One of the most frequently asked questions relates to how woodlands will adapt to the disease, and in particular which tree species will make effective replacements.
Until recently our response to this question was fairly limited and was based on a few species such as small leaved lime.
It is therefore very timely that Natural England has released a new report that may help guide practitioners in the selection of alternative tree species that are ecologically similar to Ash.
The report (Assessing and addressing the impacts of ash dieback on UK woodlands and trees of conservation importance) examines the ecological function of 11 tree species considered most likely to replace ash across the UK.
It also provides a range of case studies showing how existing management plans may be adapted to conserve ash-associated biodiversity should significant ash dieback occur.
In this post we have attempted to summarise the main findings of the report to assist with the dissemination of the findings. If proactive management action is being considered we recommend reading the report in full.
The eleven tree species looked at were:
- rowan
- birch (silver or downy)
- field maple
- sycamore
- aspen
- oak (pedunculate or sessile)
- beech
- lime
- alder
- walnut (black or common)
- wild cherry
The ecological similarity of these alternative to Ash was assessed by considering three main factors:
- their ecological functioning
- the number of ash-associated species they support
- their traits
Ideally any alternative tree should be similar to ash in all of these characteristics.
Ecological functioning
Ash lies at an extreme of the ecological range of native tree species in the UK. It produces nutrient-rich highly degradable litter that does not form a deep litter layer and which maintains a high soil pH.
For ecological functioning the 11 alternative species were assessed in terms of leaf litter decomposition rates, litter quality (chemical and physical properties), nutrient cycling and succession rates.
A change from ash to a tree species with very different ecosystem functioning (e.g. oak or beech) will result in changes in the characteristics of the woodland: slower nutrient cycling, increased carbon storage and changes in the ground flora species present.
Ash-associated species
Around 1,000 species are known to 'use' Ash trees and tree species native to the UK support more ash-associated species than non-native tree species.
Native oak species were found to support the greatest number of ash-associated birds, invertebrate, lichen and mammal species.
Elm, hazel, oak, aspen and sycamore were found to support the greatest number of the ash-associated species that are most vulnerable to ash-dieback. However elm remains susceptible to Dutch elm disease and is therefore not widely suitable as an alternative to ash.
Traits of alternative tree species
The traits of trees such as tree height, bark pH and fruit type indicate, in part, the type of habitat created by a tree species and the resources available to species that use the tree. Ideally the traits of any alternative tree should be as similar as possible to ash.
Of the native tree species assessed elm had the most traits the same as ash followed by silver birch and rowan.
Results
The study found that the alternative tree species that support the greatest number of ash-associated species are very dissimilar to ash when assessed by traits and ecological function.
- Oak supports 640 of the 955 ash-associated species and beech supports 505 ash associated species.
- However, in terms of ecological function, oak and beech have much slower rates of leaf litter decomposition and nutrient cycling than ash and their canopies cast a much darker shade which will influence the ground flora species.
- Alder is similar to ash with respect to ecological function (leaf decomposition rates, litter quality and nutrient cycling) but supports fewer ash-associated species (389 out of 955)
As such the method that is most suitable to assess how similar the alternative tree species are to ash will depend on the objectives at the site, for example maintaining ash associated species or maintaining the woodland character and ecological function.
Management scenarios
Six management scenarios that could be applied to woods infected with ash-dieback were considered over two time periods (1-10 years and 50-100 years). The scenarios were:
- Non-intervention – stands are allowed to develop naturally with no interventions.
- No felling with natural regeneration promoted – no felling but otherwise stands initially managed for natural regeneration.
- Felling – all ash trees and coppice removed in one operation with, if necessary, additional trees of other species cut to make the operation more viable.
- Felling and replanting – all ash trees and coppice removed in one operation with, if necessary, additional trees of other species cut to make the operation more viable. Then active management to replant with alternative tree and shrub species.
- Thinning – regular operations to thin stands by removing diseased and dead trees or coppicing ash, with, if necessary, additional trees of other species cut to make the operation more viable.
- Felling with natural regeneration promoted – all ash trees and coppice removed in one operation with, if necessary, additional trees of other species cut to make the operation more viable. Then active management initially to achieve natural regeneration in the stand, with subsequent management to develop overstorey species.
In the short term (1-10 years) more ash-associated species were supported under scenario (5) ‘thinning’ than scenario (6) ‘felling with natural regeneration promoted’.
In the long-term (50-100 years) there was little difference between scenarios (5) and (6) in their impact on obligate and highly associated ash species with of these species predicted to decline in abundance or be at risk of extinction.
However, the confidence level of these assessments is only low to medium suggesting that the actual impact on ash-associated species is currently only partially understood.
For species 'partially associated' the assessment is somewhat brighter:
- Species partially associated with ash are generally predicted to decline initially following the onset of ash dieback but after 50-100 years the majority of partially associated species are predicted to be unchanged in abundance compared to current population levels due to an increase in the abundance of other tree species which they utilise.
Conclusion
When woodland managers are considering which alternative tree/shrub species to regenerate or plant in order to mitigate the impacts of ash-dieback on biodiversity, the number of ash-associated species supported is only one factor to consider.
Woodland managers should also think about other information provided in this report such as the impact alternative trees might have on ecosystem function and factors which will influence the occurrence of ash-associated species in the woodland, such as: woodland structure, food availability, the size, shape and number of holes in trees for roosting bats and hole nesting birds, interactions between species, and changes in woodland ground flora composition.
Ash dieback is just one of several diseases and other potential drivers of change within woodlands within the UK. Other tree diseases and drivers such as grazing, pollution and climate change will also need to be taken into account.
Management for biodiversity will usually be considered together with the other objectives of woodland management including timber production, amenity, flood prevention and carbon sequestration.
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