By now hopefully, everyone is aware of Ash dieback (Hymenoscyphus fraxineus) and the impact this disease has had on the overall population of Ash in the UK as a tree species.
The loss of ash trees is having a significant impact on biodiversity and ecosystems dependent on ash as part of their life cycle stages, and it will continue to do so. Younger ash, those with stem diameters below 30-40cm dbh seem to be the most susceptible and can decline quite quickly once infected. Older trees can resist the disease for some time but are often then prone to secondary attack or infection by another pest or pathogen like honey fungus (Armillaria mellea) or fruit tree red spider mites (Panonychus ulmi) attacking them in their weakened state causing further decline.
During 2016, when the symptoms of ash dieback were first observed on a population of ash I was responsible for in Hampshire, there was a need to review all options as opposed to just removal and replacement. Therefore, a program of various pruning and tree surgery techniques was considered to see if the ash trees were able to respond to the pruning and whether this had an impact on their long-term health and resilience against ash dieback.
Each Ash tree was assessed and recorded on Ezytreev to capture the scale of the whole population of ash, which at this time was in the realm of around 4,000 individually recorded Ash trees, equating to around 4% of the overall tree asset population. However, in some locations, the percentage of individual ash in relation to the tree stock in that area was around 11.6%, so these areas would experience a greater impact if the ash declined and required removal.
As part of the initial assessment, the tree’s stage of decline was recorded based on the percentage of healthy leaf cover remaining in the crown, as set out in Suffolk County Council’s four-part system for assessing the health of an ash tree’s crown (published in 2015). A risk assessment was carried out to assess if intervention was required, ie was the tree in the vicinity of a target. Specific individual trees with targets were identified as important for retention due to their location, size, ecological value and amenity value. These were the trees selected for crown reduction works to ascertain if this form of pruning works would aid with their retention.
By the end of 2019, around 80 of the Ash had been crown reduced, and all retained individuals were being annually monitored. There was also an extensive phased removal and replacement plan that ran alongside the pruning works as part of the overall ash dieback Action plan. The ash population consisted of not only Fraxinus excelsior but also F. angustifolia, F. ornus, F. oxycarpa ‘Raywood’, F. excelsior ’Jaspidea’, F. excelsior ‘ Pendula’, F. excelsior ‘Westof’s Glorie’ and F. excelsior ‘Diversifolia’. It was interesting to note that F. excelsior ‘Diversifolia’ was the least affected by ash dieback alongside F. oxycarpa ‘Raywood’ and F. angustifolia.
One of the first trees to be crown reduced was a mature ash protected by a Tree Preservation Order on a council verge in Romsey. This tree had been subject to a lot of previous encroachment of its rooting area and space: a road and housing estate built around it and, more recently, a new building in very close proximity. It is a prominent tree and of high amenity value as well as ecological value. It was crown reduced in 2016 by 1-2 metres, then again in 2019 by 1-2 metres and in 2022 by up to 2 metres back to previous reduction points.
All the initial 80 ash trees that were placed under crown reduction regimes were successful in their responses and have been retained, The key observation are:
- The crown-reduced ash responded well to pruning if they were in the early stages of infection from ash dieback, i.e. class 1 with 100% – 76% healthy leaf cover at the start.
- The epicormic growth response was very high with epicormic growth rafting along branches as well as increased density of leaf cover within the canopy.
- The pruned ash trees appeared to be more vigorous and showed a stronger vitality following their recovery from the crown reduction pruning works.
- The pruned ash trees were of a more compact form but seemed to be less affected following the pruning works by a recurrence of ash dieback and showed lowering levels of infection year-on-year post-crown reduction work.
The crown reduction cycle was initially proposed on a three-yearly basis or subject to the response of each ash tree, which turned out to be around every 3 years to manage and maintain the newly reduced crown. Each tree responded in a similar way and appeared to have a robust response once pruned to resisting further infection, whereas prior to pruning, many of the ash had thinning crowns and declining crown densities. Crown density increased with the repeated crown reduction works.
This is only observational commentary but it is worth considering crown reduction pruning as a method of ash tree retention if your tree management plans allow for it. This approach was supported later by a research paper published in 2017 on a study of 38 ash trees in Sweden that were pruned: the results demonstrated that the pruning helped maintain the tree’s vitality when pruning at least 35cm from visible bark lesions. Also, the reoccurrence rate of ash dieback declined following repeat pruning.
Key lessons learnt
- Multiple factors affect the health of trees and this should be factored in when assessing tree condition ie rainfall, drought, climate, site factors of increased stress.
- Gathering data on the ash population overall was key to setting strategic goals and aims for the ash dieback management plan.
- Annual monitoring was key to understanding the longer-term impacts of ash dieback and targeting specific trees and specific locations to aid with the retention of ash trees.
- Experimenting with tree surgery techniques led to an improved understanding of trees responses not only to diseases but also to responses to pruning, and in this instance coppicing and pollarding were found to be less effective than the 1-2 metre crown reduction works
- Crown reduction is a viable method of practical tree management albeit with cost implications but worth it to aid with retaining trees of historical, cultural, amenity and ecological importance
The Crown reduction programme has led to the retention of many of the larger and older ash trees, especially in more prominent locations, as a result of their positive response to the crown reduction works.
References
Devon County Council, “Devon ash dieback action plan: An overarching plan to identify and address the risks of ash dieback disease in Devon” (February 2016) https://www.treecouncil.org.uk/Portals/0/Devon-ash-dieback-action-plan-February-2016.pdf
Forestry Commission, “Managing Chalara Ash Dieback in England” (February 2017) https://cdn.forestresearch.gov.uk/2022/02/7894_new_fc_chalara_leaflet_dft9.pdf
Marciulyniene, D., Davydenko,k., Stenlid,J., Cleary, M. (2007). Can pruning help maintain vitality of ash trees affected by ash dieback in urban landscapes?, Urban Forestry & Urban Greening, Volume 27, 2017, Pages 69-75,ISSN 1618-8667, https://doi.org/10.1016/j.ufug.2017.06.017. ttps://www.sciencedirect.com/science/article/pii/S1618866716304782
Natural England, “Report NECR151: Assessing and addressing the impacts of ash dieback pm UK woodlands and trees of conservation importance (Phase 2)” (April 2014), http://publications.naturalengland.org.uk/publication/5273931279761408
Suffolk Country Council Chalara Action Kit (May 2015) – https://www.suffolk.gov.uk/asset-library/imported/chalara-action-kit.pdf
The Tree Council, “Ash Dieback: an Action Plan Toolkit” (June 2020), https://treecouncil.org.uk/wp-content/uploads/2020/06/Tree-Council-Ash-dieback-tree-owners-guide-FINAL.pdf
I wrote this article for the Arboricultural Association magazine and it was published in the Autumn of 2024.
