Tree Health issues in Britain from the experts

Different priorities, but one joined up message: Tree Health issues in England, Scotland and Wales and what we can all do to help.

Plant Health Week, in the International Year of Plant Health, provides an opportunity to reflect upon the current and potential future challenges facing Britain’s trees and what we can all do to help protect them.

Beyond Britain’s shores

The global trade of natural resources and products has increased in recent decades and this has helped the spread of invasive alien species from one part of the planet to another, including the introduction of tree pests and diseases into Britain. Tree and wider plant health, just as we’ve seen with human health during 2020, are international issues and it is important to work with plant health colleagues around the world to help understand what has spread to where, how quickly and where the next major threat to our trees may come from. This requires an international approach and extensive networking and collaboration. Preventing new tree pests or diseases from entering Britain is something that concerns all the UK nations*, and there are many unified approaches to keeping them out. Horizon scanning, early warning and regular surveillance are important in all cases, but the priorities on which pest or disease to target within each country can differ. *Both tree health and plant health are coordinated at a UK level and therefore include Northern Ireland. However, tree health reporting in the latter uses a different system that encompasses the whole of Ireland. This blog focuses on the systems used within Britain.

Tree Health in England

Nicola Spence – Chief Plant Health Officer for Defra

"Of the home nations, England has the highest human population, the greatest level of trade, tourism, the warmest climate and the closest proximity to continental Europe. These are all factors that can play a role in the introduction and establishment of new pests and diseases. Ash dieback, oak processionary moth, oriental chestnut gall wasp, horse chestnut leaf miner and many others were first identified in England. Many were found in the south or east of the country and it is in these areas where the next unwanted threat to our trees may again occur first.

English woodlands and forests are often very diverse in tree species, providing a selection of hosts for potential pests or diseases to become established on. Many of our forests have high numbers of visitors and the risks of human transfer of unwanted pests or diseases are higher. But even our cities, parks and gardens can offer plenty of opportunities for invasive species. For example, the London plane is widespread within the city that shares its name and the trees have become a prominent feature within it. But there are pests and diseases within continental Europe that have caused the death of many trees plane trees. The loss of the plane from London parks and streets would be tragic and must be avoided if possible. To prevent such pests or diseases from becoming established, regular surveillance, monitoring, early detection and rapid reporting and intervention are essential.

Observatree volunteers have been a great help in supporting our official surveillance work in England. They have reported oak processionary moth beyond its previously known range in the southeast and reported where it was not found, providing very important baseline data for our monitoring of this pest. Towards the end of 2019, an Observatree volunteer reported a case of chestnut blight in the SE of England. This was the first confirmed report by an Observatree volunteer and is a significant finding that is helping with the management of this notifiable disease. Similarly, a volunteer located in the SW has reported an area of infected sweet chestnut, but this time with Phytophthora ramorum. More commonly known for infections on larch, this report of P.ramorum on a different host and in a new location is also a significant finding that is leading to follow investigations by Inspectors."

Tree Health in Scotland

Gerry Saddler – Chief Plant Health Officer for Scotland

"In Scotland, there are many areas that are only sparsely populated. This has some different tree health challenges. There are significant areas of productive forestry, often dominated by coniferous tree species. The likelihood of pests or diseases entering these areas due to imported materials is lower, but if and when they do, they can potentially go undetected for longer and with such high numbers of host trees, they may be able to spread very rapidly. In parts of Europe some species of wood boring insects have killed conifer trees across extensive areas. Tree health monitoring of these large areas of nationally owned forests in Scotland are usually done by aerial surveillance, a method currently being used to monitor P. ramorum, a developing problem with a substantial presence in the South West. But there are many private estates where it is important to communicate the need for vigilance to landowners.

