Aspen (Populus tremula) is an important native tree in Scotland’s Caledonian Forest and supports a unique assemblage of associated species, including a large variety of lichens, mosses and invertebrates, a number of which are rare and endangered in the UK.

However, because it very rarely flowers in Scotland today, aspen almost never produces any seeds and this prevents its natural spread to areas from where it has been lost. This problem has been compounded by the fact that aspen foliage is highly palatable to deer, which will eat it in preference to most other tree species.

As a result of these factors, aspen has suffered more from past deforestation than perhaps any other tree in Scotland. It has been reduced by historical forest loss and exploitation in most of the country to small isolated stands of a few trees, often in accessible sites where deer can’t reach them.

Award-winning work

In many cases the trees in these stands are all the same sex (an individual aspen tree is either male or female) as they have grown off the root system of a single parent, and are still connected via their roots, forming a multi-trunked organism known as a clone.

With the next nearest stand or clone potentially being many miles away, even if aspen does flower, pollination and therefore seed production is extremely unlikely.

Trees for Life became aware of this problem in the early 1990s and initiated a project to help aspen recover as part of its award-winning work to restore the Caledonian Forest.

Surviving aspen stands in the glens to the west of Loch Ness and Inverness were mapped and studied, and a programme of root collection from selected stands was initiated in early 1992.

In the absence of seed production, the only way aspen can be propagated is by taking root cuttings from mature trees in the spring and keeping them in controlled indoor nursery conditions, so that the young shoots or suckers they send up can be separated off and grown on as new young trees.

Seed production

This process is time consuming and labour intensive, but once the technique was mastered, Trees for Life expanded its work and in recent years has been growing up to 4,000 young aspens per year by this method in its tree nursery at Dundreggan Conservation Estate in Glenmoriston.

Propagation from root cuttings has a number of limitations though, making it a less than ideal method for growing aspens. Removal of root sections is damaging to the mature trees, so a period of at least five years is left between collections from an individual stand, to allow time for the trees to recover.

Another drawback is that all the propagated aspens are genetically identical to the parent tree they are grown from – the genetic variability that is the result of pollination and seed production is not achieved by this method.

Finally, because of the labour intensive nature of the work, propagation from root cuttings cannot produce enough young aspens to provide an adequate supply for tree planting projects, and the trees that are produced are much more expensive than most tree seedlings, making them less attractive for many forest restoration schemes.

For several years Trees for Life has been aiming to address this problem by encouraging aspens to flower, so that pollination and seed production can take place.

Newly-germinated seedlings

Working with the charity Coille Alba, a special aspen seed stand facility was established in the tree nursery at Dundreggan and aspen trees have been grown there using branches from parent trees that are known to have a greater propensity for flowering that have been grafted on to aspen rootstock grown from root cuttings.

In the controlled conditions of a polytunnel, these aspens have been stimulated in the hope that they would flower at an earlier age than they usually do.

In the spring of 2017 some of the trees did indeed flower, and pollination was achieved in a delicate and careful procedure, using an artist’s paintbrush to collect pollen from a male tree and transfer it to the catkins of female trees.

After pollination, the female catkins need to be watched carefully as the seeds ripen quickly and are only viable for a short period. It’s important to collect them as soon as they are ready and sow them straightaway, as germination occurs normally within 24 hours.

The tiny newly-germinated seedlings need to stay moist, so they were kept in the polytunnel, and were grown on until they were large enough, after a few weeks, to be pricked out into individual root-trainer pots.

A crucial habitat

They continued to grow well and later in the summer were transferred outside, for hardening off, in preparation for planting out. As a result, we now have just over 1,000 young aspens that have been grown from seed this year, marking a real breakthrough for our work for this scarce species.

Having proved that the technique works, we’re now aiming to scale up this work in the years ahead. So far only a handful of the 72 aspens in the seed orchard have flowered, so when more do so in future we expect to significantly increase the volume of seed that we collect and the number of young aspens we can grow from them.

While the number of aspens grown from seed this year are fewer than those we’ve grown from root cuttings, we anticipate that this will change in future, with seedling production surpassing by a considerable margin the numbers of trees we are growing from root cuttings.

In comparable native forests elsewhere, such as those in southwest Norway, aspen is much more abundant than it is in the Highlands today.

It plays a key role in ecological succession, as well as providing a crucial habitat for many other organisms, from rare lichens and invertebrates to mammals such as the European beaver.

This success with propagating the tree from seeds means that it will be possible to produce many more aspens in future, so that the species can take its rightful place again, as an important part of our recovering forests.

This Author

Alan Watson Featherstone founded the award-winning conservation charity Trees for Life in 1986, and has been promoting the importance of the aspen tree for over 25 years.