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Tree diseases in forests: prevention is better than cure

Mature ash tree suffering dieback disease.: Photo by John Healey  taken in Dorset in 2019Mature ash tree suffering dieback disease.: Photo by John Healey taken in Dorset in 2019New tree diseases are spreading to woodlands in Britain at an increasing rate causing greater damage to sustainable production of timber and the many other benefits that we get from our woods.  This is a particular concern given the Government’s commitment to a rapid increase in the area of woodland.  We don’t want to plant millions of trees that simply succumb to disease.  

Researchers in the Universities of Bangor, Strathclyde, Cambridge, Glasgow and Warwick, as well as the James Hutton Institute, have just published a full formal review of all the published evidence from around the temperate world about which options for forest management are most effective against tree diseases (Frontiers of Forestry & Global Change 3:7. doi: 10.3389/ffgc.2020.00007).  This has shown that measures taken after a pathogen has invaded a forest (such as felling diseased trees or those susceptible to infection) may only slow the spread of disease within the forest.  They rarely stop it.  Therefore, much the best approach would be to increase effective quarantine to reduce the rate of spread of new pathogens to a country or region, but this rarely seems to work.  The spores of many pathogens, such as that causing ash dieback disease, can travel far blown by the wind.

The researchers therefore focussed on the evidence for how forests should be managed to make them more resilient against any future diseases that have not yet arrived.  They found strong support for keeping individual woodlands isolated from others at the landscape scale to reduce the spread of pathogens between them.  It is also beneficial to remove the cut stumps of felled trees because they can act as a potent source of pathogens to infect the remaining trees in a forest.  But these measures present a dilemma because making woodlands better connected and retaining “dead wood habitat” are considered beneficial for biodiversity.

In contrast, combining a diversity of tree species within a forest, rather than just a monoculture, and having the trees more spread out at a lower Newly planted woodland with ash trees already suffering dieback disease: Photo by John Healey  taken in Dorset in 2019.Newly planted woodland with ash trees already suffering dieback disease: Photo by John Healey taken in Dorset in 2019.density should both increase resilience against tree pathogens and conserve biodiversity, according to the evidence.

However, first author of the study, Dr Michaela Roberts pointed out:

“What we don’t know is at what scale different tree species need to be mixed to achieve these benefits.  Is it OK to have small blocks, each a monoculture of a different species, or is there a big advantage in having trees of different species in a more intimate mixture, despite the greater costs this poses to managing the forest for timber production.”

In conclusion, John Healey, Professor of Forest Sciences at Bangor University and senior author of the study, commented:

“There are huge gaps in the base of existing evidence about how these management options affect the resilience of different types of forest to different tree pathogens.  We need to generate new collaborations between experts in pathogen biology and epidemiology, forest ecology, management and economics, and modellers who can integrate all of these components, to improve the advice for forest managers.  This will be crucial so that old and new forests are managed to be as resilient as possible to all the future challenges that they will face of new tree diseases, while the climate markedly changes within the life span of a tree.”

Publication date: 10 February 2020