Because ash dieback and the horse chestnut leaf miner first appeared in southern England, it has taken time for them to spread north and there remains some uncertainty on how the typically cooler and wetter Scottish climate will affect them. A recent Scottish ash dieback workshop with Scottish Forestry, Tree Council, Local Authorities, utility companies and other stakeholders, concluded that ash dieback in Scotland is only a few years behind England in terms of progression and severity and that affected trees are most common on roadsides, urban areas etc. and less prevalent in natural ash woodland. There remains a need for vigilance and possible interventions as the disease progresses. Volunteers from Observatree and Scottish Natural Heritage are monitoring and reporting on ash and horse chestnut trees to see how ash dieback and the leaf miner develop and whether the climate or tolerance by some individual trees can help to slow the rate of spread.

There are also ports, airports and similar points of entry to those in England and volunteers have a vital role to play in monitoring trees across the Scottish central belt, and area of higher levels of urbanisation where there the higher movement of goods and people increases the risks of new pest or disease introductions."

Tree Health in Wales

Martin Williams – Head of plant health and biotechnology unit for Wales

"Wales shares many of the concerns of both England and Scotland, having areas of upland conifer plantations and mixed broadleaf lowland and some of the milder temperatures of the former. Similarly to southwest England, Phytophthora ramorum is a concern within areas of larch. And like Scotland the Great spruce bark beetle is a pest of many upland Welsh forests and ash dieback has advanced across England and into Wales.

We now have a better understanding of the distribution of the diseases in Wales, partly thanks to one Observatree volunteer who took the mapping of this diseases for the first time in dozens of 10Km2 grid squares across Wales as a personal challenge. Others are now following his example in Scotland. Reports of tree pests or diseases from anywhere in Britain are submitted via the online tool Tree Alert. This is available to the public and is used by them, landowners, foresters and other professionals to report unhealthy trees. There reports are received by tree health scientists in Forest Research for checking and escalated to the necessary authorities where relevant. Of all the Tree Alert reports received from Wales between April 2019 and March 2020, 25% were submitted by Observatree volunteers showing what an important contribution they are making to tree health in Wales.

Many of the Observatree volunteers across Britain monitor Sentinel trees that they have chosen on a regular basis. This concept is being employed across Wales where Sentinel Gardens are being monitored by volunteers and professionals to examine the impacts of pests, diseases and climate change. These gardens are a new concept and it is hoped that as they are taken forwards, Observatree volunteers can assist with the monitoring of the trees that are present."

The role of citizen science in supporting tree health throughout Britain

"Projects like Observatree have and continue to show that citizen scientists can make a positive contribution to tree health. And there are many ways in which this can be achieved. Acting as an early warning system is the most obvious. And when it works, it can be very successful. As seen with the Asian longhorn beetle outbreak in Kent some years ago, early interception and rapid action can successfully eradicate a pest.

The implications of these significant reports can be large, and the value of the ecosystem services protected by rapid eradication can be difficult to estimate. If citizen science can help to reduce the threats from xylella or emerald ash borer, that alone would be very welcome. But citizen science can and does offer more. Many of the Observatree volunteers are providing important background information on healthy trees. These are important data that can help to track the spread of pests or diseases. Some volunteers monitor sentinel trees and provide regular reports on their condition. And others participate in tree health related research, providing vital support to scientists who are working to understand the pests and diseases and hopefully slow their spread or reduce their impact.

As mobile devices become increasingly sophisticated and technology continues to advance, the use of Apps and similar web-based tools will enable more people to access the information they need and the tools to submit tree health information. It is essential that these reports are quickly processed and that the relevant officials are informed of significant cases in time to allow them to mitigate accordingly. But equally important are the negative data where trees remain healthy. These may identify natural tolerance that is vital in protecting the gene stock of some tree species.

Tree pests and diseases are continuing to head our way, both at country and GB levels. Citizen science can continue to help us at both levels and provide an example to colleagues elsewhere around the world of what we can collectively achieve. And we are keen to share that experience with them during this International Year of Plant Health."