From surviving ash trees to a viable population

Since it was first detected in Sweden in 2001, ash dieback has caused extensive damage and mortality among European ash trees. The disease has consequences not only for the ash trees themselves, but also for species that depend on this tree species, as well as for ecological functions and cultural landscapes associated with them.

From surviving ash trees to resistance breeding

When the ‘Save the Ash’ project began in 2013, help from the public proved crucial in identifying unusually healthy ash trees in areas where many other trees were severely affected by ash dieback. The project has thus identified about 1,300 potentially resistant trees. These have provided material for testing and preserving genetic material for breeding.

The long-term research has since given cause for optimism. Resistance to ash dieback has been shown to be heritable. Some of the most resistant clones identified early on in the research have remained remarkably healthy despite the presence of ash dieback in their surroundings for more than 25 years. This has since laid the foundations for a breeding programme aimed at developing plant material that can better withstand ash dieback.

“Thirteen years ago, the focus was on finding and preserving the ash trees that were still alive. Now we need to build on that foundation and work out how to restore viable ash populations in the landscape,” says Michelle Cleary, Associate Professor and Senior Lecturer in Forest Pathology at SLU.

Resistance is not the same as restoration

Resistant trees are not immune to ash dieback. Even trees with improved resistance can become infected, suffer damage or die.  Their long-term performance also depends on growing conditions and other pressures, including root diseases and browsing. Producing resistant plant material is only one part of the process of restoring ash trees to our landscapes. To make this transition, long-term and nationally coordinated efforts are needed. Resources are also required to preserve genetic diversity, identify suitable sites and management practices for ash trees to be successfully re-established, and investigate whether they can survive, reproduce, and give rise to subsequent generations.

‒ Restoration means taking the knowledge from breeding and research and putting it into practice – testing approaches in different landscapes, monitoring their development over decades and adapting our efforts as we learn more, says Michelle Cleary.

Nor is restoration simply about bringing back the tree itself. An important part of it is preserving or rebuilding the biodiversity and ecosystem services associated with ash. Protecting surviving mature trees remains particularly important because their habitats cannot quickly be replaced by young plantings.

The next step requires a sustained support

For most of the past 13 years, efforts to conserve the Swedish ash have relied on individual research projects, foundations and public engagement. These efforts have allowed some progress to be made, but the Perspective article also highlights the limitations of short-term funding and individual initiatives.

Compared with several other European countries, where larger and more coordinated efforts have been introduced, Sweden’s fragmented funding has limited the work to a smaller scale and with less continuity. Earlier intervention would probably have preserved more valuable conservation opportunities as trees have continued to disappear.

The Government’s latest initiative on long-term breeding and research into ash and elm represents a significant change and provides a considerably stronger foundation for future recovery. However, translating these investments into population recovery will also require sustained support for practical restoration, long-term monitoring and coordination among researchers, landowners, forest managers and conservation organisations.

A new threat has reached Europe

At the same time, yet another threat to the European ash tree has reached Europe. During the summer, the emerald ash borer (Agrilus planipennis) was first detected in Hungary and later in Slovakia. This invasive beetle has caused widespread mortality among ash trees in North America. Resistance to ash dieback cannot be assumed to provide resistance to emerald ash borer, making research into both threats essential for long-term conservation, making the work of preserving genetic diversity even more important.

Hope remains – the opportunity is still there

Surviving mature trees and continuing natural regeneration provide a foundation for recovery. But survival today does not guarantee viable populations in the future. The ash has a future in Sweden but only if efforts relating to breeding and research are allowed to continue, and be complemented by a well-funded, long-term national restoration programme that supports ash populations and their associated biodiversity over the long term.