Introduction

The failure of trees, especially in urban and landscaped settings, poses significant risks to public safety and property. Understanding the relationship between decayed wood and sound wood is crucial for accurately predicting the likelihood of tree failure. This article focuses on oak (Quercus sp.) trees and explores the ratios of decayed to sound wood necessary to maintain structural stability.

We will discuss the mechanisms of slow and fast decay organisms, examining how they differentially degrade the key structural components of wood – lignin, cellulose, and hemicellulose. This provides insights into the indicators of “creeping failure,” where trees exhibit gradual signs of decline leading up to a catastrophic collapse. Additionally, we will examine cases where trees fail without exhibiting any external symptoms, highlighting the importance of advanced assessment techniques for evaluating internal decay.

By understanding the complex interplay between sound and decayed wood, arborists, urban foresters, and other professionals can make more informed decisions about managing hazardous trees and mitigating the risks they pose to people and property. Ultimately, this knowledge is essential for preserving the health and longevity of our urban tree canopies while ensuring public safety.

Relationship Between Decayed and Sound Wood

Predicting Tree Failure

The likelihood of tree failure due to decay is a complex assessment, involving various factors such as the extent, location, and type of decay within the tree stem. Research indicates that the Moment of Capacity Loss (MCL) of a tree’s cross-section is a critical factor in predicting failure. MCL is the point at which the tree’s structural integrity is compromised, and its capacity to resist bending moments is reduced, 1.

The MCL is influenced by the size and position of decay relative to the direction of sway, with concentric decay having a more significant impact on MCL than peripheral decay. Studies have shown that in landscape oaks, internal decay is almost always present to some degree, with an average of 41% of the cross-sectional area affected. The strength loss associated with this level of decay was approximately 35%, highlighting the importance of accurately assessing decay to predict tree failure, 2.

While determining the exact amount of sound wood required for a tree to remain standing is challenging due to variability in decay patterns and tree species, maintaining a substantial proportion of sound wood is essential to ensure structural integrity, particularly in older trees where decay is more prevalent. Arborists and urban foresters must carefully evaluate both the extent and location of internal decay to accurately assess a tree’s failure risk.

Required Sound Wood for Stability

Determining the precise amount of sound wood required for a tree to maintain stability is a complex challenge, as it depends on various factors, including the tree species, the pattern and extent of decay, and the tree’s overall structural characteristics.

However, research has consistently shown that retaining a significant proportion of sound wood is crucial for ensuring a tree’s structural integrity, particularly in older specimens where decay is more prevalent. Studies have found that even a relatively modest 35% reduction in strength due to internal decay can significantly compromise a tree’s structural capacity and increase the risk of failure.

In a comprehensive study of mature landscape oaks, researchers found that 73% of the examined trees had detectable internal decay, with an average of 41% of the cross-sectional area affected. This highlights the ubiquity of decay in older urban and landscape trees and the importance of accurately quantifying the remaining sound wood.

While the exact safe threshold for sound wood varies, arborists and urban foresters generally aim to maintain as much sound wood as possible, prioritizing the preservation of structurally important regions of the stem. Techniques such as non-destructive testing, including sonic and electrical resistance tomography, can help assess the internal condition of trees and guide decision-making around retention, pruning, or removal.

Ultimately, the guiding principle is that a substantial proportion of sound wood is essential for a tree to remain stable and withstand the forces it encounters, especially in the face of progressive decay. Careful evaluation and monitoring of a tree’s internal structure are critical for predicting and mitigating the risk of catastrophic failure, 2.

Decay Organisms and Wood Digestion

Slow Decay Organisms

Slow decay organisms (fungi) degrade the lignin component of wood, which provides rigidity and structural support to the cell walls. These organisms often cause a type of decay known as white rot, which selectively removes lignin while leaving the cellulose and hemicellulose relatively intact.

The degradation of lignin by slow decay organisms can result in a spongy or softened texture in the affected wood, but it may not immediately compromise the tree’s structural integrity. This is because the cellulose and hemicellulose, which are the primary load-bearing components of the wood, remain largely unaffected.

The gradual nature of this type of decay can be deceptive, as the external appearance of the tree may not clearly indicate the extent of internal degradation. Slow decay organisms can operate within the tree for extended periods, slowly eroding the lignin and gradually reducing the tree’s overall strength and stability.

