Ash trees have been part of North American and European forest landscapes for millennia. Yet, they are now under threats because of invasive pests and pathogens such as the ash dieback in Europe and the emerald ash borer in North America. In this post I would like to give you some updated information on ash’s conservation status, to better understand if the next generations will be able to enjoy ash trees as part of our landscapes or if they are really set to extinction.

Widely distributed throughout temperate regions of the northern hemisphere, the 43 tree species belonging to the genus Fraxinus are an important part of European and North American identity. The origin of the name ash seems to derive from a proto-Indo-European word, used to mean spear as the wood is good for making shafts. The wood of ash trees has been used for centuries to make common items such as furniture, sport equipment, paddles, doors, and windows. Most importantly, ash trees constitute a large portion of trees in urban areas, providing important services such as recreation, cooling effects and shade in our ever-sprawling cities. Outside of cities, ash trees are an important part of our native ecosystems. In North America, for example, fallen ash leaves are a critical food source for frogs, feeding tadpoles in lakes and other water bodies [1]. A study found that more than 40 native insect species rely on ash trees for food or breeding, insects that are the food supply for birds and other animal species [2].

During the past two-three decades, ash trees have been under attack both in Europe and in North America. Their decline is happening so fast that scientist are wondering whether ash will be soon compared to other species that have been almost wiped out from forest landscape during the 20^th century (for example, American chestnut due to the chestnut blight, and elm due to the Dutch elm disease). In the Old Continent, the major ash species, Fraxinus excelsior, has been affected since the mid-1990s by the fungus Hymenoscyphus fraxineus, more commonly called by the disease that is causing, the ash dieback. In the New World, a tiny insect is rapidly wiping out most of the native ash species: the emerald ash borer.

EUROPE: Fungus VS Ash

Ash dieback is caused by a fungus that originate from East Asia. In its native range, the fungus colonizes leaves of Asian ash trees, but there it has never caused extensive mortality. It is still not very clear how and when the fungus has been introduced to Europe, but the first symptoms were observed in the early 1990s in Poland. In a few years, the fungus rapidly spread across the whole range of ash trees in continental Europe, reaching Britain in 2012, where is now in an epidemic phase. The infection starts in summer, when spores of the fungus can move from the forest floor to the leaves from where the pathogen develops towards the annual shoots and twigs, causing blackening and wilting. If the infection progresses to the branches and the trunk, it causes reduced growth and eventually tree mortality [3]. The disease has shown high rates of infection and mortality and only between 1% and 5% of ash trees show high levels of tolerance to the disease.

There are several review articles that describe in great details the history, development and the fight against ash dieback during the latest years [4, 5]. Between 2012 and 2016 there has been also a collaborative COST Action called FRAXBACK which has involved scientists from all over Europe. The results of the project are summarized in a book which is publicly available for download, focusing on the implications of ash dieback disease in terms of the historical uses made of ash, the impacts of this disease on society, overviews of the spread of ash dieback in Europe and how society might address the issues arising from loss of this tree species. According to the research carried out in the past years, there seems to be no practical and effective method to stop ash dieback, thus is not really possible to prevent the spread of the disease. Spores of the pathogenic fungus develop in the litterfall of ash forests and it would be impossible to remove all infected plant material in the affected regions. The use of chemical products like fungicidal is highly controversial and usually not authorized in natural forests. The only strategy is to preserve and promote ash trees that show none or little symptoms, which are likely to be resistant to the disease and that may be able to transmit this trait to their descendants [3].

Ash dieback is currently being studied in a number of research projects across Europe, to better understand how to limit the impact of this pathogen in the continent. Among the latest scientific results, researchers in northern France found that the landscape characteristics strongly affect the development and spread of ash dieback: the disease seems less severe in forests in which ash density is low or in open canopies such as hedges and isolated trees [6]. In Switzerland, latest research using tree-ring analysis showed that trees with smaller crowns and slower growth already before the arrival of the fungus are more susceptible to dieback and mortality; fungal-induced defoliation not only reduces growth, but also directly decreases  earlywood  vessel  size, which in turn  reduce  the  efficiency  of  water transport, and negatively  affects  photosynthesis  and  radial  growth [7]. In a recent study carried out in Britain, scientists from Oxford University presented a method based on functional traits to locate the most ecologically vulnerable area to loss of common ash trees and identify the resultant damage to distinctive ecosystem properties [8]. The method appears particularly interesting, as it allows mapping the change in occurrence of traits across the landscape, and therefore to assess areas at higher risk of looking ecosystem services, which might be targeted early on with specific management interventions.

Read more: Aliens are coming! Threatening invasion or new opportunities from introduced tree species in Europe?

Unfortunately, the fungus is not the only problem for European ash. A well-known wood-boring beetle native to Eastern Asia is slowly moving westward [9]. This insect is the Emerald Ash Borer – Agrilus planipennis – has already caused extensive damages in North America (see details below) but it has not yet reached maximum distribution in Europe. The insect was first detected in the European continent in 2003 in the Moskow area, Russia. Ten years later, in 2013, the pest has been recorded in 9 regions of European Russia where it highly damaged green ash, Fraxinus pennsylvanica, an ash species native to North America but widely planted in Europe and affected common ash – Fraxinus excelsior – too. A recent study by researchers of the Russian Academy of Science showed that in the last 16 years, the emerald ash borer spread over an area of 600.000 km2. As of 2020 it has reached Ukraine and eastern European countries and it will be just a matter of a few more years until it will have reached central and southern Europe [10]. Nevertheless, it seems that the ash borer is not causing the same damages on F.excelsior as on North American ash trees (e.g., F.pennsylvanica, F.nigra). A recent study showed that that saplings (i.e., young trees) of European ash are much less susceptible to the borer than the North American black and green ash [11]. Although carried out on sapling only – therefore caution should be taken to extrapolate results to mature and old trees growing in forest communities – these results are encouraging. This is due to the fact that common European ash is perhaps more genetically related to the Asian Manchurian ash – the species that pacifically coexist with the insect in its native range – as they are both Eurasian species. Similar results were found in a 2017 study carried our on the fungus causing ash dieback [12].

