Visitors of our campus may have recently noticed tree removals near the intersection of Dearborn st and 33rd St. Two linden trees were removed at the site in late October. Another linden at the intersection of Federal st and 33rd has also been removed. The removals may have been shocking to passers-by – why would we decide to remove such incredible mature trees? This decision was necessary to improve the safety of the campus arboretum. The trees pose a significantly heightened risk due to compromised internal wood structure and active decay. Our mission as an arboretum includes preserving trees in our landscape. Removal is always a last resort, but sometimes a justified one in keeping our students, staff, and faculty safe. So, you may be thinking: how is the safety of a tree assessed? What signs signal the need for pruning or removal of a tree? Let us review these linden trees as an example of the process and share some insight into this decision.
Risk assessment
As trees mature, it is very common for them to develop cavities and defects in their woody tissues. These defects may be benign or compromising. A risk assessment is used to determine the associated risk with retaining a tree in the landscape. Of course, no tree is completely free of risk, and even the best-informed assessment cannot predict the future. The risk tolerance is up to the property manager. Sometimes risk can be mitigated, and sometimes removal is the best option.
Risk assessments are both qualitative and quantitative; some factors can be directly measured, and others are based on the experience and knowledge of the assessor. Assessing the tree’s health condition is the first part of the process and includes looking for compromised wood internally or externally, observing canopy distribution and fullness, seeking any visible disease symptoms, evaluating surrounding soil health, and more. This assessment includes the above-ground parts and the below-ground parts of the tree. Assessing the health of a tree can tell us if the tree will rebound from a significant defect or continue spiralling into poorer health, further weakening the structure. An unhealthy and declining tree is more likely to fail.
The second part of a risk assessment is evaluating the nearby potential targets which could be impacted by a failure, and how regularly those targets occupy the space. Occupancy rate can be anywhere from never to continuous. A tree in the center of an empty remote space has no object to strike, but a tree growing directly alongside a permanent structure, such as a building, always has a target in range of a potential failure.
The occupancy rate and tree health are combined with a third factor: the severity of the consequence for a potential impact. For instance, a failure with the potential to strike a person and cause significant harm would be a severe consequence. On the other hand, a failure with the potential to cause negligible property damage is a negligible consequence. Generally, the severity of an incident striking a person is rated higher than an incident striking property, even if the harm may be minor. People are always a more important consideration than property, and our decision-making process reflects that.
These factors combined create the risk assessment. Assessed risk may be low, moderate, or high. Most formal risk assessments tend to fall into the low category, so an assessment of high risk usually warrants prompt response. These 3 removed trees were all assessed as high risk.
Lindens by the 33rd Street Prairie
Let’s look at these specific Linden trees and why the removal decision was made. First, I want to share some historical background courtesy of our landscape architecture professor, Ron Henderson:
These trees were planted with the completion of the Galvin Library (formerly The John Crerar Library) and Hermann Hall in 1962. Including perhaps 5-10 years of nursery growth at the time of planting, this would make these trees around 70 years old. Galvin Library and Hermann Hall frame rectangular landscape spaces that flank 33rd Street. These were intended to be the center of campus in the Mies van der Rohe master plan of the early 1940s. Following the completion of Galvin Library and Hermann Hall, these Tilia americana, or linden/basswood, were planted along the east and west sides in allées, or parallel rows of trees. This planting configuration departed from the campus landscape idiom of pseudo-random planting established by Alfred Caldwell. In the 1999 Campus Landscape Master Plan, these allées of trees were protected, but their canopies were significantly crown-thinned in the early 2000s to increase sunlight to reach the ground.
The linden trees removed all showed signs of advanced decay and significantly compromised structural stability. Some had fruiting bodies (mushrooms/conks) present on tissues, indicating an advanced degree of decay. Several of the trees have Polyposous squamosus conks, a fungus which causes white rot. This fungus isn’t the sole cause for removal, but it is a positive indicator of active decay.
This specific tree also had a significantly compromised root structure. The tree was inspected in summer, and the presence of a cavity at the root flare was noted. The cavity progressed throughout the summer, and by October, almost half of the trunk’s base was completely degraded. This loss of sound wood at the root flare significantly compromised the tree’s structural stability and created a high risk of failure at the base.
The second tree at this location was closer to Dearborn St and also had significant signs of decay. The bark was showing a bumpy and swollen shape. This is a typical indicator of internal decay, where the bulging swollen points are created by the tree trying to seal off decayed wood on the interior of the trunk. The horizontal splits in the bark are another sign that a break is beginning to form within the xylem. As the tree begins to break apart, splits in the bark slowly become visible until the remainder of the tree snaps apart.
The third linden tree across the street had extensive brown rot, causing internal decay and compromising the structure. The causal fungus is likely Turkey tail, Trametes versicolor. Brown rot is a decay of the cellulose, taking away flexibility and leaving only the lignin of cell walls behind. This tree had lost a large limb just a few weeks prior, likely resulting from the brown rot, creating a very brittle structure. Active root decay was also observed, potentially Gandonerma species, which had created fruiting mushrooms just above the soil surface, radiating away from the trunk. These fungi can be very serious and may infect healthy wood and tissues, further accelerating the decay process.
These 3 trees were severely compromised, and immediate action was warranted. The remaining linden trees are being further evaluated for mitigation options. The Illinois Tech staff will continue working to keep our campus safe.
Alphawood Arboretum is making strides in enhancing the safety of everyone visiting our campus, but also in our capacity to identify and diagnose these issues. Our goal is always to make good decisions to prevent or mitigate risk before any harm occurs. We are currently exploring options for new equipment for the advanced technical diagnosis of compromised wood inside trees in our collection. This will give us a massive leap forward in our mission of preserving our urban forest, and we will be sharing more news as these updates begin to materialize. Thanks for reading, and look for more updates shortly!