Case Study: Fungal Fruiting Bodies - American Elm
A landscape manager at a small university noticed that the specimen American elm (Ulmus americana) in one of the university-owned lawn areas had white fungal fruiting bodies on its low stem and root flare. Because the neighboring tree had shown branch flagging associated with Dutch elm disease (Ophiostoma spp.) (The Morton Arboretum, n.d.) the previous summer and a lab test returned a positive identification, the manager was concerned that the larger tree was infected too, despite receiving preventative treatments for the disease.
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The subject American elm is 53 inches in diameter at standard height, and approximately 70 feet tall with a crown spread of approximately 60 feet. A narrow union which likely has included bark is present at approximately 15 feet, but shows robust response growth which indicates at least some compensation for the weak union (Slater and Ennos, 2016). Similarly, the low trunk has developed buttressing which is sometimes an indicator of internal decay as the tree attempts to compensate for loss of interior strength (Smiley and Lilly, 2013, pg. 111). Black fungi, in addition white fungal fruiting bodies are visible in the valleys between the buttresses around the base of the tree from the ground up to approximately 15 feet. The tree is dormant so an assessment of overall vigor and signs of branch flagging from Dutch elm disease was not possible, but the Dutch elm disease presents as vascular streaking in the extremities of the tree (University of New Hampshire, n.d.), and not large fruiting bodies in the low trunk.
A Level 2 Tree Risk Assessment returns a “Low” level of risk, despite the popularity of this lawn area for picnickers and dog walkers. However, the fungal fruiting bodies appear to be two separate facultative pathogens - Ganoderma applanatum and Kretzschmaria deusta (Luley, C., 2022). Further testing in the form of a lab ID for samples of both fungi, as well as sonic tomography to show the extent of interior decay is recommended. Restriction of the area within 70 feet of the trunk (approximately 1x tree height) is also recommended while additional testing and assessment are carried out. Long term mitigation actions are to restrict the target zone and monitor annually, or remove. Common recommendations to manage decline are crown reduction or “veteranization”, or reduction to a snag, but in this case they are not practical because of the presence of elm bark beetles carrying Dutch elm disease spores in the area which are attracted to open wounds on elms (City of Portland Urban Forestry, n.d.), and best practices for the complete removal of dead elms and proper disposal of elm wood (City of Portland Urban Forestry, n.d.), respectively.
The fungal activity is unusual in that there is extensive Kretzschmaria, but the majority of the Ganoderma conks appear to be very young - possibly in their first year of growth - despite being on all areas of the tree which also showed Kretzschmaria. Identifying the two separate pathogens was initially a surprise because fresh Kretzschmaria growth is also white, but the assessment was at the wrong time of year for new growth and the shape was not right. A small year-plus old Ganoderma conk was found near the base almost covered by grass which helped with identifying the white fungi elsewhere on the tree.
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Additional considerations in this case include the species’ large summertime sudden limb drop failures (Harris, 1983; personal experience); frequent use of space for picnicking, dog walking, and events; budget for monitoring and future maintenance on top of treatments for Dutch elm disease; and the status of the tree as a focal point for the area. Winter weather is less of a concern because it is deciduous and has less wind loading during dormancy, and the use of the field is less frequent.
Pending further information about budget from management, the recommendation is to fence the area around the tree and install arborist wood chip mulch to improve site conditions and protect maintenance staff by reducing time spent mowing inside the fence. The lawn is large enough to continue to accommodate the amount of traffic it sees, and the specimen tree should be retained until it absolutely must come out. Tomography and lab testing will inform management going forward and if the buttressed areas in combination with the remaining shell wall area are not enough to support the wind load, the management strategy can be adjusted. While the site of this tree played favorably into its fate, the same tree with immovable high value targets would receive a “Moderate” or “High” risk rating, and receive a recommendation for removal. Recognizing that multiple conditions within the tree can contribute to its likelihood of failure, and weighing against site conditions and other considerations such as the property manager's budget can be complex, and all components should be considered carefully before making a management recommendation. |
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This project was built as an assignment for the Tree Risk Management module within the MSc Arboriculture and Urban Forestry course at Myerscough College. All photos and content are my own unless otherwise attributed or cited. All names, addresses, and dates are made up, but accurately reflect season and site conditions at the time of assessment.
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