The editor’s choice for our October issue is ‘Neighbours matter and the weak succumb: Ash dieback infection is more severe in ash trees with fewer conspecific neighbours and lower prior growth rate’ by David Cracknell et al. Here, Associate Editor Anne Kempel discusses the importance of this research:
For nearly three decades, European woodlands have grappled with a formidable adversary – Ash Dieback Disease (ADB). Caused by the ascomycete fungus Hymenoscyphus fraxineus, ADB has spread relentlessly from East to West across the continent, leaving a trail of devastation in its wake. Originating from Asia, it was first observed in Poland in the early 1990s where it caused significant damage to ash trees. From there it spread to other European countries, primarily through the movement of infected ash plants and wood products.
The disease’s arrival in Great Britain in 2012 marked a significant turning point, as it signaled the disease’s arrival in a region with extensive ash populations. ADB is characterized by the progressive dieback of foliage in the crown, with leaves wilting, yellowing, and browning from the tips inward. Research on ADB has projected an average cumulative mortality rate of approximately 60% for ash trees in natural woodlands across Europe, albeit with wide confidence intervals (Coker et al., 2019).
But what determines the severity of Ash Dieback Disease in ash trees? Environmental conditions, such as wet and humid climates, or a high density of trees leading to strong competitive interactions, could be contributing factors to the growth and spread of the fungus. Poor plant vigor or a high density of conspecific trees in the vicinity might provide further advantages to this specialized fungus. Leveraging an extensive monitoring dataset from an ancient woodland nature reserve – the Lady Park Wood – Cracknell and coauthors delve into these complex factors in this Editor’s Choice article.
Lady Park Wood spans 36 hectares of woodland in Gloucestershire and Monmouthshire, UK, set aside by the Forestry Commission in 1944 to study natural woodland development. Over the years, a unique long-term study has meticulously recorded individual trees and shrubs at intervals, accumulating detailed records of more than 20,000 individual species, including beech, sessile oak, ash, wych elm, small-leaved lime, large-leaved lime, birch, hazel, yew, and others. The extensive history of monitoring at the site provides rare longitudinal records from repeated censuses of all trees in permanent sample plots.
In the decades since the study’s inception, the woodland has changed; trees have grown, died and regenerated, while drought, disease and other events shaped its destiny. Ash dieback disease also made its presence known in Lady Park Wood. In 2019, Cracknell and coauthors surveyed more than 450 ash trees across ten permanent transects, assessing the incidence and severity of ash dieback disease symptoms. They then correlated disease variables with an array of potential explanatory factors, many of which were only available due to the extensive historic plot records at Lady Park Wood.
Their findings underscore the successful spread of the fungus. Ash dieback disease had infected 81% of all ash trees by 2019, with an increasing trend in 2020. Disease severity was evidently linked to site environmental factors, with higher severity in areas with moist soils. However, contrary to previous research, disease prevalence was not higher in denser forests.
Additional findings corroborated results from earlier studies (Klesse et al., 2021) and emphasized the pivotal role of tree vigor in mitigating the severity of infection. Cracknell and coauthors explored whether the condition of an ash tree prior infection influenced the severity of the disease. Indeed, their investigation unveiled that trees which had exhibited a high growth rate over the eleven-year period preceding the initial disease observation displayed fewer disease symptoms, rendering them notably more resilient to the disease. Such a profound analysis was made feasible solely due to the extensive historical monitoring records preserved at Lady Park Wood.
Surprisingly, disease incidence was unrelated to ash tree density at the stand level, and disease severity was even higher in trees with a greater species mingling, i.e. a lower proportion of conspecifics in the nearest neighbourhood. This positive effect of conspecifics challenges the widely held view and a vast body of ecological research (Halliday et al., 2020; Keesing et al., 2006) stating that diverse plant communities tend to be more resilient and resistant to pathogens compared to monocultures. The mechanism behind these findings remains unresolved in the current study, but future analysis of the identity and size of neighboring trees could offer promising avenues to explore. Certain tree species might exert stronger competitive effects on ash trees or influence ash vigor or pathogen dispersal through leaf litter chemistry, conceivably overriding potential positive diversity-disease relationships.
Ash Dieback Disease continues to pose challenges to European woodlands, emphasizing the critical importance of comprehending its dynamics for effective management. While genetic selection and breeding offer hope for the future, ongoing research is shedding light on the complex interplay between environmental factors, tree metrics, and neighboring trees in influencing ADB severity. Although many questions remain, studies like the one by Cracknell et al., demonstrate the power of extensive historical spatial and temporal data combined with extensive near neighbour spatial analysis, offering invaluable insights into understanding the underlying factors in driving disease severity in natural ecosystems.
Read the full article online: Neighbours matter and the weak succumb: Ash dieback infection is more severe in ash trees with fewer conspecific neighbours and lower prior growth rate’
References:
Coker, T. L. R., Rozsypálek, J., Edwards, A., Harwood, T. P., Butfoy, L., & Buggs, R. J. A. (2019). Estimating mortality rates of European ash (Fraxinus excelsior) under the ash dieback (Hymenoscyphus fraxineus) epidemic. PLANTS, PEOPLE, PLANET, 1(1), 48–58. https://doi.org/10.1002/ppp3.11
Halliday, F. W., Rohr, J. R., & Laine, A.-L. (2020). Biodiversity loss underlies the dilution effect of biodiversity. Ecology Letters, 23(11), 1611–1622. https://doi.org/10.1111/ele.13590
Keesing, F., Holt, R. D., & Ostfeld, R. S. (2006). Effects of species diversity on disease risk. Ecology Letters, 9(4), 485–498. https://doi.org/10.1111/j.1461-0248.2006.00885.x
Klesse, S., Abegg, M., Hopf, S. E., Gossner, M. M., Rigling, A., & Queloz, V. (2021). Spread and Severity of Ash Dieback in Switzerland – Tree Characteristics and Landscape Features Explain Varying Mortality Probability. Frontiers in Forests and Global Change, 4. https://www.frontiersin.org/articles/10.3389/ffgc.2021.645920
