The benefits of planting trees as a climate change abatement measure are widely reported in the literature, but rarely is there a mention of invasive alien tree diseases and the impact these are having on planting programmes, writes Dr Frances Giaquinto

The benefits of planting trees as a climate change abatement measure are widely reported in the literature, but rarely is there a mention of invasive alien tree diseases and the impact these are having on planting programmes, writes Dr Frances Giaquinto.

Climate change is no longer regarded as a possibility for some time in the future but as a phenomenon that has already manifest and may destroy millions of livelihoods within the next few years (Bendell, 2018).

In Ireland, the government has proposed planting trees as a new policy measure towards climate change abatement (DAFM, 2014); the global benefits of this approach were described by Bastin et al. (2019) and Woroniecki (2019).

However, there is a major obstacle to the use of trees for mitigating climate change: as fast as they are planted, they are likely to succumb to highly invasive and lethal alien diseases.

Japanese knotweed and Giant hogweed

Most people have heard the terms invasive alien species (IAS) and non-native invasive species, which refer to plants, such as Japanese knotweed and Giant hogweed, and animals, such as mink and grey squirrel, defined as IAS because they have been introduced to a country where they do not belong and proceed to colonise rapidly, often leading to serious economic and environmental damage.

Indeed, IAS are regarded as the largest cause of global biodiversity loss next to climate change (IUCN, 2018). Less known are insect and microbial IAS, which remain hidden initially but can then explode into a massive problem.

Ash dieback, a fungal disease of our European Ash (Fraxinus excelsior), is one such example. The fungus, originally known as Chalara fraxinea and now called Hymenoscyphus fraxineus, is thought to have originated in east Russia and China where it lives harmlessly on Manchurian ash (Fraxinus mandschurica) without causing disease.

It found its way to Europe, probably via live plant and timber imports, and was first identified in plantations of European ash in Poland in 2009.

From there it switched from harmless endophyte to deadly pathogen and, by 2012, ash dieback disease had spread to 25 European countries, including Ireland, an unprecedented rate of spread for any organism.

It arrived in Co Leitrim on a commercial batch of ash saplings imported from continental Europe in 2012.

Six years later, in 2018, Forestry Minister Andrew Doyle concluded that, because of the extent of disease spread, control was no longer feasible.

The same situation has occurred in the UK: a recent survey confirmed infections across 80 per cent of Wales, 68 per cent of England, 32 per cent of Northern Ireland, and 20 per cent in Scotland (Hill et al., 2019).

A tragedy on many levels

The loss of our ash trees is a tragedy on many levels. European ash has grown in Ireland since before the last Ice Age, which means that it has evolved here over thousands of years, making countless connections with other organisms and creating a niche environment for other species to flourish.

Danish mythology describes the ash as the ‘Queen of the Forest’ with her branches reaching to the sky and her roots traversing the earth. Indeed, mature ash, is one of our most magnificent native trees, reaching 40m in height, distinctive by its graceful foliage, its characteristic white bark with black diamonds and its soft open canopy.

It is also one of our most common trees, representing more than 35 per cent of hedgerow species in some counties such as Co Clare.

Along with its beauty and presence, European ash fills a unique and vital ecological role in our natural and farmed ecosystems.

It produces a readily degradable leaf litter which balances the pH of soil. The leaf litter is high in nitrogen, phosphorus, potassium, and sulphur which generate rapid nutrient flows that support a rich biodiversity.

Its leaf buds are late to break, which allows high light penetration in early spring, and this shapes ash woodland’s distinctive ground flora of wild garlic (Allium ursinum), bluebell (Hyacinthoides non-scripta), wood sorrel (Oxalis acetosella), and male fern (Dryopteris felix-mas).

In addition to the ground flora, ash trees represent a hotspot for other kinds of biodiversity. Mitchell et al. (2014a) identified 955 species of birds, small mammals, mosses, fungi, lichens and invertebrates that are associated with ash; that is, they rely on ash for part or all of their lifecycles.

Of these, more than half (536 species) are lichens and some of these are obligate, which means that they are entirely dependent on their ash host to survive.

Ideal environment for lichens

In 2018, the British Lichen Society described how the light dappled shade beneath ash canopies combined with the high pH of ash bark make it an ideal environment for lichens and there is growing concern that ash dieback has or will lead to multiple lichen extinctions (BLS, 2018).

Loss of biodiversity is one aspect of ash dieback disease. Loss of ecosystem function is another, particularly leaf litter decomposition and nutrient cycling.

Mitchell et al. (2014a, 2014c) and Broome and Mitchell (2017) conducted systematic studies to examine which tree species can best replace the biodiversity and ecosystem value of ash.

