The use of exotic tree species in forests appears to some to be an essential solution to cope with rapid climate change . More resistant to high temperatures, they would make our forests less fragile, for example in the face of episodes of drought .
However, this practice is controversial on several grounds. Not only because these species sometimes prove to be invasive, dispersing in an uncontrolled manner. But also because the flora and fauna associated with these species are often poor, especially since they are generally planted as monocultures in France.
Beyond these risks for ecosystems, this practice would also be responsible for the introduction of pathogenic microorganisms and insect pests. The white paper from the French Botanical Society on the introduction of exotic tree species into forests develops this idea, illustrating it in particular with the case of ash chalarosis, a disease which has emerged in recent decades.
If true, this strategy would pose a very serious threat: invasive pathogens represent approximately 50% of disease cases reported by the Department of Forest Health , and this proportion is increasing.
But is this assertion founded? Do we know precisely how microorganisms attacking trees enter forests? Let us look at three emblematic cases.
Among the most severe epidemics affecting European forests, elm blight comes first. Two successive episodes were in fact induced by two neighboring fungal species ( Ophiostoma ulmi and O. novo-ulmi ).
The first epidemic occurred at the beginning of the 20th century. Its mode of introduction remains unknown, although it has sometimes been suggested that the wooden packaging of American military equipment arriving at the front during the First World War could have been responsible.
The second, caused by O. novo-ulmi , caused much more damage, since it largely eliminated the elm from hedges and forests in France. In Western Europe, its origin is better known: the fungus arrived from North America , which had previously been invaded. We do not know precisely the initial source of the introduction of the parasite. The epidemic started around a few large ports in the south of England: an inspection at the port of Southampton in 1973 showed that the parasite was present on North American elm logs coming from Ontario.
According to a retrospective study, imports of the latter had already occurred in the 1960s, precisely in the areas where the first outbreaks of graphiosis had appeared.
Now let's look at ash blight , which heavily affects common ash trees across Europe. First reported in Poland in the 1990s, the disease is caused by a fungus, Hymenoscyphus fraxineus , which was only identified in 2006.
A few years later, this invasive fungus was shown to have originated in the Far East, where its native host is the Manchurian ash tree. Its arrival here is linked, according to Estonian scientists , to the repeated introductions of Manchurian ash trees into the Baltic countries during the Soviet period – generally in botanical gardens, arboretums or parks.
But most often these were imports of ash tree seeds. However, the risk of introducing parasites through the seeds is very low: in the case of chalarosis, it is impossible for the pathogen to have been transported in this way. In 1975, an introduction of plants from Soviet Manchuria into the Tallinn Botanical Garden could, however, be responsible, the pathogen having also been detected there on a herbarium sample of Asian ash in 1978. is the first known presence of the parasite in Europe.
The time between the first known presence (1978) and the outbreak of the epidemic (1990s) may seem very long, but it corresponds to the time taken by the pathogen to adapt to a host and a new environment. And if we can point the finger at the Tallinn Botanical Garden, it is thanks to the conscientious traceability work carried out by its team – what happened there surely happened in other gardens or arboretums. It should be noted, however, that Manchurian ash has never been used in forests in Europe.
The sudden death of the oak
The route of introduction of Phytophthora ramorum into Europe is much better known. In the 1990s, this disastrous disease affecting American oak trees was reported in California, and called sudden oak death. At the same period, a new Phytophthora affecting rhododendrons was described in Germany. The scientists will subsequently show that the two diseases are caused by the same microorganism, P. ramorum , quickly found on rhododendrons in California .
In Europe, the fear of seeing the disease develop on our local oaks grew: we quickly noticed the wide dissemination of the parasite on rhododendron and laurel in garden centers and ornamental nurseries throughout Europe, but also sometimes in parks, particularly British heritage gardens.
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Ultimately, it is not the oaks that will be impacted in Europe: in 2010, the British reported that the parasite was the cause of a severe epidemic in forests on larch, before reporting a few years later that it can also develop on chestnut. In France, a first outbreak was reported in 2017 on larch trees in Finistère by the Forest Health Department : an eradication procedure was carried out in the affected stands and the outbreak appears, for the moment, to be under control.
The methods of introduction of P. ramorum are also much better known thanks to the molecular characterization of the parasite: two variants of the pathogen initially circulated on the Pacific coast of the United States, while two other different ones circulated in Europe. After a few years, the main North American variant was found in rhododendron nurseries on the east coast of the United States. As for the main European variant, it appears in Oregon on rhododendron in nurseries, then in urban areas and finally on native forest species in natural environments close to urban areas.
There is therefore little doubt about the spread of this microorganism, visibly caused by international trade in rhododendrons. According to genetic studies, it is believed to be native to the mountains of northern Vietnam where it was found recently , infecting local rhododendron species.
The role of ornamental plants
For each of these three examples, the planting of exotic tree species in forests is not responsible for the introduction of parasites, contrary to what the Botanical Society of France maintains. Which is quite logical: exotic species selected for forestry use have generally already demonstrated at length their adaptation to the climate and their good growth in our regions, through tests carried out in botanical gardens or arboretums. If an invasive parasite were to be introduced this way, the damage would already have been done...
However, let us not absolve foresters of all responsibility. Once an exotic parasite is introduced and has adapted to a local species, it will be more easily dispersed in the forest by planting it – as in the case of ash blight. Although the disease is naturally dispersed by wind, it has been shown that the introduction of the pathogen into the British Isles was partly through the planting of European common ash.
Certain diseases can also be introduced by foresters, particularly when spread by seeds. This is the case, for example, of sticky pine canker in South Africa. Absent in French forests, it has been detected occasionally in nurseries and is present in Spain. Other cases are known, such as eucalyptus trees in the Indian Ocean area.
More generally, let us highlight the role played by the trade in ornamental plants in the global dispersal of tree pests – this is the case of the boxwood moth. European cities are in fact home to several thousand woody species, including many exotic species, which represents a diversity unlike that present in our forests.
Exotic pathogenic microorganisms find there not only a possible route of introduction, but also a rich choice of potential hosts to be able to establish themselves.
Claude Husson, Department of Forest Health, Ministry of Agriculture and Food Sovereignty – DGAL, contributed to the writing of this article.