In reply to wintertree:
Depressingly, there is nothing that can be done about ash dieback, except to fell badly infected trees and wait upwards of 50 years for disease resistant trees to establish a foothold. The UK would probably have been vulnerable to ash dieback anyway, but the spread was greatly accelerated by tree imports from other infected countries prior to 2012.
In the meantime, other tree species with ash-like traits can help fill the spaces left by the felling of dead or dying trees, which as near as possible replicate woodland ecosystems in which ash formed a big part. These trees can help conserve functional characteristics in ecosystems affected by the loss of ash.
In vulnerable regions in the UK, trees like Poplar can fill vacant gaps relatively quickly and can act as a nurse species for slower growing Alder, Field Maple, Sessile Oak, Small Leaved Lime etc, which are less vulnerable to known diseases.
Research article from the British Ecological Society on this subject:
https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2664.1325...