Metla Bulletin

International plant trade – an unreasonable risk to forests

Over the last few decades, the growth in international trade of plant material has resulted in a severe increase in forest damage. Expressing their concern about this, scientists from 25 countries signed a declaration (Montesclaros declaration, IUFRO) proposing restrictions to trade in plant material.

"Pitch canker, (Fusarium circinatum), appeared in Spain in the mid 1990s and has spread there in spite of control measures to most pine plantations" (photo: Michael Müller).

While authorities in various countries have taken several measures to ensure the health of commercial plant material, significant damage has been caused by plant diseases and pests that migrate with the help of humans. Scientists increasingly share the view that the risk of plant damage cannot be sufficiently reduced through control alone.

A reason for this lies in the fact that an immense number of microbes and insects live on plants; it is impossible to predict which will constitute a risk of damage in a foreign environment.

Individual plants form complex biotic communities, where hundreds of microbe species live in interaction with the plant. For instance, more than a hundred species of fungi have been found to live on the above-ground parts of a healthy fir tree in southern Finland. To this, we must add bacteria and viruses as well as the various species living in the roots of the tree.

Over the course of their history, most microbe species living on plants have adapted to a shared existence with their host. However, if a microbe or an insect is transported to an environment to which it has had no natural access previously, predicting its behaviour will be impossible.

An example of this is the cultivation of eucalyptus, imported from Australia, in South America. In 1912, it was discovered that a rust fungus had migrated to eucalyptus from a guava plant, later causing severe damage to eucalyptus plantations.

In 1977, the guava rust was detected on allspice plants in Florida, from where it made a further host jump to melaleuca trees in 1998. Despite all precautions taken, the rust fungus spread to Hawaii in 2005 and Australia in 2010, where damage is already visible. At present, the question is what will happen to Australia’s native eucalyptus forests.

The best known forest pathogens dispersed via trade in plant material include an oomycete, causing widespread destruction of oak forests in the western coast of USA and larch forests in Great Britain, the Dutch elm disease that destroyed a large part of Europe's elm trees, as well as the chestnut blight that caused the near extinction of the American chestnut tree in North America.

Mortality of Japanese larch in south-west England due to lethal stem infections by an oomycete, Phytophthora ramorum, an introduced species in UK. The picture was taken in 2010 (©Crown Copyright Forestry Commission).

As new pathogens, dispersed via plant trade, are detected at an increasing speed, one could give dozens of such examples. According to the latest studies, tree nurseries and beech forests in Central Europe are widely infected by non-native oomycete species, putting the future use of beech in forestry at stake.

In their declaration, the undersigned scientists propose a phasing out of all trade in plants and plant products found to be of high risk in terms of spreading forest pests and pathogens, but of low overall economic benefit. An example of such activity is transporting tree seeds from Finland to a country with lower labour costs for seedling production, after which the seedlings are transported back to Finland for planting.

Importing live plants, for instance from East Asia to European parks and gardens, is especially dangerous, as microbes dispersed to Europe in this way, from a completely different vegetation zone, share no developmental history with local plants.

Unlike most other countries, Finland has so far been spared from major forest disasters caused by pests that have been transported by humans. Preventative action should be taken now to ensure that this remains the case.

Our forest nature has few main tree species, which makes it prone to risk: over 80% of Finnish forests are dominated by pine and fir trees. If a pest or pathogen killing half of pine trees entered Finland, the damage would be immeasurable both ecologically and economically (such agents might be, for instance, the American mountain pine beetle and Western gall rust).

Bearing this in mind, it is of vital importance that Finnish policy-makers add the banning of plant trade that is of little economic significance but involves several risks, to their international agenda.

Further information

• Researcher Michael Müller, ph. +358 29 532 5451, michael.mueller(a)metla.fi
• Professor Jarkko Hantula, ph. +358 29 532 5419, jarkko.hantula(a)metla.fi
• Montesclaros declaration, IUFRO