Kiel Plant Center

Katharina Mausolf - Ash dieback disease

Ash dieback is a lethal fungal disease affecting ash trees across Europe. Katharina Mausolf, a postdoctoral researcher working with Alexandra Erfmeier and Joachim Schrautzer at the Institute for Ecosystem Research at Kiel University, is working to develop a better understanding of the factors influencing the health and resilience of the ash and their ecosystems. Here she talks about the research she and her colleagues are doing within the framework of the multi-partner project 'FraDiv' and explains why ash dieback disease is a concern for the survival of many rare and endangered fungi species found in these forests.

Scientist crouches on forest floor
Katharina Mausolf coordinates the multi-partner project 'FraDiv' which aims to understand the impact of the ash dieback disease on forest ecosystems. Copyright: A. Erfmeier.

What is ash dieback disease?

Ash dieback is a fungal disease affecting the common ash tree - Fraxinus excelsior. The disease is caused by a fungus called Hymenoscyphus fraxineus which originally comes from Asia. It was first observed in Poland in 1992 and was likely imported in a really tiny amount; some studies have actually been able to show that genetically only two individuals were involved! From Poland it then moved west and reached Schleswig Holstein, north Germany, at the beginning of the 21st century. Since then it has spread across the entire state, as well as moving further south so now we have a European-wide distribution of the disease.


How does the fungus infect the trees?
There seem to be two independent paths of infection. The first is via the leaves, where the fungi spores enter the stomata. Vital pipelines within the leaves are then blocked by the fungal hyphae, preventing important processes like photosynthesis so the leaves wilt and die. The fungus continues moving down through the tree via twigs and branches, and proceeds in the woody tissues. While the first visible symptom is the brown discoloration of infected leaves, later during the course of the disease, the bark will also die as the other tissues die off and finally kill the tree.

More recently scientists have become aware of a second infection route via the roots. This seems to be really severe. During this winter as we’ve had many climatic extreme events including heavy rainfalls and strong wind here, we have observed lots of trees falling down in the forests which do not have any roots anymore. The whole root system is just gone, destroyed by the fungus and secondary pathogens, and the trees have no stability at all. This is currently one of the major reasons why the trees are falling at such a high frequency. I think this is even worse than infection in the tree crowns.


Ash forest with fallen trees
Fallen ash trees infected by the fungus Hymenoscyphus fraxineus. Copyright: A.Erfmeier

Why is ash dieback such a problem?

Common ash is not among the most common tree species in Schleswig Holstein. At a guess, ash makes up something like 4% of the total forest area in the state, however, ash-rich forest sites belong to the species-richest forests we have in Central Europe. All the important, rare and endangered species of fungi and herbaceous plants are associated with ash-rich forests. For example, many rare orchid species we have in Central Europe seem to be bound to ash forests. We assume that the ash dieback disease will lead to dramatic changes in these ecosystems. Especially these red listed and endangered species might get into trouble as the forest changes. There are some beautiful species growing in these forest sites and it would be a pity to lose them, also from an aesthetic point of view.

From an economic perspective, ash is a highly valued timber – it is light and flexible and used in many products including furniture. In the current situation, forest owners tend to harvest ash trees as quickly as possible for fear of total economic loss because of the disease. While the reason for this is understandable, it adds to the rapid transformation of ash-rich forest ecosystems.

Red Fungi
Hygrocybe splendissima, one of the rare fungal species often associated with ash rich forests. Copyright: Tanja Böhning

What is the focus of your research?

We want to understand more about what influences the health of the ash trees, as well as the health of the whole forest ecosystems. Within the framework of our research project “FraDiv”, we will monitor and study ash-rich forest sites across Schleswig Holstein. On the one hand, we want to learn more about the vulnerability of the ash depending on site conditions and species interactions. On the other hand, we aim to understand how the species composition within the ecosystem changes so we can identify what we can do to protect and restore the biodiversity of these highly vulnerable forest ecosystems.  We hope to be able to provide recommendations for future forest practice with more emphasis on aspects of biodiversity conservation.


How are you doing this exactly?

There are several parts of the project assessing the biodiversity of these sites at different levels. The biggest part involves twelve planting sites in different formerly ash-rich forests across Schleswig Holstein. In a total of 264 plots (22 on each site), each ten square metres in size, we have planted different mixtures of tree species. Some we have planted as monocultures, and in others we have planted mixtures of two, four, and even five indigenous tree species. These are ash, elm, maple, lime and hornbeam. We want to see if other species can functionally replace the ash in affected ecosystems, and whether ashes in combination with other tree species are resistant towards the fungus. We also have a bare plot as a reference to see what will take hold if not controlled. Despite all rationale expectations, we must keep in mind that nature might still provide better solutions!

3 scientists and dog stand in forest clearing
Katharina Mausolf (left) together with student assistant Franziska Fischer (centre) and PhD student Katharina Haupt (right) during the FraDiv planting campaign. 25,200 trees were planted in December 2019 and January 2020. Katharina's dog Lotte is often around to lend a helping paw. Copyright: Karena Hoffmann-Wülfing / CAU


In addition to these planting plots we have about nearly 140 monitoring plots where we will observe how the ecosystem changes over time. We will continue to monitor these plots for the duration of the project to study the long-term dynamics. A third part of the project involves the reciprocal transplanting of ash saplings at ten sites in Schleswig-Holstein differing in their infection history and extent. We want to see if the saplings taken from highly infected forests have more resistance compared to those taken from forest with a low infection rate. Here, we address the evolutionary dimension and hypothesize that the fungus infection acts as selection filter for increasing resistance. For all of this we work closely with many different partners including some 50 private forest owners, large landowners, the state-run forestry commissions and ministries and nature protection organisations. This is something that concerns and affects them all and it is important to have them on board.


What have you observed so far?

We only started last year but already there is a dramatic change visible. We lost around 20% of the marked adult trees in our monitoring plots within the last four weeks because they were just falling over. We have some sites where the ash completely died off several years ago and there are forest sites that show very low infection rates. It is clear that some trees in some areas are more resistant than others, but whether that’s due to genetics or environmental site conditions for example, is hard to know right now. There is indeed a rapid and dramatic change going on. However, we do not think the ash will disappear completely from our forests, so there is hope for the rare species that might rely on the ash. The FraDiv project is funded within the federal program Biologische Vielfalt of the Federal Agency for Nature Conservation (BfN) and runs until 2025 but we hope we can continue to monitor some of the sites beyond that to gain a longer-term perspective.


Read more about the FraDiv project here