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Why lightning strikes
Because electricity seeks the path of least resistance and the moisture (sap and water) inside a tree is a much better conductor than air, a tree is a preferred path for lightning to reach the ground. The moisture is instantly heated far beyond the boiling point and the resulting steam creates an explosion whose effect varies depending on the location in the tree.
Tree susceptibility
Tall trees are the most vulnerable, especially those growing alone in open areas or near water. Moisture content is also a significant factor. Consequently, some species are struck more frequently than others. Note that while conifers are less likely to be struck than hardwoods of the same size, conifers will sustain more damage.
Likelihood of lightning strike
| Low | Medium | High | Very High |
|---|---|---|---|
| beech birch | elm maple pine spruce | ash oak | tulip poplar |
Tree damage
Within the tree, lightning seeks the area with the most moisture because water is a better electrical conductor than wood. Because the area just under the bark layer usually contains the most moisture, lightning tends to travel just underneath the bark. The exploding steam blows off a bark strip along the lightning path and often leaves a ½″ wide groove inside the scar. However, if a tree already has dying or rotted areas deep in the trunk, these may become the preferred path and the entire tree can be blown apart. On the other hand, if heavy rain has soaked the outside of the tree, a lightning strike may flash over the outside of the bark, leaving little or no damage.
Potential for tree recovery
If only one side of the tree shows evidence of a lightning strike, often it will survive and eventually close the wound. However, a strike that completely passes through the trunk, with splintered bark and exploded wood on each side, is usually fatal. Unfortunately, a tree that has lost a significant amount of bark is highly vulnerable. It cannot withstand dry spells or freezing winters and the injury leaves the tree exposed and defenseless against disease and parasites.
If current stays within the tree it will seek ground through the roots, especially if the soil is dry. Trees growing in rocky or dry soil are therefore highly susceptible to root damage. Because rock is a poor conductor, the charge will travel greater distances within the root system. Major root damage from electricity can cause decline even though there was no significant above ground damage.
