Managing Chalara fraxinea, dieback of ash, for selection of tolerant individuals

It is difficult to be positive when considering the developing impact of Chalara on the British ash population.  Where suited to the site, ash has been a tremendously important element in our woods, landscape and culture.  As a timber, it has been used extensively in production and in our industrial history.  It has also been a fantastic silvicultural component in woodland, coppicing well and seeding freely.  It has a shade tolerance in its early years which allows it to develop seedlings under canopy or the protection of bramble, by the hundreds and thousands.  This easy natural regeneration enables selection from an abundant population.

And there is considerable genetic diversity between individuals within populations.  This therefore gives rise to the hope that tolerant trees and seedlings will be found despite Chalara progressing across the country.  This advance pulse of the disease will leave a landscape of mature ash dying back over time, with infected leaf litter on the woodland floor, puffing up its spores and increasing the density of inoculum in the atmosphere.  But in a situation where, say 99% of the population succumbs to Chalara, the remaining 1% of a prolific quantity still represents a significant number.

The aim of managing ash must therefore be to create open, light sites to allow seedling development from parent trees.  And because seeds are constantly germinating from the previous year, this advanced regeneration is a helpful start.  Where coppicing operations have occurred in the recent past, seedlings will be developing in relatively clean, pre-Chalara sites, with reduced leaf litter.  These sites may be important for monitoring and identifying tolerant individuals. 

Increased light will allow thousands of seedlings to germinate and establish in the normal way.  Infection by atmospheric spores will then affect individuals according to their susceptibility.

Practical management

In the medium term, practical management for ash high forest is likely to be heavy thinning for ventilation, reducing the fungal environment, and to create enough light for seedling regeneration.  Female trees should be identified and retained.  Stored coppice stools can be singled, which will mean retaining the full genetic complement for cross pollination. 

An accelerated programme of gap regeneration can be started from the time of thinning, allowing the planting of different species to diversify the stand without losing forest cover.  The aim would be to have patchy regeneration while the canopy of the matrix thins over time. 

Where coppicing is concerned in reasonably species diverse sites, it is important not to lose sight of the reasons for this kind of operation in the first place.  The benefits of rejuvenation, increasing resilience including supplementary planting of different species, habitat diversity, carbon sequestration and establishment of young light demanding standard timber trees, are all of primary importance.  The impetus of this work should not be lost, for fear of putting a wrong foot forward.  Nonetheless, we are expecting a very high impact from Chalara, and if we don’t manage our sites, we will minimise the ability to select from widespread genetic diversity, and we will prolong the period during which we are without resistant generations which can be developed and planted for future resilience.