base

The findings at autopsy depend on the circumstances, and range from severe multiple injuries involving all body cavities to blunt head trauma. In some cases there may be evidence of crush asphyxia [12, 28, 29].

As can be seen from the data on lethal cases accrued over a 20-year period in the current study these events are not common. For example, Schmidlin reported only 407 deaths from wind-related tree failures in the United States from 1997 to 2007 [23] with 273 deaths over 160 years in Australia [22]. In the United Kingdom 64 deaths occurred between 1999 and 2009 from falling trees or branches, representing an average of 6.4 deaths per year [30]. In a clinical setting it was estimated in one report that only 0.26% of hospital admissions for trauma over an 8.5-year period involved trees and that only 0.15% were associated with tree failure [31].

This study has demonstrated that deaths due to tree failures, although uncommon, may be regularly seen in forensic practice. A variety of different tree species may be involved and these events may not necessarily occur during high winds or storms. The major findings at autopsy are blunt force injuries, sometimes with evidence of crush asphyxia.