Ancient trees have secrets

“Ash is an important mythological tree in Sweeden, often there is an ash tree planted at the front of the house for protection.”

A recent talk by Dr Vicky Bengtsson of the Ancient Tree Forum at the Ashscapes conference in Dorset offered some positive views on the ancient ash pollards that are common in Sweeden.

Pollarding is a tree management technique that has been used since medieval times to keep the tree at a certain height and to make use of the young growth. Ash “hay” or young growth from ash pollards has been used in traditional farming in Sweeden as fodder for cattle. Subsidies are provided to farmers to keep their pollards in a healthy cycle of about 5 years.

Dr Bengtsson conducted a survey to investigate whether the pollarding is having an impact on the chalara infection rates of ash trees. From the random sample of 330 trees that were surveyed across the country every two years since 2009 only 35 of those trees have died (some of which have been felled). The early results show that the mortality rates for the pollarded trees is slowing as is the rate of infection.  It could be that removing the young green growth from the trees helps to remove the infected leaf and plant material from the tree which could keep the older parts of the tree healthier. Other studies have shown that natural endophytes slowed the spread of the chalara fungus, which led some to speculate that perhaps the fungus is is not good at getting past tree branch unions and if so could contribute to the health of pollards.

The survey also showed that girth was an important factor in a trees health and that no tree under a 140cm girth was healthy. This follows other research findings and has serious implications for the next generations of ash trees.  Dr Bengtsson points out that ancient trees have secrets and that there is much we are yet to learn about how they work within the complex ecosystem of fungus, endophytes, mycorrhiza, and soil so the outcomes of the Ash Dieback infection can not yet be predicted, especially in older tree populations.

Dr Vicky Bengtsson has the following recommendations for managing veteran ash trees in the UK:

• Avoid all restoration work on old pollards
• If pollards are healthy and managed in a in cycle,  continue with cutting
• Avoid cutting all trees in the same year
• Do not fell as a preventative measure
• Replace or plan for succession with native deciduous trees

See more about the Ancient Tree Forum.