On December 21st, a brief wind storm ripped through my hometown of Longview, Washington. Over 200 city-owned trees as well as uncounted residential and rural trees were destroyed.
Lake Sacajawea, a 173 acre park in the downtown area was the hardest hit, showing some of the most dramatic damage. The park contains a great number of very large trees, many of them close to 90 years old. The devastation was astounding.
I had meant to go down and get some photos of the destruction, but with Christmas just around the corner, I didn’t get the opportunity until after the new year, when almost all of the mess had been cleaned up. There was however, one fallen tree that had not been attended to yet.
Besides satisfying my own curiosity, I wanted to get some tree damage photos for my Uncle, who is an arborist in California. Uncle Bruce co-wrote a book called, Oaks in the Urban Landscape: Selection, Care and Preservation. Since Lake Sacajawea has an abundance of oak trees, I thought that some documentation might provide my uncle with an opportunity to study what factors might have contributed to bringing some trees down, while others stood fast.
My Uncle visits the area occasionally and has often pointed out some of the problems faced by specific trees as we walked around the lake. Of course, he noticed these hollow trees, but he also detected other hidden issues revealed by the growth of tiny fungi for example.
Even with the meager amount of material I did sent to my uncle, namely the photos in this post, he had some interesting things to say.
He said,”I’m always amazed to see that trees that are more than 90% hollow often stand for so long. Most of a tree’s structural strength is in the outer “shell”. A tree could lose 50% of its radius to decay and lose less than 10% of its strength. Many large trees can stand with a shell thickness of 10% or less.” But he went on to say, “trees that are hollow should always be considered risky.”
My Uncle said that a postmortem serves to help predict failure of similar trees nearby exposed to similar forces. Here is a tree standing maybe 50 feet away from the fallen one. It is the same species, I believe. Besides a couple broken limbs, it still appears to be in pretty good shape. Or is it?
A closer inspection from the back side reveals this gaping hole. This tree has nearly the same issues as the first. Although the cavity appears fairly small in the photo, I could have easily climbed inside and possibly stood up, but instead I chose to activate my camera’s flash, reach my arm in pointing the lens upward and take a few shots.
My Uncle offered me one more thought without the advantage of seeing the shot at the bottom of the page. He said,”the other concern is root stability due to root rot (Armillaria mellea). If the lower trunk is hollow, the roots underneath are missing and the larger laterals are probably decayed to some extent.”
I would say he hit the nail on the head. It appears to my untrained eye that the lack of a viable root system combined with heavy wind brought this tree down.
There is also the possibility of one additional factor. The week preceding the the sudden wind storm brought a considerable amount of rain to the area, super-saturating the soil surrounding tree root systems. In this case there wasn’t much of a root system left. But other trees in the area appear to have been pushed over, roots and all.
I wonder how much longer the hollow standing tree or others in the area with similar conditions will be able to hold out, before they too succumb to the forces that work against them.