Caption: Severe ash dieback in Denmark
Forest management with ash dieback- Summary from Iben Margrete Thomsen's presentation at the Cumbria Woodlands’ ash conference
The much-quoted statistic (in the UK) that >90% of Denmark’s ash trees have been killed by ash dieback is WRONG! In fact, 90% of Denmark’s ash trees in forests show signs of infection, but it is mainly young stands that die. For ash trees outside forest settings (e.g. park land, urban, pasture) the situation is quite different, as only a small proportion are severely affected, and large old trees cope well with the disease.
This vulnerability of woodland ash is because the conditions in (cool, moist) are more conducive to ash dieback infection, and because of the presence of honey fungus, which often kills the trees as a secondary infection.
Young trees are by far the most at risk. This is because of their smaller girth and thinner, smoother bark, which is more vulnerable to infection around the root collar.
Once a forest tree is affected by ash dieback, it will cease to add any significant girth to the trunk, because all its energy will go into replacing the canopy and fighting honey fungus.
Diseased trees may be left standing, but once epicormic growth appears on the main stem/trunk, the fungus will rapidly enter and stain the wood of the trunk, causing economic loss for the forest owner.
The initial rate of death from the infection throughout the UK may seem rapid and alarming. This is because the young and highly susceptible trees will die, and trees that are already stressed succumb fast. Trees avoiding this initial wave may linger for decades, and this will give woodland managers some breathing space to initiate strategies for replacement of affected ash trees and stands.
In order to identify truly tolerant individuals, it is necessary to allow for at least 10 years of incessant infection pressure. Before this time, trees may show a varying degree of tolerance, but most will eventually develop dieback symptoms and decline. It is important to retain ash stands until the few percent truly tolerant trees stand out, so they can be selected for breeding.