Within the arboriculture industry it is not rare to see the odd lightning struck tree on the job.
Damage from lightning may be scars (Figure 1), a loss of branches or bark, or, in extreme cases, death. However, other trees may survive a lightning strike unscathed, this article will explore parameters that can aid in the evaluation of both the probability of a tree to be struck by lightning, and the individual trees risk of failure due to lightning.
The vertical or spiral scar on a tree is a typical sign of lightning damage, where the shape of the scar depends on the grain of the tree’s fibre (Taylor 1962). A lightning strike on a tree may cause the water in the tree to turn into steam and cause a branch shattering explosion which may lead to death or a secondary attack that declines the tree (Jim, 2017). An example of a secondary attack may include damage by insects or fungal diseases that eat at or rot the wood (Yanoviak et al 2015). Larger trees will have a greater damaged area of bark or wood from lightning, however smaller trees have a higher percentage of damage (Taylor 1962).
The damage caused by lightening to the sapwood of a tree can have a flow on effect which results in the dehydration of the tree. A preliminary study theorised that in a multi-stroke lightening attack, the lightning plasma has a heating effect on the water in the tree (Cornish et al 2014). Fungal decay on sapwood can be caused by lightning (Luly 2021). A watering regime can be utilised to mitigate the dehydrating effect of lightning (DeRosa 1983). A preventative technique in lightning prone trees and areas may be the use of a copper rod, in such a way that the rod reaches the full extent of the tree height and is planted in the earth (DeRosa 1983).
The largest predictor of lightning in an area is climate. In a study of fires caused by lightning, trees in areas with higher rainfall are more likely to have fires caused by lightning than other factors during the wet season (Ramos-Neto and Pivello, 2000). Tree species can also be a predictor of lightning induced fires, conifer trees are prominently associated with lightning fires in the literature (Muller et al, 2013).
A tree may be more at risk to lighting exposure if the tree is tall, isolated or otherwise outstanding from a group of trees (Jim, 2017, Yanoviak et al 2015). McCaw and Read (2012) found that tall mature trees with dead wood or decay elevate fire risk associated with lightning. Topography also has an effect on tree lightning risk, with trees on ridge tops or elevated plateaus at higher risk (Ramos-Neto and Pivello, 2000, Yanoviak et al 2015). A large Eucalyptus pilaris (Blackbutt) at Cherrybrook was hit by lightning on a ridge and did not have the marks apparent but when cut, it was obvious dehydration on one side from the strike. (McArdle 2021).
The relative height of larger trees susceptible to strike can be reduced with lightning rod conductors, although in the field installations within trees are limited. When removing trees with lightning strike they are not suitable for wood fires because of the loss of gases and loss of mineral. They are not utilised for timber planks or milling as the density of the timber presents smaller pores than a seasoned timber plank and are easily fractured. Personal experience from a remnant dead tree Eucalyptus melliodora (Yellow Box) located at Upper Turon Bathurst of which was hit by discharge fractured the tree at 14 metres and destroyed the tree with force throwing wood splinters of greater than two metre x40xm shards greater than 300mm (McArdle 2021).
The damage lightning causes to a tree can also be predicted by parameters. The conductivity of the earth is correlated with the ability of the lightning to pass through the tree, and thus cause less damage (Makela et al 2009). As such, the higher the ground conductivity, the less chance of tree damage by lightning (Makela et al 2009). The ground conductivity is strongly influenced by moisture content, with saturated soils being highly conductive, and influenced to a lesser extent by soil texture, with sandy and rocky ground being highly conductive compared to clay or agricultural soils (Makela et al 2009).
In assessing tree health and determining lightning strike, key features to observe are leaves dehydrated and retained from sporadic discharge, spiral tracking around the stem, patches of bark (not always) detached or missing, dehydration of the wood nutrient phloem and xylem cells according to Kucera et al (1985), lightning strokes lead to quick desiccation within the wood, causing the collapse of any not fully stabilized cells, including cambium. Additionally discharge to metal conducting surfaces may indicate positive and negative discharge marks or in some cases puncture marks in metal where the discharge as routed to the quickest path to earth. Canopy dehydration and mortality are common but some trees can respond and regenerate depending on the damage to transporting vessels within the stem.
In summary, an arborist can make informed decisions about lightning strikes and trees to their clients. When discussing a trees risk in the context of lightning, it is important to comment on the climate and topography of the area, as well as the soil type. Dependant on symptom severity, lightning struck trees can be remediated, however sometimes the only safe option is tree removal. When looking at the tree, it is important to assess the level of damage already done by the lightning, the probability of lightning to occur again, the maturity, height and species, and condition of the tree.
For more information get in touch with Tree Contractors Association Australia by calling 1300 660 379 or via email – admin@tcaa.com.au