NORTH AMERICA: Borer VS Ash

In North America, instead, the story is the other way around. Differently from Europe that has only three native ash species, North America is home of 16 to 18 native ash species, and some of them are widely distributed across the continent. Their habitat spans from the shores of the Gulf of Mexico to Canada. The four major species are green, white, black and blue ash, all of them important and abundant across the Eastern portion of the continent but unfortunately all susceptible to damage by the emerald ash borer (EAB). Studies showed that the Asian beetle first became established in the suburbs of Detroit in the early 1990s, but it was only discovered in 2002 when damages on urban trees in the city and in southern Ontario, Canada, became apparent. Despite regulations to limit the spread of the pest (e.g., extirpation from infected nursery, ban to move firewood), EAB has expanded dramatically across the USA and Canada, and is now present in 35 US states and five Canadian provinces. The insect has now killed millions of ash trees in North America, with extensive (and expensive!) damages especially in urban environments, where ash have been widely planted in the last decades.

Although the emerald ash is often depicted as a shiny, green beetle, the insect spends most of its life cycle concealed beneath the bark of host trees in the larvae stage. The females lay eggs in bark cracks and, once hatched, larvae progress through four instars feeding on the phloem of ash trees, creating distinct, serpentine-shaped galleries [9]. In large numbers, the larvae can also damage the outer sapwood and disrupt the transport of water and nutrients, sentencing the ash tree to defoliation and eventually death in a few years.

There appear to be differences in the host preference of emerald ash borer, though. According to a 2015 study by the Michigan State University, the insect has high preference for green and black ash, which are by far the preferred hosts, but it is also at home with white ash, considered an intermediate host, while blue and Manchurian ash (the last one used mostly as a control, since it is the species that coexist with the pest in East Asia) are mostly ignored [13].  Another study showed that some ash trees have varying degrees of resistance to EAB [14]. The research took place at a plantation of ash trees planted on Penn State’s University Park campus in the mid-1970s, in which only 13 trees remained of the 1 762 that were alive when the emerald ash borer arrived in 2012. Results suggests that some ash genotypes can survive with lower densities of emerald ash borer beetles on the landscape. A comprehensive review on the life history, impacts and management options for EAB is given by McCullough 2019 [15].  Many other source of information from US and Canadian institutions are available from the web, if you are interested (see here and here).

Read more: Giant tree hunting and old-growth forest conservation in Vancouver Island

Are there any solutions to reduce the spread and mortality on ash trees? Yes, options exist but unfortunately, they are hardly implementable across all forests in North America. In urban area, mature trees can be treated effectively with systemic insecticides. Products like Azadirachtin (TreeAzin, Azasol) can be injected in the trunk and protects the tree from the borer for a few years (see video). Some cities support this system for preserving large and historical trees in parks, and even made this treatment for free. For example, where in Montreal, Canada, the city administration supports the treatment of ash against the borer, and citizens can get their ash treated for free. However, this can be done only for those individuals that are worth to save, for example those large trees in the middle of public squares or along characteristic boulevards or on private yards. For example, of the 10.000 ash trees present in the largest city public park, the park of Mt. Royal, 4.000 were found to be dead or dying due to the borer while the remaining 6.000 are highly susceptible to the disease. Being impossible to treat 6.000 trees every 1-2 years, the city took the decision to start felling most of them, replacing with other tree species.

Biological control by releasing natural enemies of the borer are also controversial because it is about releasing another exotic insect in an ecosystem without knowing much what other consequences this might bring. Studies for releasing such natural enemies are usually long and require lots of trials. Some larval parasitoid could help provide biological community resistance to EAB invasion [16] but these natural enemies alone will likely not slow down EAB population growth and ash mortality.

The future of ash trees

So, are ash trees really doomed in Europe and North America? Will they become the new American chestnut, a species that was once widely present but that is now so-called functionally extinct so that is not marked as Critically Endangered by the IUCN? Well, it is hard to say, but research results from both continents give some hopes. First of all, not all ash tree species seems to be highly susceptible to these pests, so there is the chance that they will survive at lower densities in forested landscapes. Some species might decline drastically, like green and black ash in North America, but likely resistant genotypes will survive. Scientists suggest that there is no silver bullet to defeat ash dieback and emerald ash borer. However, one thing we know for sure, that nature will eventually sort things out. As fewer and fewer host trees will become present in a given area, the fungus/borer population will eventually die off. What we can do in the meanwhile is to monitor hot spots areas and collect genetic material and diversity that will help us in the future to re-establish a healthy ash tree population back in the landscapes. For the moment, early detection systems, single-tree treatments in urban areas and area-wide management programs tailored to local conditions can help us buying critical time for a gradual replacement of ash in landscapes and to develop new scientific advances, such as resistant cultivars acting like a vaccine against these pests or new and more effective biological controls.

Sources

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Main photo source: Ash tree in urban area, city of Boulder, CO, USA.  

Marco Mina is a forest ecologist, postdoctoral fellow at the Centre for Forest Research at the Université du Quebéc à Montreal, Canada. His research interests include the dynamics of temperate forest ecosystems and anticipating the impacts of global and climate change using mathematical models of forest dynamics to explore interactions of trees with their changing environment and to investigate management strategies to enhance forest resistance and resilience to future challenges. His personal webpage here.