Arborists and urban foresters must be vigilant in identifying the presence of slow decay organisms, as the symptoms may not be immediately apparent. Visual inspections, along with non-destructive testing techniques, are crucial for detecting the early stages of this type of decay and determining the appropriate course of action to mitigate the risks posed by the affected tree, 4.

Fast Decay Organisms

In contrast to slow decay organisms, fast decay organisms primarily target the cellulose and hemicellulose components of the wood, leading to a type of decay known as brown rot. This type of decay can rapidly reduce the wood’s strength and structural integrity by breaking down the polysaccharides that provide the essential load-bearing capabilities.

The accelerated degradation of cellulose and hemicellulose by fast decay organisms can cause significant and rapid strength loss in the affected wood. This is a critical factor in the risk of tree failure, as trees can go from appearing relatively sound to catastrophically collapsing in a short period of time (for trees – this can be several  years).

The rapidity of this decay process is a significant concern for arborists and urban foresters, as it can outpace the visible external symptoms and make it challenging to accurately assess the true state of a tree’s structural stability. Often, by the time outward signs of decay become apparent, the internal degradation may have already reached a critical point, increasing the likelihood of sudden and unexpected failure.

Monitoring for the presence of fast decay organisms, such as through the identification of fungal fruiting bodies or other external indicators, is essential for proactive risk management. Additionally, the use of advanced assessment techniques, like sonic or electrical resistance tomography, can help provide a more comprehensive understanding of the internal condition of the tree and guide decisions around remediation or removal.

Understanding the mechanisms and impacts of fast decay organisms is crucial for developing effective strategies to mitigate the risks posed by trees affected by this type of rapid degradation. Vigilance, comprehensive assessments, and timely interventions are key to preserving the safety of urban and landscape tree populations, 4.

Indicators of Creeping Failure

Body Language of Trees

Trees exhibit various indicators of creeping failure, which arborists can use to assess the risk of collapse. These indicators include, 3, 4:

• Leaning: A noticeable lean can indicate root or stem instability.
• Cracks and Splits: Visible cracks in the trunk or branches suggest internal stress and potential failure points.
• Cavities and Decay: External cavities and signs of decay, such as fungal fruiting bodies, indicate internal wood degradation.
• Dead Branches: The presence of dead or dying branches can signal overall tree health decline and increased failure risk

Symptoms of Creeping Failure

The most obvious symptoms of creeping failure include, 3, 4:

• Mushroom Growth: The presence of mushrooms or fungal conks on the trunk or roots indicates internal decay.
• Bark Changes: Peeling or missing bark can reveal underlying decay or insect damage.
• Root Plate Movement: Soil heaving or exposed roots suggest root instability and potential failure

Failure Without External Indicators

One of the most concerning challenges in predicting tree failure is the phenomenon of trees or tree parts collapsing without exhibiting any visible external symptoms. This situation can occur when the internal decay within a tree is extensive, but not outwardly apparent through typical visual inspections.

Advanced assessment techniques, such as sonic and electrical resistance tomography, have proven invaluable in detecting internal decay that is not visible from the outside. These methods provide a more accurate and comprehensive picture of the tree’s internal condition, allowing for better risk assessment and more informed decision-making by arborists and urban foresters.

The absence of external indicators can be particularly troubling, as it can lull observers into a false sense of security about a tree’s stability. Trees that appear healthy and structurally sound on the outside may, in fact, be suffering from advanced internal degradation that has not yet manifested in visible symptoms.

This hidden decay presents a significant challenge, as traditional visual inspections may fail to identify the true extent of the problem. Without the use of advanced diagnostic tools, the risk of sudden and unexpected tree failure remains elevated, putting people and property in jeopardy.

Addressing this issue requires a multi-faceted approach, combining routine visual assessments with the strategic deployment of non-destructive testing techniques. By developing a deeper understanding of the tree’s internal condition, professionals can make more accurate determinations about the appropriate course of action, whether it involves remediation, pruning, or removal.

Ultimately, the ability to detect hidden internal decay is critical for effectively managing the risks posed by trees in urban and landscape settings. Continued research and the adoption of advanced assessment methods will be essential for improving the reliability of tree failure predictions and enhancing public safety, 3, 4.

