March did not go out like a lamb, nor did it end with a whimper. No, this lion ended with a grand BANG!
A lightning strike from the massive thunderstorm that roared through Seattle yesterday was a direct hit on one of our largest trees in the Washington Park Arboretum.
A Grand Fir located in the Oak grove at the north end of the Arboretum was obliterated with one flash. All that remains of a tree that was easily over 100 feet tall is a jagged snag and a circular field of debris extending at least 150 feet in all directions.
Electricity always takes the path of least resistance, so arborists in places where lightning is common will install tree protection systems. These usually are metal rods affixed to the top of the tree with a metal cable running down the tree to a ground rod buried deep in the soil. This system allows the tree to avoid catastrophic explosions like the one we had yesterday. Lightning is relatively uncommon in the Seattle area, so none of our trees have lightning protection systems.
So why did the tree explode instead of just breaking or cracking? Good question. A lightning bolt is hotter than the surface of the sun and has a strong electric current. The current is carried through the tree by the sapwood below the bark. This sapwood is composed of mostly water and when the bolt’s heat and electrical charge hit the tree, the water boils instantly and turns to steam; just like a pressure cooker, except the tree doesn’t have a steam release valve on top. So the result of the excessive heat and pressure causes the tree to explode. This is not common, but the results are spectacular!
We will never know why this tree was hit, but we have had a day full of speculation and mitigating safety hazards. Was the lightning attracted to this metal bolt inside the tree from a former cable?
Was it just a case of being in the wrong place at the wrong time? Was it the high volume of spring sap running? Was it because it was the tallest tree in an open area near water? Was it all of these factors and some unknown? We may never know but we will never forget.
One odd bonus of this amazing event is that lightning strikes are one of a few (non-synthetic) ways to fix nitrogen in the soil. Along with nitrogen-fixing bacteria and algae, the heat of a lightning flash causes atmospheric nitrogen to combine with oxygen to form nitrogen oxides. These oxides then combine with atmospheric moisture and are then delivered to the soil by rain, where it is transformed by microorganisms into nitrates that can be taken up by plant roots. Fascinating.
We know you never need an excuse to visit the Washington Park Arboretum, but we plan to keep the debris field intact for a few more days so any curious onlookers can come and check out our exploding tree. For your own safety, please stay behind the barriers, and enjoy the show.