The airy fluttering of compound leaflets is characteristic of the ash tree. The stout sapling in front of me was only about three metres tall, less than a tenth of its potential and – thus far – a picture of verdant vivacity. Its growth had been fast, much of it witnessed within my own lifetime. Perhaps this will be one of the lucky survivors. The chances, however, are slim: I wouldn’t bet on it living out the decade.
The European ash (Fraxinus excelsior) is a common tree in the olive family, Oleaceae. It has fascinating reproductive biology, with trees being either monoecious (male or female) or dioecious (male and female flowers on the same tree), and the occasional ability to switch between them. The ash ranges from Europe to western Asia as far south and east as Iran, and features in various aspects of folklore and industry.
The world tree
The fundamental significance of the ash is nowhere greater than in Norse mythology. For ancient Nordic people, a giant ash tree known as Yggdrasill was the world tree, at the centre of the cosmos, its branches reaching into heaven. Yggdrasil was the meeting point for daily counsels of the gods.
Ash had abundant uses in folk medicine, with preparations of leaves, bark or sap used to treat a range of ailments. It featured heavily in the magical ritual of ‘passing through,’ in which sick children were lifted through clefts in tree trunks to cure them. Its dense, hard and elastic wood has also made it a favoured material for toolmaking, sports equipment and axles into modern times.
Ash tree ecology
The ecology of ash is rather distinctive. It grows fast, and can thrive in a range of soil types and light levels, making it an early pioneer species in the development of new woodland. Ash are among the relatively few large trees widespread across the South Downs. Their dappled shade creates a more variable pattern of light intensity beneath than denser canopy trees, thus potentially enabling a more diverse range of woodland plants to grow.
Ash trees tend to shed their leaves early in autumn while they’re still fairly green. This leaf litter is highly degradable relative to that of other trees, returning high concentrations of nutrients into the topsoil, reducing its acidity and resulting in high plant productivity.
Its seeds are a food source for birds, its bark is habitat for a wide range of lichens and mosses, and various parts of the tree are used by invertebrates. In total, around 955 species are associated with ash to some extent – mostly invertebrates and lichens.
This is somewhat problematic given that the UK is predicted to lose over 80 per cent of its ash within the next couple of decades due to ash dieback.
Ash dieback is a fungal pathogen native to East Asia, first identified in Britain in 2012 (but likely present at least a decade before then) which spreads widely via airborne spores, infecting new trees through the leaves. The fungus grows within the tree, blocking its vascular system and usually killing it.
Dieback was probably introduced to Europe (and subsequently the UK) via international trade in plants. The value of this trade to the British economy in 2017 was £300mn, yet the cost of managing ash dieback alone (to say nothing of other introduced pests and diseases) is estimated up to £15bn, mostly within the next decade. This cost will fall primarily upon local authorities, landowners and conservation NGOs, while the tree nursery industry whose operations introduced it keeps conspicuously quiet.
The collapse of ash populations will radically alter ecosystems, with strong implications for those species associated with it. It is unlikely, according to research, that any other single tree species could support all ash-associated wildlife (the best alternative being English oak, supporting 69 per cent of it), though combinations of other trees could stand in reasonably well. In a bitter twist of irony, the best alternative tree for those species most heavily dependent on ash is elm – already mostly lost from the British countryside due to Dutch elm disease.
Reforestation and responsible tree procurement
Conserving ash-associated wildlife will therefore require large scale reforestation of much of the country with a sufficient diversity of tree species to cater to it.
A sliver of hope for the long-term is that some ash trees show genetic predispositions to greater tolerance of the disease than others. There’s effort underway to identify such individuals for use in selective breeding, aiming to produce tolerant seed and eventually repopulate the landscape.
For the foreseeable future though, we’re going to lose most ash from the UK. While this may have some silver linings, such as an increase in dead wood habitat (unlike with elm disease, there’s little point cutting down and removing infected ash trees unless they present health and safety risks), it will have serious adverse biodiversity impacts.
The urgent lesson here is that we must invest in biosecurity and prioritise domestically grown plants over imports, rather than continuing to sacrifice our wildlife (and our wider environment) to the altar of free trade and profit. We know that the introduction of plant pests and pathogens correlates with increased reliance on imports, and the UK Plant Health Risk Register lists over 100 more that could have high impact if they are introduced to the UK.
By prioritising assurance schemes like UK and Ireland Sourced and Grown, we can ensure that the risks of new diseases repeating the devastation of ash dieback can be minimised in future. What could be a more alarming metaphor, or a more acute warning, than the mass death of the world tree?