Lightning and Trees

Lightning and Trees seem to go together. Many fires in the Western US are started by lightning strikes on trees. Here in the Bluegrass and Nashville Basin, we find fire scars on a very large number of trees. Lightning rarely kills our trees, but it is an important factor in their lives. Lightning-struck trees lose branches,part of their crown, sometimes the entire crown and have to rebuild. Decay becomes an important factor, but if the tree is healthy, growing well and stays ahead of the decay fungi, it will live a very long life.

We should never assume that a lightning-struck tree will decline or die, even if it appears severely damaged. Lightning struck trees are often over-managed by tree care workers eager to take action (and make a few bucks). Since lightning damage may take several years to fully reveal itself, observation is the best course. Obviously, this is not the case if the tree has an immediate target such as a home, street or playground. Even then, if possible, observation is best unless the risks are too great.

A street tree near my home was struck and lost some bark. A local tree company recommended that the tree be taken down, or that it be heavily pruned. The owner chose to do nothing and 30 years later, the tree is still in excellent shape. I have also seen trees that were struck, showed no damage and a year later were dead, probably because the roots were killed. Observation is the key.

The fate of a tree depends on the course that the lightning takes. During a thunderstorm, with water coursing down the bark, especially on smooth-barked trees like beech, the electricity may flow down the outside of the tree and do no harm (to the tree – anyone standing near it is at great risk). In drier condition, the current may travel through the wood, and either cause a crack or explosive rupturing of the wood. A cracked tree may not be seriously harmed, and again only time will tell. Old trees with significant decay are more likely to explode or be severely damage.

During the spring, the path of least resistance is the vascular cambium. The high salt and sugar content of the cambium and adjacent developing phloem and xylem provides a path for electricity. This may result in a strip of cambium being killed and the bark popping off, often in a spiral pattern. As with the cracked tree, damage may not be fatal. Sometimes the path boils the entire cambial layer, killing the tree. This may be immediately obvious if the bark pops off or explodes, or it can be subtle, with the tree dying some time later the same season or during the dormant season. I suspect that more trees are killed by lightning in the spring than summer, but this is difficult to prove, and there has been little research on the subject.

We find a certain number of very large old trees in the Bluegrass that die suddenly for no apparent reason – no signs of dieback, insects, diseases. I have long suspected that some of these trees have cryptic lightning damage, and that the cambium or fine roots were killed.

Fire is not a significant factor in the Bluegrass woodland pastures, though lightning-struck trees occasionally burn. We do not find fire scars in the wood, or charcoal in the soil. I have seen two blue ash trees struck by lightning that burned. Both were extensively decayed before the lightning strike and the fire traveled slowly down the interior of the stem. In neither case was a ground fire ignited, though there was grass scorched by the initial strike.

If we compare our woodland pastures with those of Europe, we can see a very different role of lightning. Lightning is much less common in most of Europe than it is here in the Bluegrass. It is probably a less important factor in the growth and development of European wood pastures than the woodland pastures of the Bluegrass. The maps below show the annual frequency of lightning strikes in the US and Europe (Data from NASA’s Lightning and Atmospheric Electricity Research at the Global Hydrology and Climate Center (GHCC).  We do not know the frequency of lightning damage (e.g. stem scars) in European wood pastures compared with those in the US.

Bur oak with lightning scar
Bur oak with lightning scar
Bur oak with lightning scar and lost crown
Bur oak with lightning scar and lost crown
Bur oak with deep lightning scars
Bur oak with deep lightning scars
Lightning Strike Frequency, Annual
Lightning Strike Frequency, Annual