Iconicity in isolation

David Treanor

The Argyle Street ash in Finnieston, Glasgow, despite showing signs of Hymenoscyphus fraxineus infection, has demonstrated a notably slow decline compared to other ash trees, remaining resilient while many others, even within a few hundred metres, have succumbed rapidly to ash dieback.

As you may be aware, according to the categorisation matrix used in assessing the progression of ash dieback, which is detailed in Forestry Commission publications, trees affected by the disease are classified in four stages of decline based on canopy dieback percentage. This particular tree is at Stage 1, and retains well over 76% of its canopy, indicating only minor symptoms and a comparatively healthy structure.

We have monitored this tree for the last five years and visited once before to remove significant deadwood over one of Glasgow’s busiest streets. It clearly stands as an arboricultural anomaly. Its slow decline highlights a unique ability to endure, possibly due to the distinctive urban conditions around it that interfere with the pathogen’s life cycle. The resilience of the Argyle Street ash not only underscores the impact of microclimatic and spatial factors that influence disease progression but also enhances its status as a cultural and environmental icon within Glasgow, embodying hope amid the wider devastation of ash populations across the UK.

This tree, isolated amid the sandstone facades of tenements and the hustle of traffic in a prominent thoroughfare, exemplifies how environmental factors in a cityscape may actually play a role in shielding it from H. fraxineus.

The Argyle street ash.

Limited leaf litter and disrupted disease cycle

A key factor likely contributing to the Argyle Street ash’s resilience lies in the way its leaf litter is managed by its surroundings. As H. fraxineus relies on infected leaf litter to complete its life cycle, trees with decomposing, overwintering leaves at their base serve as a consistent pathogen reservoir. However, the Argyle Street ash has minimal opportunity for leaf litter accumulation. The wind and constant traffic disperse its fallen leaves, preventing them from piling up and overwintering – a critical step in breaking the cycle of infection.

With only a small patch of exposed soil and grass at the tree’s base, there’s little substrate for fungal spores to survive in high concentrations. This aligns with observations in the literature, which indicate that H. fraxineus struggles to build and maintain a viable reservoir of spores in urban areas where leaf litter is disturbed or removed.

Isolation from other ash trees

This tree’s isolation adds another protective barrier. Positioned without other ash trees within a 100-metre radius, the Argyle Street ash is less exposed to potential sources of airborne ascospores, reducing the likelihood of infection. Research suggests that isolated trees in open or urban settings suffer less from ash dieback than those surrounded by other ash trees, which create a localised source of infection.

Furthermore, the lack of lower branches up to around 8–10 m could also limit the tree’s exposure. Lower branches on other trees often become the first points of infection, where spores settle and find entry into the tree. By contrast, the Argyle Street ash’s height and clearance may reduce spore accumulation and entry points, protecting its main structure from initial infection.

Influence of urban microclimate on disease progression

The tree’s resilience may also be shaped by its urban microclimate. Although Glasgow is known for its damp climate, the tree’s positioning next to paved surfaces and hard landscaping impacts water runoff, reducing humidity levels at its base. Excess moisture typically fosters fungal growth, so the reduced soil humidity due to impermeable surfaces may create conditions that are less favourable to H. fraxineus.

There is also a possibility that radiated heat from the surrounding sandstone buildings influences the tree’s microenvironment. Sandstone absorbs and radiates warmth, potentially raising temperatures near the tree slightly during warmer weather. While it’s uncertain if this effect reaches the threshold to impact the pathogen directly, warmer and drier conditions have been documented to limit the development and viability of H. fraxineus.

Another factor to consider is airflow. With airflow somewhat restricted on one side by the tenement building, the remaining exposure may create a channelling effect, accelerating airflow around the tree. Increased air circulation could help to reduce moisture levels on the tree’s surface and within the canopy, further discouraging fungal spore establishment.

Cultural and environmental icon

In many ways, the Argyle Street ash has become an icon of resilience not only against natural threats but also as a symbol of survival amid urban challenges. Its ability to thrive in isolation, removed from the support network of a woodland or park, enhances its cultural significance. As Glasgow’s only ash standing prominently in such a bustling area, it represents both a biological outlier and a landmark of persistence, showcasing how trees can adapt and survive under unique pressures.

The Argyle Street ash stands as a living testament to resilience, and as Tree Preservation Order Number 1 it’s a notably cherished part of Glasgow’s urban ecosystem, defying the odds against a deadly pathogen and adding a sense of natural majesty to its cityscape.

This tree’s story reinforces the broader narrative of how city trees, shaped by particular microclimatic and spatial factors, may possess advantages in the face of environmental challenges. This hypothesis is inspired by a recent literature review by Combes, Webber and Boddy (2024) that links environmental factors to disease progression in ash populations, suggesting that certain isolated or distinct ash trees may have an advantage in the face of this pathogenic threat.

Leaf litter and sporulation

Research by Hietala et al. (2013) and Kirisits (2015) has demonstrated that H. fraxineus relies on ash leaf litter from previous years, as ascospores are dispersed from apothecia on infected rachises that accumulate within the leaf litter layer. The Argyle Street ash’s relatively isolated position in a dry, urban environment could mean its leaf litter decomposes rapidly due to warmer, drier conditions, potentially diminishing the pathogen’s ability to sustain a sporulation reservoir around the tree base. As Dahlsjö et al. (2024) observed, ash dieback severity is influenced by the decomposition rate of infected leaf litter; sites where litter decomposes in less than a year show reduced pathogen buildup, interrupting the cycle of infection.

