One of the most common species in Britain, growing to 35 metres at maturity, ash trees can be widely seen growing in woodlands, hedges and fields. They can live up to 350 years, and their long lifespan means they are vital for a huge a range of species – from birds like woodpeckers and nuthatches using the trunks to nest in, to lichens and mosses on the bark, and deadwood specialists like bugs and beetles. 

In fact, it is estimated that the average ash tree can support up to 955 species, 45 of those being species that are only found on ash trees!

The mighty ash tree is a staple species of the Yorkshire Dales, found in swathes of ancient semi-natural ash woodland that supports many rare species, but for the last 10 to 15 years it has been under threat from a non-native invasive fungus. 

Ash dieback disease

Originating in Asia, ash dieback is a fungus that first entered the UK around 15 years ago. Since then, it has spread across the UK and reached Yorkshire – including a number of our nature reserves, including Grass Wood in the Yorkshire Dales – by 2015. The fungus produces small white fruiting bodies on the ash leaf litter, and the spores are then blown for miles, sticking to ash leaves, branches, and trunks along the way. The fungus grows inside the trunk and stops the tree from absorbing water, causing internal rot and eventually killing the tree. 

How are we managing this on our reserves?

As the disease causes a decline in the tree’s vigour, it starts to display stress signs such as producing excess shoots called epicormic growth, shedding limbs, and a reduction in the tree’s canopy. These are indicators that the tree is no longer healthy - and it soon becomes a health and safety risk if the tree is adjacent to a footpath, boundary or road due to the unpredictable nature of it dropping limbs. 

As a result, the Trust completes a tree safety survey twice a year and any ash trees that display more than 50% dieback in their canopy are marked with a dot and later felled in the winter. As Grass Wood nature reserve is predominantly ash woodland, ash dieback management forms a very large part of the management as many days are required to mark and later fell the trees so the woodland remains safe for public use. There is also large cost associated with this work, particularly when contractors are required to fell larger or more complex trees. 

What impacts is the disease having on Grass Wood nature reserve?

Because so much of Grass Wood nature reserve is ash, the continual increase in felling has begun to change the character of the wood. There are some positive outcomes from this change in management; more open spaces and woodland glades have been created, which has exposed ground flora like woodland wildflowers to more light. There is now an abundance of deadwood throughout the woodland, feeding nutrients back into the soil and supporting other important fungi, deadwood specialists, and birds like woodpeckers. The log and brash piles left on the forest floor have become important habits too, providing additional nesting opportunities for birds and hibernacula for reptiles like slow worms.

It is impossible to tell what the long-term impact of ash dieback disease will be on our woodlands. 80% of Yorkshire Wildlife Trust reserves contain ash trees, and a lack of cure for the disease as well as a certainty of death once the tree is infected means we expect to see ash dieback management work expanding across most of our reserves – and a corresponding increase in costs. 

Although there are some benefits to wildlife from the space and deadwood the disease creates, the loss of one of our most iconic woodland species is tragic. It remains to be seen whether ash trees are able to develop an immunity to the disease, and as a Trust we are leaving space for this hope in our woodlands – as well as considering what species to replace our ash trees with where necessary, particularly in the face in potential new infectious diseases each year.