Conclusion

Understanding the relationship between decayed and sound wood is crucial for accurately predicting the risk of tree failure, particularly in urban and landscape settings. The ratio of decayed to sound wood, as well as the specific types of decay organisms present, are key factors that influence a tree’s structural integrity and likelihood of collapse.

Slow decay organisms primarily degrade lignin, while fast decay organisms target cellulose and hemicellulose, leading to rapid strength loss. Trees require a substantial proportion of sound wood to remain stable, with studies showing that even 35% strength loss due to internal decay can significantly compromise a tree’s structural capacity.

Arborists and urban foresters must carefully assess both external and internal indicators of decay to accurately evaluate a tree’s failure risk. While visible signs like leaning, cracks, and fungal growth provide important clues, advanced techniques like sonic and electrical resistance tomography are needed to detect hidden internal decay. By understanding the complex interplay between sound and decayed wood, professionals can make more informed decisions about managing hazardous trees and mitigating the risks they pose.

Ultimately, the ability to reliably predict tree failure is essential for protecting public safety, preserving urban canopies, and ensuring the long-term health and viability of our trees. Continued research into the biomechanics of decayed wood and improved diagnostic methods will be crucial for advancing this critical field.

References and related papers

1. Ciftci, C., Kane, B., Breña, S., & Arwade, S. (2013). Loss in moment capacity of tree stems induced by decay. Trees, 28, 517 – 529. https://doi.org/10.1007/s00468-013-0968-8.
2. Brazee, N., & Burcham, D. (2023). Internal Decay in Landscape Oaks (Quercus spp.): Incidence, Severity, Explanatory Variables, and Estimates of Strength Loss. Forests. https://doi.org/10.3390/f14050978.
3. Okun, A., Brazee, N., Clark, J., Cunningham‐Minnick, M., Burcham, D., & Kane, B. (2023). Assessing the Likelihood of Failure Due to Stem Decay Using Different Assessment Techniques. Forests. https://doi.org/10.3390/f14051043.
4. Soge, A., Popoola, O., & Adetoyinbo, A. (2020). Detection of wood decay and cavities in living trees: a review. Canadian Journal of Forest Research. https://doi.org/10.1139/cjfr-2020-0340.
5. Frank, J., Castle, M., Westfall, J., Weiskittel, A., MacFarlane, D., Baral, S., Radtke, P., & Pelletier, G. (2018). Variation in occurrence and extent of internal stem decay in standing trees across the eastern US and Canada: evaluation of alternative modelling approaches and influential factors. Forestry, 91, 382-399. https://doi.org/10.1093/FORESTRY/CPX054.
6. Musah, M., Diaz, J., Alawode, A., Gallagher, T., Peresin, M., Mitchell, D., Smidt, M., & Via, B. (2022). Field Assessment of Downed Timber Strength Deterioration Rate and Wood Quality Using Acoustic Technologies. Forests. https://doi.org/10.3390/f13050752.
7. Loader, N., Robertson, I., & McCarroll, D. (2003). Comparison of stable carbon isotope ratios in the whole wood, cellulose and lignin of oak tree-rings. Palaeogeography, Palaeoclimatology, Palaeoecology, 196, 395-407. https://doi.org/10.1016/S0031-0182(03)00466-8.
8. Kennard, D., Putz, F., & Niederhofer, M. (1996). The Predictability of Tree Decay Based on Visual Assessments. Arboriculture & Urban Forestry. https://doi.org/10.48044/jauf.1996.038.
9. Xu, F., Liu, Y., Wang, X., Brashaw, B., Yeary, L., & Ross, R. (2019). Assessing internal soundness of hardwood logs through acoustic impact test and waveform analysis. Wood Science and Technology, 53, 1111 – 1134. https://doi.org/10.1007/s00226-019-01122-y.
10. Marais, B., Schönauer, M., Niekerk, P., Niklewski, J., & Brischke, C. (2023). Modelling in-ground wood decay using time-series retrievals from the 5th European climate reanalysis (ERA5-Land). European Journal of Remote Sensing, 56. https://doi.org/10.1080/22797254.2023.2264473.