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Diversifying broadleaved woodlands for the future – A way forward from ash dieback

15 October 2020

In the Spring 2020 issue of Farm Woodland News, we looked at diversifying woodland tree species for growing softwood timber. What about native broadleaved woodlands, and semi-natural woodlands? How do you manage trees suffering from ash dieback?

Why diversify woodlands?

Making sure woodlands are diverse in terms of species (as well as spatial and age structure) is beneficial for biodiversity, and the long-term health of the woodland as a whole. Here we’ll focus on management in response to ash dieback but this is just one threat to our trees. There are many other pests and diseases that could arrive in the UK and the principles discussed here could be beneficial for any semi-natural woodland.Photo looking upwards through the canopy of an open broadleaved woodland

A mix of trees and shrubs that mimics the composition of a naturally occurring woodland type, suited to the local conditions, can support a wide range of woodland plants and animals, and function as a whole ecosystem. Greater diversity of species also means resilience in a changing climate, effectively spreading your bets on how trees will adapt to future climate conditions.

Managing ash dieback

Dutch elm disease had a huge impact on the rural landscape in the 1960s and 1970s, although less so in Scotland than south of the border. European ash (Fraxinus excelsior) was planted to replace many lost elm trees, never anticipating the future appearance of ash dieback disease (often referred to as ‘Chalara’) which looks set to wipe out 80% of the species.

As explained by Clarinda Burrell on page ?, dead or dying ash trees that are unsafe should be felled by a fully trained professional; the particularly brittle wood makes felling these trees very unpredictable. If infected trees don’t pose a safety risk, eg away from roadsides, people and property, they should be left alone. The aim is to retain trees that can tolerate and survive the disease.

Dying or dead trees also remain valuable as deadwood. Holes and hollow trunks provide nesting for birds, and roosts for bats. The deadwood itself feeds decomposer species of invertebrates and fungi that are essential for a healthy functioning ecosystem. If felling is necessary, leaving some or all of the timber on site will benefit biodiversity.

When the ash trees are in leaf, identify any that appear to be less affected by dieback than others nearby. Prioritise these trees in your management plans and encourage regeneration around them. Retaining genetically tolerant trees to produce seed will be critical in helping the species survive.

Major efforts to find and breed Chalara-tolerant trees are ongoing. However, as the decline of ash trees becomes more evident with each passing year, so does the urgency of replacing them.

If you lose ash trees to dieback, what can replace them?

Alternative species for biodiversity

Ash are a keystone species, known to support around 1000 different species of plants and animals, from birds and insects, to lichen and mosses.Young broadleaf trees planted in rows, supported with tree guards and stakes on a gently sloping grassland field with high mountains visible far in the background

They also indirectly support many species of understory plants. The canopy of an ash casts less shade than many other trees, and they’re one of the first species to shed their leaves in autumn. This allows more light-demanding plants to grow beneath them than could tolerate the shade cast by other trees.

Ash leaves also decompose faster than those of most other native trees, playing an important role in nutrient cycling, and creating rich soils.

Replacing ash in native and semi-natural woodlands

Ash is the third most common broadleaved tree in UK woodlands. Although it is rarely a dominant species in Scottish woods, it plays a unique and important role in woodland ecology. Unfortunately, this means that no single tree species can replace ash like for like. In many cases, using a mixture of species will more closely replicate the characteristics of ash. The most suitable replacement species, or species mix, will vary between different settings.  This choice depends on the composition of the existing woodland, and the site conditions in terms of climate and soil type. Additionally, if site conditions are suitable for a wide range of alternative trees, you may be able to choose species based on whether your priority is to support ash-dependent species or fulfil the trees’ role in nutrient cycling.

Natural regeneration

If your primary objectives are restoring a native woodland or supporting biodiversity, encouraging natural regeneration may be the first choice. This still requires active management, for example thinning to open the canopy, controlling invasive species such as Rhodedendron and protecting young trees from herbivores.

Natural succession will occur where lost trees create openings in the woodland canopy. This gives any tolerant ash the chance to self-seed. Where ash is a minor component of the woodland, the gaps left may not be sufficient for significant regeneration of any species – the crowns of adjacent surviving trees may grow to fill the gaps. Consider thinning to encourage regeneration.

Otherwise, what is most likely to replace ash will depend on what other tree species are present in the woodland, and nearby. Initially shrubs such as hazel, hawthorn and elder, often associated with ash woodlands, are likely to colonise. After 10-15 years tree species such as sycamore or beech the lowlands, or birch in more upland areas, are most likely to fill the gaps in the canopy.

In Scotland, Sycamore and beech are classed as non-native although ‘naturalised’, meaning they are well established here. They are particularly good at self-seeding and are likely to out-compete regenerating ash or other native species. If non-native species make up a large proportion of the woodland, it may be better to retain some of these trees initially, in areas where they are less likely to spread. Phase their removal gradually to avoid losing large areas of woodland canopy. They can also serve as ‘insurance’ for the lost ash trees, in terms of woodland cover, and timber revenue if that is a secondary objective.

