Stressed out trees helping charities restore valuable aspen forests

On a nature reserve deep in the Scottish Highlands there is a polytunnel which houses a small forest of slender grey aspen trees. It is known as the “torture chamber”.

The aspen is one of the UK’s scarcest but most valuable trees. And to produce the tiny, delicate aspen seeds being harvested by the charity Trees for Life, these 104 specimens are deliberately made to suffer.

They may be water-starved, have their limbs trimmed, or have their trunks sliced and ringed, the slivers of bark rotated or put back upside down. And despite the ice-cold chill and the snow falling outside the tunnel, leaf buds are beginning to form.

It seems paradoxical but it works: being stressed helps these aspen flower and produce the short-lived seeds which rewilding charities and foresters need in their efforts to restore the aspen forests which once thrived across Britain’s uplands.

In a little understood quirk of nature, the UK’s aspen rarely flower in the wild and very rarely cross-germinate each other. Most live isolated lives. They often cling to crags or rocky slopes to escape sheep and deer, the male trees too far apart to naturally fertilise with females.

“We treat them with lots of love for most of the year, but we can see in the wild that they respond to stress by flowering,” said Heather McGowan, an assistant at Trees for Life’s rewilding centre at Dundreggan near Loch Ness.

“So for example when there was a mass flowering in 2019, it followed a very hot and dry spring the previous year. We think that’s a stress response.

“And you can see if a limb has been damaged then next year it’s likely it would flower. So again, the stress response. We’re trying to mimic that in the tunnel by putting them under a bit of duress.”

The British aspen’s idiosyncrasies have perplexed the forestry community. Some liken it to the panda: scarce in the wild, and slow to breed. Like the black and white bear, the aspen has a very narrow window of fertility, in a few weeks each spring.

In Norway, the nearest cousin to the British aspen flowers annually and procreates quite happily. In the UK, however, natural cross-fertilisation is so infrequent aspen instead normally spreads through its roots, creating large stands of trees all derived from a single parent.

While individual aspen may flower more often, there have been only two mass flowerings in Scotland in the past four decades: in 1996 and 2019. Its seeds are so light and have very little longevity, they need to have immediate contact with bare, disturbed earth to take hold.

Yet the aspen is known as a pioneer species of critical importance to upland biodiversity. Fast-growing, its roots and leaf litter reinvigorate nutrient-poor soil.

McGowan’s supervisor Jill Hodge said: “It’s one of the trees that has the highest biodiversity benefit to other species. It is literally up at the top of the list for providing habitat for rare mosses, lichens, hoverflies, dark bordered beauty moths. It’s absolutely amazing for biodiversity and it can also be used for timber production.”

Hodge believes Scotland’s aspen may be losing fertility due to their age. Kenny Hay, tree nursery and seed resource manager for the government agency Forestry and Land Scotland (FLS), believes the reason they stop flowering regularly and instead spread by cloning and self-replicating is a response to their scarcity.

“Nobody knows for sure,” he said. “But we suspect their fragmentation in Scotland has basically forced them to put their energies into suckering from their roots rather than produce seeds.”

The Trees for Life tree nursery is the only one in Scotland producing aspen seed – other aspens are grown from root cuttings and clones, but the effort to restore the tree is now occurring across the UK.

Its saplings are being snapped up by FLS and used for private native woodland projects. Its progeny have also been sent to conservation nurseries at Thetford in Norfolk and in Surrey, where England’s warmer climate may help them rediscover regular flowering.

There are recently planted aspen forests at Dundreggan and at nearby Loch Affric. And in the Cairngorms, a major new aspen recovery project was launched in early November to help map and restore it in the wild.

Hay said the ultimate goal is to restore the aspen so successfully that they naturally spread across Britain’s over-grazed uplands. “What we need in the uplands of Britain is 200 years of pioneer birch, aspen and rowan just cycling the soil and leaf-dropping,” he said. “It’s a very long-term project.”