Why Scotland's native aspen trees tremble – and have good reason to do so

Aspen trees with Scots pine in the background under a stormy autumn sky at Dundreggan, Glenmoriston | Alan Watson Featherstone

For some people, the word ‘aspen’ may be more likely to conjure up images of a ski resort in Colorado rather than a tree indigenous to Scotland. However, Populus tremula, as aspen is known in scientific terms, is an important species in our native forests, albeit one that has perhaps suffered more from past deforestation than our other trees.

A member of the poplar family, it is a fast-growing deciduous species with a number of interesting features that supports a large range of other organisms, including mosses, lichens, insects, birds and mammals.

Aspen has distinctive, almost-circular leaves with flattened petioles or stems. This flattening causes the leaves to tremble or flutter in the slightest breeze, thereby giving the tree one of its most obvious distinguishing characteristics.

A red deer favourite

Another feature of aspen leaves is that they are borne on the tree for a shorter time than any of our other deciduous species. Aspen is invariably the last tree to get its new leaves in spring, with them often not opening fully until early June in the Highlands, and it loses them in late September or early October, before the leaves of birch, rowan, oak etc are shed.

It therefore has a shorter time in which to absorb the sun’s energy through its leaves each year, but compensates for this in an unusual and perhaps unexpected way. Careful examination of an aspen trunk reveals that it is slightly green in colour, and this indicates the presence of chlorophyll there. This enables the tree to absorb a small amount of the sun’s energy through photosynthesis in its trunk and branches – a feat which other native trees are incapable of.

In addition to being short-lived, aspen leaves are highly palatable to large herbivores, such as red deer, which will preferentially browse on aspen – along with rowan, another native tree with very edible foliage. This is the main reason why aspen is so scarce in much of Scotland today.

It has suffered extensively in recent centuries from over-browsing by the country’s large number of deer. As a result, it only survives in many areas in inaccessible sites that are out of reach of the deer, such as rock outcrops and steep gullies.

Woodpeckers like making their nests in the soft wood of aspen trees | Alan Watson Featherstone

Two means of reproduction

An interesting attribute of aspen is that it has two methods of reproduction. It is a ‘dioecious’ species, meaning that male and female flowers occur on separate trees – most of our trees such as oak, birch, and Scots pine have both male and female flowers on every tree – and pollination by the wind results in the production of seeds on the female trees.

However, aspen also has the ability to send up new young shoots, or suckers, off the root system of an established tree, sometimes up to 30 metres away, if the tree is large. These suckers are technically known as ramets and, if they survive without being browsed, they grow to become mature trees in their own right.

In Scotland today, most aspen reproduction occurs by this method, creating stands of genetically identical trees connected by their shared root systems, forming a single multi-stemmed organism. It appears that individual aspens have an inherent preference for either flowering or root-suckering, and the theory is that those trees which would naturally reproduce by flowering have been selectively removed from the population by centuries of excessive browsing pressure, leaving only those able to spread by suckering.

This, in turn, means aspen cannot readily recolonise areas from where it has been lost – most of Scotland – because very few seeds are produced.

Can you spot the caterpillar on this aspen leaf? | Alan Watson Featherstone

A good host

Aspen is palatable to more species than just deer, and there is a large range of insects whose larvae feed on the leaves, including caterpillars of the poplar hawkmoth (Laothoe populi), which are camouflaged to look like rolled aspen leaves, and host-specific species like the dark-bordered beauty moth (Epione vespertaria), which has only been recorded at two sites in Scotland. Another rare species specific to this tree is the aspen hoverfly (Hammerschmidtia ferruginea), the larvae of which feed in dead aspen wood. In the UK, it is confined to Scotland and is classified as an endangered species.

Aspen is a good host for various types of moss and lichen, including the aspen bristle moss (Orthotrichum gymnostomum), only known from a few sites in the Highlands, and a pinhead lichen (Phaeocalicium praecedens), another rare species. The aspen bracket fungus (Phellinus tremulae) is host-specific to aspen trees, causing heart rot in the trunks and producing large distinctive brackets that can persist for years.

As is often the case in nature, there are various levels of interactions between these different species and the effects they have on aspen. It has been shown, for example, that great-spotted woodpeckers (Dendrocopos major) preferentially excavate their nests in aspen because the wood is soft, and will select, in particular, those trees where the wood has been weakened by the aspen bracket fungus.

The author Alan Watson Featherstone with an aspen tree partially chewed by a beaver | Alan Watson Featherstone

Another species that takes advantage of aspen’s soft wood is the European beaver (Castor fiber), recently restored to Scotland. It chews on aspen with its large incisors, in preference to other trees, felling them to build its dams and lodges, which provide a habitat for many species of aquatic organisms.

World’s largest and oldest organism

Aspen has a very large native range, occurring from Japan and China across Asia and Europe to the UK, and from Scandinavia to North Africa. In North America, the closely related quaking aspen (Populus tremuloides) is found in much of Canada and the northern USA.

In Utah, one stand of aspen has been identified as the world’s largest organism, with over 40,000 separate trunks all growing off the same root system. Known as ‘Pando’ (from the Latin for ‘I spread’), it has recently been shown to be the world’s oldest living organism, perhaps 80,000 years in age.

Given that aspen, like all Scotland’s native trees, has been greatly reduced from its previous distribution, this raises the question of whether we might have had aspens as large and old as that here too in the past.

Alan Watson Featherstone is an ecologist, nature photographer and founder of the Trees for Life charity