Planting

Planting may be necessary where there is a high density of ash or particularly high vulnerability to dieback. In these circumstances regeneration by ash is likely to be limited. Diversifying a woodland is not a case of planting as many different tree species as possible. As with creating new woodlands, it’s crucial that you choose species suited to the site conditions and climate.

The National Vegetation Classification (NVC) system categorises naturally occurring types of woodland habitats. Five of the native woodland types found in Scotland include ash. The table lists the major and minor species and gives general descriptions of where each woodland type can grow.

If it’s a new native woodland created fairly recently, the species mix planted will likely be based on one of these NVC woodland types, and you may already know which one. Whether you’re replacing ash or just wanting to increase species diversity, adding new suitable species will help to enhance the woodland. Consider including some of the minor tree species and shrubs that aren’t already there. These lists serve as a starting point – it is essential to check that each new species you want to plant will be suited to the site conditions. A forestry agent can help you assess the woodland and select suitable species.Young broadleaf trees planted in rows, supported with tree guards and stakes on a gently sloping grassland field with high mountains visible far in the background.

If your woodland was planted with grant funding and is still under contract, you are obliged to maintain the original stocking density, replacing any trees lost to ash dieback or other pests and diseases. Contact your local Scottish Forestry Conservancy office for specific advice.

Replacing ash outside of woodlands

Ash is the UK’s most common tree species outside woodlands, a very common sight in hedgerows, field boundaries and parklands. These trees play an important role in connecting habitats. The species that depend on ash in these situations are particularly vulnerable to individual trees being lost.

If you have mature trees of any species in these situations, are there also younger trees in between to eventually replace them? Could you plant new hedgerows and/or trees to join up existing woodlands to increase the habitat network for biodiversity?

Although mid-aged to mature ash appear more tolerant to Chalara than younger trees, older trees have the greatest ecological value so succession planting should be done as soon as possible. Ensuring there will still be trees there in the future will benefit the animal species that depend on hedgerow trees and maintain continuity in the landscape.

Woodland type
(NVC code)
 Lowland mixed broadleaved woodland with dog’s mercury
(W8)
Upland mixed broadleaved woodland with dog’s mercury
(W9)
Lowland mixed broadleaved woodland with bluebell/wild hyacinth (W10)Alder-ash woodland with yellow pimpernel (W7)Alder woodland with stinging nettle (W6)
Typical terrainLowland valley slopes; mainly eastern.Ravine and valley sides and heads; often rocky.Valley bottoms and gentle valley slopes on lowland coastal margins; mainly eastern.Mainly valley sides and hill-slopes with flushes: streamsides.Alluvial terraces in mature river valleys, disturbed and enriched floodplains, silting loch margins.
Soil typesBase-rich brown earths and base-rich groundwater gleys.Calcareous and basic brown earths and base-rich surface water gleys.Brown earths and base-poor ground water gleys.Base-rich gleys and flushed brown earths.Moist alluvial soils, enriched fen peats.
Characteristic tree & Shrub SpeciesMajorAsh
Pedunculate oak
Sessile oak
Wych elm
Hazel
Hawthorn
Ash
Downy birch
Rowan
Hazel
Pendunculate oak
Sessile oak
Silver birch
Hazel
Hawthorn
Alder
Ash
Grey sallow
Hazel
Hawthorn
Alder
Grey sallow
Elder
The most suitable alternatives
to ash for biodiversity are in bold
MinorDowny birch
Silver birch
Rowan
Holly
Crab apple
Gean
Grey sallow
Aspen
Elder
Guelder rose
Blackthorn
Goat willow
Sessile oak
Wych elm
Alder
Bird cherry
Pendunculate oak
Hawthorn
Elder
Grey sallow
Rowan
Holly
Downy birch
Wych elm
Ash
Gean
Crab apple
Aspen
Elder
Guelder rose
Blackthorn
Whin/gorse
Broom
Downy birch
Goat willow
Pedunculate oak
Sessile oak
Rowan
Holly
Bird cherry
Elder
Guelder rose
Blackthorn
Bay willow
Ash
Downy birch
Pedunculate oak
Holly
Goat willow
Hawthorn
Guelder rose
Blackthorn
Purple willow

For further information see the Scottish Forestry guidance: Management of native ash in Scotland  and NVC field guide to woodland

Leona Baillie, Forestry Consultant, SAC Consulting


This article has been published in the Autumn2020 edition of the Farm Woodland News.  Download a copy to access all articles.  Subscribe to receive newly published editions via email by using the form here.

Photo looking upwards through the canopy of an open broadleaved woodland

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