Their findings showed that some tree species, such as oak and beech, can provide alternative hosts for a high number of ash-associated species, but they do not replicate well ash’s superb ecosystem function.

Other tree species, such as alder (Alnus glutinosa) and small-leaved lime (Tilia cordata), have rapid leaf litter decomposition rates but do not support many of ash’s associated species.

Alder emerged as the one species that can best support both functions: provide a home for 89 per cent of ash-associated species and serve a similar ecosystem function to ash.

International trade policies

Is there anything that could have been done to stop ash dieback disease from its disastrous course? Given current international trade policies and national land management practices, the answer may well be no.

However, Ireland has demonstrated its capacity to deliver co-ordinated and well-planned actions against disease. In 2001, an outbreak of foot-and-mouth led to widescale spread across the UK, including a farm in south Armagh close to the Irish border, followed by further outbreaks over the following months.

This led to cross-border co-operation to prevent the spread into Ireland. An analysis by Dr Patricia Clarke in 2002 (Centre for Cross Border Studies, 2002) concluded that the skilled, proactive actions taken by the two agriculture ministers north and south of the border, along with other state agencies, successfully contained the spread and stopped what could have been the catastrophic consequences of a widescale outbreak.

Woodland management becomes very complex in the face of climate change, alien tree diseases, and biodiversity loss.

Which tree species can we plant to best maintain the biodiversity and ecosystem function of ash woodlands and hedgerows?

Other invasive alien tree diseases

Which species will be able to adapt to climate change? What about other invasive alien tree diseases: will our selected ash-alternative species succumb to the next microbial invasion?

Sessile and pendunculate oaks, supporters of the greatest number of ash-associated species, are already succumbing to a brutal invasive alien disease.

How do we prepare? What do we focus on: mitigation to prevent further spread of ash dieback, planting with alternative tree species to maintain ecosystem function and biodiversity, or breeding for a future species that can withstand disease pressure?

What is critical is an approach that builds resilience in our environment so that ash and other tree species can withstand the onslaught of disease and stresses they may face in the near future.

Perhaps we are too late for the majestic ash, but can we stop other tree diseases? Sudden oak death, caused by the fungus Phytophthora ramorum which belongs to the same genus as potato blight, is already established in at least 14 locations in six counties.

Four other Phytophthora species have been reported which attack larch, beech, alder, willow, and firs. Red band needle blight was first detected on Scots pine at two locations in Ireland in 2016 (

Brown spot needle blight has been found on Scots pine and mountain pine. The green spruce aphid is rapidly multiplying because of climate change. The large pine weevil, which attacks Douglas fir and young Sitka spruce is already well established.

Four other insect pathogens, including the dreaded oak processionary moth, whose hairs can cause irritation and dermatitis, are established in the UK and hovering at Ireland’s doors. If no action is taken, we may wake to a landscape of dead trees.

What we can do

There is no such thing as ‘can’t do’. It has been possible to control even the deadliest infectious human diseases, such as TB, smallpox, leprosy, and HIV, by adopting a systematic approach based on high standards of hygiene, the development of new medicines, and building resilience within the population.

Stopping the spread of foot and mouth disease into Ireland is a fine example of what can be achieved. It is up to the government to take action at national level, to prevent the importation of infective materials, to introduce measures that prevent spread, to raise awareness and education, and to provide incentives to landowners to monitor tree health and to act promptly and appropriately when disease is found.

On an individual and community level there is also much that can be done and there is a growing number of internet resources and research publications which provide guidance and tools.


Our most important action is to introduce hygiene (‘biosecurity’) into all our activities. Just as we use decontaminating hand washes on a regular basis in hospitals and when travelling, so we should adopt automatic and routine hygiene outdoors in order to break the pathways that pathogens use to spread.

Tree diseases are transported within and between countries and regions via different routes. A commercial assignment of ash saplings from Europe introduced ash dieback disease to Ireland.

Wood packaging material (for example, palettes) can easily harbour insect diseases. The main pathways at local and regional levels are dirty equipment (for example,, chainsaws, boots, vehicles), infected soil and leaf litter, brush (for example, from hedge cutting), and natural methods such as wind and water.