The Argyle street ash from above showing little sign of dieback.

Arboricultural work being carried out on the Argyle Street ash.

Drought and temperature tolerance

The pathogen’s sensitivity to heat and drought could further protect the Argyle Street ash. According to Grosdidier et al. (2018), high temperatures can inhibit apothecia production. In Glasgow, where summer temperatures occasionally reach levels detrimental to H. fraxineus, this tree may be less exposed to consistent, spore-friendly conditions compared to trees in cooler, shaded environments. Additionally, Combes (2022) noted fewer apothecia produced in dry, heatwave years, suggesting that warm, urban microclimates may create challenging conditions for the fungus.

Proximity to potential infection sources

Unlike ash trees in dense forest stands, which can be heavily impacted by ash dieback, isolated trees in agricultural or urban settings experience reduced disease pressure. Research by Bengtsson et al. (2021) highlights that ash trees in open landscapes suffer less dieback than those in closed-canopy forests, likely due to better airflow and lower pathogen concentration in the vicinity. The Argyle Street tree is distanced from large populations of other ash, which limits nearby sources of airborne ascospores. Grosdidier et al. (2018) also found that apothecia abundance could be limited by the proximity of mating partners for the pathogen, which would further reduce infection risk.

Genetic resistance and phenotypic selection

Finally, research on heritable resistance to ash dieback (McKinney et al., 2011; Sollars et al., 2017) suggests that a subset of European ash trees possesses genetically based resistance. The Argyle Street ash, having potentially survived in relative isolation for some time, might represent a phenotype with inherent resilience. Although genetic selection for resistance is complex, this tree’s survival amidst widespread dieback hints at both genetic and environmental factors at play.

Implications for arboriculture

The Argyle Street Ash serves as a reminder of the complexity of urban tree management. Key lessons include:

• Adapting to the urban environment: Strategies for managing ash dieback must account for microclimatic variables specific to urban trees.
• Preserving genetic diversity: Trees that demonstrate resilience in the face of disease should be studied and potentially integrated into breeding programmes.
• Reframing urban trees: Beyond their ecological roles, trees like this ash are symbols of continuity and resilience, offering opportunities for public engagement and education.

Conclusion: environmental and genetic factors at play

The Argyle Street ash appears to stand as a testament to the interplay between environmental conditions and disease resilience. Its isolated location, relatively high temperatures, lower humidity, and distance from dense ash populations could create an environment where H. fraxineus struggles to establish, while the possibility of an underlying genetic resistance strengthens its ability to endure. This unique case reinforces the idea that urban trees, benefiting from specific microclimatic conditions, might hold valuable insights for developing management strategies that foster resilience in threatened ash populations across the UK.

David Treanor works as an arborist in Paisley, Renfrewshire.

References

Bengtsson, V., Stenström, A., Wheater, C. P., & Sandberg, K. (2021). The impact of ash dieback on veteran trees in southwestern Sweden. Baltic Forestry 27(1), 2–9.

Combes, M. J. (2022). The ecology and pathology of ash dieback disease. PhD thesis, Cardiff University.

Combes, M., Webber, J., & Boddy, L. (2024). Current understanding and future prospects for ash dieback disease with a focus on Britain. Forestry 97(5), 678–691.

Dahlsjö, C. A. L., Atkins, T. and Malhi, Y. (2024). Large invertebrate decomposers contribute to faster leaf litter decomposition in Fraxinus excelsior-dominated habitats: Implications of ash dieback. Heliyon 10(5), e27228. Available at: https://doi.org/10.1016/j.heliyon.2024.e27228

Grosdidier, M., Ioos, R., & Marçais, B. (2018). Do higher summer temperatures restrict the dissemination of Hymenoscyphus fraxineus in France? Forest Pathology 48(4), e12426. Available at: https://doi.org/10.1111/efp.12426

Hietala, A. M., Timmerman, V., Børja, I., & Solheim, H. (2013). The invasive ash dieback pathogen Hymenoscyphus pseudoalbidus exerts maximal infection pressure prior to the onset of host leaf senescence. Fungal Ecology 6(4), 302–308.

Kirisits, T. (2015). Ascocarp formation of Hymenoscyphus fraxineus on several-year-old pseudosclerotial leaf rachises of Fraxinus excelsior. Forest Pathology 45(3), 254–257.

McKinney, L., Nielsen, L., Hansen, J. et al. (2011). Presence of natural genetic resistance in Fraxinus excelsior (Oleraceae) to Chalara fraxinea (Ascomycota): an emerging infectious disease. Heredity 106, 788–797.

Sollars, E. S. A., Harper, A. L., Kelly, L. J., et al. (2017). Genome sequence and genetic diversity of European ash trees. Nature 541(7637), 212–216.

This article was taken from Issue 209 Summer 2025 of the ARB Magazine, which is available to view free to members by simply logging in to the website and viewing your profile area.