The destruction of tree strength at the cellular level can be a silent killer to the untrained eye. Tree owners should have large trees evaluated on a regular basis to fulfill their duty of care in owning large specimens. When extensive decay is present, fungal fruiting bodies can often be identified by a professional on the exterior of a stem or main pedestal trunk.
Brown-Rot Fungus
The white spruce tree shown above was located on municipal lands in a natural area on the edge of a neighbourhood. The tree had begun to fail, and displayed evidence of both structural deficits from extensive decay and crown decline. The tree had begun to tilt by a few degrees, and was approximately 70 feet tall. A Board Certified Master Arborist evaluated the tree and recommended its immediate removal because of its condition and proximity to neighbouring homes. The tree was on municipal lands so the order to remove the tree had to come from city staff who were called in to evaluate the tree. They stated the tree was not at any immediate risk of failure, and that they would monitor it annually. Shortly after their visit the tree ultimately failed in a mild wind gust event. Luckily no one was injured, and the only damage was to the neighbouring properties. The homeowners decided to go through their own insurance to repair their homes, and not pursue legal action against the municipality. When the tree failed it exposed the extensive internal decay, which appeared in the typical block like structure of wood that has been weakened at the cellular level from brown-rot fungus.
This failure could have easily been avoided, and the municipality could have been held accountable. At minimum it was a learning opportunity to bring to light the issue of unreasonably applied assessments of risk. Any tree that has begun to deviate from its original line of development is in a confirmed state of failure. Even if the tree was not slated for removal based upon the visible decay and sounding of wood, the fact that it was tilting off its growing angle should have been enough evidence to warrant the removal.
The decay organism was identified in the field to be brown-rot fungus, although this was not confirmed by a laboratory. Brown-rot breaks down the cellulose and hemicellulous of the wood, causing it to turn blocky and brown in appearance. Brown-rot does not break down the lignin, which is believed to cause the brownish colouration of the wood that remains. Brown-rot can proliferate from fruiting bodies that spread billions of spores from conks on infected trees. The spores that become suspended in the air, can travel great distances in the wind. The spores can inoculate a wound that has exposed the heartwood of a tree. Sapwood has protection mechanisms in place to make the environment inhospitable for fungal growth to take hold. More recent research has shown that spores have already inoculated the heartwood of many mature trees, and once the conditions are right the fungus can start to flourish. The fungal colonization could be instigated by a change in oxygen levels in the heartwood, such as the occurrence of a wound to the trunk.
Identifying Heart Rots
Much to the homeowners surprise the tree shown above failed in a windstorm. The initial reactions from the owners was that it was hit by lightening or that strong winds had blown it over. Looking at the extensive decay and large fruiting bodies along the trunk, a much different story is being told. The tree previously had two codominant stems, one of which that failed many years earlier. Instead of removing the whole tree at that time, the remaining single upright stem was retained. Over time the large open wound left by the codominant stem failure became an ideal environment for decay to set in. Eventually the weakened structure of the remaining stem gave way. The fruiting bodies could have been an indicator for the home owner to contact a professional prior to the failure occurring. A greater awareness could be promoted for property owners to know when to contact a tree care professional to fulfill their duty of care in owning a large tree. The wood structure of the failed tree clearly displays white rot fungus in the majority of the stem cross section. In many cases of white-rot fungus colonizations, the sapwood remains viable keeping the canopy full and healthy. When the canopy remains in full leaf, the tree is at an increased risk of failure from a combination of the resulting large canopy wind sail effect, and the compromised decaying stem structure.
White-Rot Fungus
Tree structure and tree health can be related yet very different things. The 100 year old poplar tree displayed in the images above, had a full canopy and was producing many resources from its crown despite its age. The full canopy was masking the extensive decay that had proliferated in the trunk. The tree was supporting its vast size through a network of a few remaining well placed roots, and a band of sapwood running up one side of the stem. The end result was a catastrophic failure that crushed a 30′ spruce tree into oblivion, and knocked the roof off of the house. Thankfully the tree failed away from the busy roadway to the south, which could have ended in dire consequences.
The tree was dissected to establish the cause of failure, and extensive decay was visually identified in most of the lower 20 feet of the trunk and root flair. Large conks were also visible along the main stem, clearly flagging the decay occurring inside. The tree was also subjected to many redevelopment construction activities in recent years. Conceivably these could have damaged the rooting structures and provided a pathway for decay to flourish.
The large root flair of this tree was approximately 10 feet across, and had extensive signs of decay in it. The magnitude of this tree falling was immense, and it completely obliterated 30′ spruce tree was not even known about until the removal of the failed section was virtually complete. The magnitude of forces involved in a failure such as this are almost inconceivable. Yet the fear of removing this large and rare established tree, was greater than the fear of its failure. When a hazardous tree is also is part of the fabric of a neighbourhood, the risk comfort of condemning a tree tends to lean to the retention side of the fence. It is not a logical decision to ignore signs of decay in large trees with significant targets. Yet it is an all too common occurrence when historic trees are involved, people tend to rely on emotional decision making.
White-rot fungus functions differently than brown-rot fungus in how it breaks down the composition of the wood. In addition to breaking down the carbohydrates in the wood’s structure like brown-rot, white-rot also breaks down the lignin as well. This completely destroys the composition and structure of the wood, leaving it extraordinarily weak. The exposed root flair of the tree at the point of failure, clearly shows a wide spread infection of white-rot fungus and the weakened wood. Decay was also found in a majority of the stem when the tree was dissected during the removal process. A small band of sapwood running up one side of the trunk was enough to keep the canopy in full leaf; which increased the wind loading on the compromised structure. Fruiting bodies along the stem were also present, and these perennial conks were easily identifiable.
Had a reasonable evaluation been done of the tree prior to failure, the decay would have been an easily identifiable risk. The massive size of the tree and its targets, created a significant risk no matter which direction the tree could have fallen.
Click on the link below for further information on white-rot & brown-rot fungus:
Western Canadian Tree Defects 