In 2018, the UK government produced a useful biosecurity toolkit, ‘think Kit, think Transport, think Trees’. The most relevant general guidance is as follows:
• Think kit;
• Make sure all equipment, including boots and clothing, are free from soil and organic materials before entering or leaving a site;
• Routinely disinfect equipment, such as pruning saws and chainsaws;
• Think transport;
• Remove any build-up of soil and organic materials on vehicles and machinery, including wheels and footwells, before leaving an area or site;
• Clean your vehicle regularly;
• Think trees, plants and materials;
• Develop relationships with nurseries you can trust so they can help you responsibly source plant and tree stock from pest- and disease-free areas;
• Regularly monitor the trees and plants in your gardens and communities and report any suspect symptoms to the National Parks and Wildlife Service ( and the Forest Service: (

Other biosecurity steps include:

• Be careful about transporting soil: it may be infested with microbial or insect tree diseases and invasive alien plants such as Japanese knotweed and Giant hogweed. If you purchase soil, ensure that it is certified as clean;
• Ash logs can be transported as they are regarded by the government as unlikely to carry the disease ( However, brush is highly likely to be infected. If you cut hedges, burn brush on site or transport it in a sealed trailer to prevent pieces from falling off;
• Recent research suggests that H. fraxineus may not be the agent that kills, but it weakens ash trees to the point that secondary invaders such as the deadly honey fungus root pathogen can get a hold. This means that high standards of woodland and tree management could help to build sufficient resilience to slow the disease down until a cure is found.

Breaking the link in the life cycle

Many fungi have complex, little understood life cycles and H. fraxineus, the causal agent of ash dieback, is no exception.

When we consider control measures against a fungus, the starting point is to identify those phases in the lifecycle when it is at its most infectious, and then to identify if there are weak links in the life cycle when it is at its most vulnerable.

Fungal and bacterial diseases require what is known as ‘inoculum potential’ in order to get established as a disease. Inoculum potential is the spore load, which is the volume of spores that are required in order to overcome a tree’s defence mechanisms.

It is probable that the spores produced on fallen ash leaves in autumn develop in sufficient numbers to create this inoculum potential: think of the sheer volume of leaves that fall from a single mature ash tree.

Therefore, it is possible that the removal of fresh leaf litter could keep the spore load sufficiently low to limit the capacity of the fungus to cause disease.

From the author’s experience, raking up and burning leaves from a mature ash on a daily basis is an arduous task, yet those trees which have been treated in this way are still healthy in an otherwise disease-ridden location.

This may be a worthwhile approach for individual ash trees in gardens and amenity areas and for high value specimens.

There is some evidence that silica builds up the cellular strength of a tree’s water and nutrient transporting channels. Seaweed contains high levels of silica and also represents a highly alkaline environment which fungi cannot tolerate.

So, for those living near the sea, applying a thick layer of fresh seaweed around the base of the tree and from there out to the outer edge of the canopy may be beneficial (but not against the trunk).

It is unproven, but herein is the basis of a citizen science project if a few hundred people tried it on a few thousand ash trees.


Marciulyniene et al. (2017) examined if pruning could halt the spread of ash dieback within individual trees. Their research showed that within 5cm of a lesion, H. fraxineus was found in 80 per cent of every investigated branch, but at 10cm from the wound this dropped to below 50 per cent.

They concluded that the infection could be cut out of a tree by pruning at least 30cm from each lesion. In theory, the approach can work but, in practice, it may be impossible and the wounds created by pruning may provide a gateway for other infections.

Fungicide treatment

Dal Maso et al. (2014) tested six fungicides for their potential to control ash dieback. Their systematic research showed that copper sulphate and potassium phosphite were ineffective, but treatment with thiabendazole and allicin applied by low impact methods resulted in significant slowing of the disease.


The benefits of planting trees as a climate change abatement measure are widely reported in the literature, but rarely is there a mention of invasive alien tree diseases and the impact these are having on planting programmes.

Over a period of seven years, ash dieback disease has swept across the country and may kill 95–99 per cent of our European ash population.

As our ash trees die, it is not only Ireland’s traditional source of hurley sticks that will be lost; the health of our ecosystems will decline, as will our biodiversity and our economy.

Can we stop the disease? Probably not: recent research in the UK indicates that H. fraxineus is becoming more virulent and spreading more rapidly, not less.

Is there anything we can do to limit the rate of spread? Yes, these are listed in the article with the most important being hygiene.

There are 12 other serious alien tree diseases that have arrived in Ireland but, unlike ash dieback, the consequences of these are not yet evident.

We urgently need a national strategy to build resilience in the environment and stop diseases at point of source.

But, until these are implemented, we must take action at individual, community, and organisation levels to raise awareness and introduce biosecurity and hygiene into all our activities outside.

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. To view a copy of this license, visit or send a letter to Creative Commons, PO Box 1866, Mountain View, CA 94042, USA.

Author: Dr Frances Giaquinto CEnv MCIEEM, Ecological Research and Practice, Co Clare. Email:


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The benefits of planting trees as a climate change abatement measure are widely reported in the literature, but rarely is there a mention of invasive alien tree diseases and the impact these are having on planting programmes, writes Dr Frances Giaquinto. Climate change is no longer regarded as a possibility...