On the morning of June 27, 2018, a woman got off a bus and walked over to sit on a nearby bench under a huge tree. She did not often sit on this bench because homeless people were usually encamped around it. But for whatever reason they were not there today, and she decided to sit for a while as she waited for her next bus. The morning was so beautiful that she decided to remove her headphones and fully take in the morning.
Just then she heard a cracking sound and looked up to see a foot and a half diameter tree stem falling at her from the canopy above. She dove out of the way and the stem came crashing down on the bench she was sitting at. Toady was a lucky day, had several little things been different she or some homeless people could have easily been killed.
The location of this incident was a small developed park on the top of a hill in a large Pacific Northwest city. Our subject tree was an Acer macrophyllum, bigleaf maple. This is a native, deciduous hardwood tree. Our subject tree had a diameter at breast height of about 44 inches and was roughly 85 feet tall. The tree separated into five codominant stems at about 10 feet up on its trunk.
This tree was one of the most impressive trees of its species that I had ever seen. The tree was so large that it engulfed the park in which it sat. The tree was deeply loved by the neighborhood residents. They had named it long ago and held summer block parties in the shade of its canopy.
While I would describe the weather that occurred that days as mild, I did find some interesting information while looking up more specific weather data. According to a nearby weather station, right before this failure occurred the winds which were blowing at 8 mph from the southwest turned. They then began blowing from the southeast and settled on blowing at 5 mph straight from the east. I don’t know how much this wind speed and direction change contributed to the failure, but I do find it interesting.
Some long terms effects that I consider somewhat relevant to this tree failure are the low rain fall totals which occurred this month, the abnormally high temperatures the month before, stressors associated with the lawn that surrounded this tree and the presence of a wood decay organism Kretzchmaria duesta.
First off, historical June rain fall averages for this area are 1.42 inches. This June there had only been .63 inches of rain. The stress caused be the lack of water was increased by the soil compaction around the tree and the competition from for what available water there was with the lawn. This complex situation was promoting the growth of the wood decay organism K. duesta. This organism’s effectiveness was likely promoted by the previous months high temperatures. High temperatures correlate with a lack of water in trees and a more gaseous environment in their wood. This environment promotes wood decay.
The wood decay organism effecting this tree was Kretzchmaria duesta, or brittle cinder fungus. K. duesta is a native wood decay organism of North America. It is very common in mature bigleaf maple. This organism is quite flexible in that I have seen it perform as a root rot, butt rot, stem rot and canker rot. This organism generally moves up from the soil into the lower heartwood of the tree. Overtime it can extend into the wood high up in the tree canopy. In our subject tree, heart rot of the wood was seen very high up into the tree’s canopy.
As K. duesta moves from the lower parts of the tree it poke holes out of the sapwood and bark. It does this to gain access to new sources of oxygen which help it advance up the tree. Once the fungus opens a hole in the wood and gains access to oxygen it fruits. K. duesta fruiting bodies start out as a whitish fungal mat which turns black as it ages. Eventually it resembles charcoal in appearance and texture. This is where the organism gets its common name, brittle cinder fungus.
The relationship of K. duesta and Acer macrophyllum is common in our urban and traditional forests. This relationship is part of a healthy forest where older trees are culled to make room for new trees and return nutrients to the soil. This relationship can also promote the longevity of A. macrophyllum where older trees shed large stems and then sprout a new small canopy from the remaining stump. This process is call retrenchment or natural crown reduction.
Acer macrophyllum has a proclivity to grow fast and shed large branches and stems as it ages. This equates to it being the tree that the PNWTFD has the most failures reports of. This pattern of sheding large branchs and stems at maturity is evident in the PNWTFD data with the adverage size of a failed A. macrophyllum is 34 inches in DBH.
Most of these branch and stem failures happen in winter but we do see a bump in June branch and stem failures. Lastly, of the 183 A. macrophyllum failures where data was collected about the contributing factor including the presence of a decay organism, 52 failures (28,4%) listed K. duesta as being present at the time of failure.
The last contributing factor to this tree’s failure was not location or condition based, it was societal. After assessing the post failure tree, it was determine that the tree was to be removed. During the removal process, a local resident entered the work site and refused to let work continue. They were in shock and could not accept that the tree had structural issues. This person only allowed work to continue after other community members arrived on scene and helped the onsite arborist explain the situation to the subject resident. During this time, one resident mentioned that years ago, they and the subject resident stopped the local municipality from cabling and pruning the tree. The City planned to cable and prune the tree after an inspection which included the use of a resistance drill. These two community members vocally fought the City in order to have them not prune the tree because they considered the work unsightly and they loved their tree the way it was.
After their past transgressions were brought to light, they asked the onsite arborist, “ If we had let the city prune and cable the tree in the past would it have failed today?” The onsite arborist responded that “Probably not.”
So, what caused this tree to fail? Like usual, many things. Somethings we could have changed and others we would have had a hard time changing. Things that were sort of out of our hands were; weather patterns, the species proclivity to fail and re-sprout, and the presence of K. duesta. The things we could have changed were; the stressors related to the lawn, soil compaction, and the presence of targets under the tree.
Lastly, the thing that should have happened but didn’t; the municipal entity should have stood up to the community pressure and made the hard choice to manage and preserve their unique and loved veteran tree. But in the end, this is another case of what my mentor Torrey Young would say, “Someone loved their tree to death”.