Ash dieback is less severe in isolated ash trees

Ash dieback is less severe in isolated ash trees - British Ecological Society

New research published in the British Ecological Society’s Journal of Ecology finds that ash dieback is far less severe in the isolated conditions ash is often found in, such as forests with low ash density or in open canopies like hedges, suggesting the long term impact of the disease on Europe's ash trees will be more limited than previously thought.

The research looked at a 22km2 area in North-eastern France, where ash dieback was first observed in 2010. Although the environment had little impact on the initial spread of the disease, the researchers found that after ten years, the disease remained mild in many places.

“We found that the disease had spread to virtually all ash present in the studied landscape within two years. Nevertheless, in many areas ash trees remained relatively healthy” said lead author of the study Dr Benoit Marçais, French National Institute for Agriculture, Food, and Environment (INRAE). “We see that in many environments not favourable to ash dieback, the proportion of ash that remain heathy is closer to 80-95% than to 5%, although the disease may be locally very severe.” added Dr Marçais.

As well as being further apart from each other, isolated ash trees or those in open canopies tended to have higher crown temperatures than those in shaded forests. This is less favourable for the development of the fungal pathogen that causes ash dieback, Hymenoscyphus fraxineus. High summer temperatures, even in temperate climates like North-eastern France, could further help to reduce the severity of the disease. Climate change is expected to increase the frequency of summer temperatures above 35°C, which the disease cannot survive at.

Reducing the density of threatened tree species by promoting diversity in forest stands could be a valuable strategy in limiting vulnerability to invasive pathogens like ash dieback. With these invasive pathogens occurring more frequently, mixed forest stands may offer protection without the need to target specific pathogens.

You can read the research free (for a limited time) here: Grosdidier, M, Scordia, T, Ioos, R, Marçais, B. Landscape epidemiology of ash dieback. J Ecol. 2020; 00: 1– 11. https://doi.org/10.1111/1365-2745.13383