Aspen is one of the lesser-known tree species in the Caledonian Forest. However, ecologically it is very important, as it provides the habitat for a suite of associated organisms, including mosses, lichens and invertebrates, many of which are now rare and endangered in Scotland.

These range from the aspen hoverfly, which is a priority species for conservation in the UK and whose larvae feed in dead aspen wood, to the European beaver (Castor fiber), which relies on aspen as one of its main food sources in winter.

Aspen has probably suffered more from deforestation than any other native tree in Scotland.  This is because it rarely produces seeds, so that when it is lost from an area, it is highly unlikely to recolonise it again of its own accord. Instead, aspen reproduces mainly by new shoots, called ramets or suckers, growing from the root systems of mature trees. As a result, it is constrained to producing new trees only on the periphery of existing aspen stands, where the roots of established trees can extend up to 50 metres from their source trunk. Adding to its challenges in reproducing, aspen is one of the most palatable of all tree species for red deer, so any new shoots are eaten, unless they occur in locations that are out of reach of the deer.

The consequence of this is that aspen has been reduced to small fragmented stands, sometimes consisting of a handful of old trees all growing off the same root system, that are geographically isolated from each other. As such, they are unable to provide the habitat for the species that depend on aspen. In many cases the aspens that do survive are growing in difficult and inaccessible locations, such as the edge of cliffs or in steep rocky gullies. Those are the only sites where deer can’t reach them – they do not necessarily represent the best sites for aspen to grow in.

Recognising both the ecological importance of aspen, and the difficulties the species faces in re-establishing itself in the forest, we launched a project in 1991 to help restore aspen in the Highlands. There are four main elements to the project: surveying and mapping of existing aspen stands; protection of ramets or suckers at existing stands to facilitate their natural regeneration; propagation and planting of young aspens to establish new stands; and research into the ecology of aspen and its associated organisms.

We began recording data on aspen stands in 1991, initially in Glen Affric, and subsequently throughout a 1,000 square mile area to the west of Inverness and Loch Ness. The information gathered at each site includes data on: soil types; aspect; elevation; number, height and stem diameter of trees; numbers of ramets observed; evidence of grazing; accessibility of roots for collection and propagation; and associated vegetation and tree species. This information is stored in a database, and to date, over 450 aspen stands have been surveyed, with the mapping now expanding to sites outside our original project area.

Throughout this work, it has become apparent that aspen occurs more widely than we had first thought, and 68 aspen stands have now been identified in Glen Affric alone. The stand sizes vary from some that contain only 1 tree to others with more than 200, and with heights that range up to over 20 metres. Extensive aspen stands, with some of the tallest trees, are in riparian areas in Glenmoriston and on the RSPB’s Corrimony Nature Reserve. The largest stand surveyed to date is in the gorge of the Cannich River in Glen Cannich, where there are 330 trees, with one measuring 2.7 metres in girth, giving it an approximate age of 270 years. All the data we collect is transferred on to our GIS dataset, and this provides a valuable tool for planning the strategic expansion of aspen in the areas where we work.

The surveys have also produced some interesting data, including the discovery of an aspen site in Glen Affric that consists solely of young ramets measuring 30 centimetres or less in height, with no evidence of a parent tree anywhere nearby. This is an example of the ability of aspen roots to survive underground for many years after the death of the parent tree, with the photosynthesis from the ramets’ leaves providing enough nourishment to keep them alive. This phenomenon has been known of since the 19th century, and in the case of this example in Glen Affric, we suspect that the parent tree(s) may have been drowned when the construction of the Beinn a’Mheadhoin dam in the 1950s raised the level of Loch Beinn a’Mheadhoin by 6 metres, with only the roots and ramets surviving on higher ground.

The surveys have also recorded seasonal phenomena associated with aspen. These include the widespread blackening and dying back of leaves that happens in some years, and is caused by Venturia, an ascomycetes fungus which non-lethally infects aspen, particularly under stressful conditions such as drought. Flowering of aspen is known to occur irregularly, and we have built up records of this at some sites during the past two decades.

Our surveys of aspen stands confirmed our casual observations that very little natural regeneration of aspen was occurring in them, and the ramets showed clear evidence of grazing damage from deer that was preventing their growth. Many stands had profuse numbers of sprouting ramets a few centimetres in height, but no young trees whatsoever, indicating that recruitment, and therefore successful regeneration of the stands over time, was being completely inhibited.

Beginning in 1992, we instigated a programme of protection for ramets at selected sites, using a variety of methods. These include the protection of individual ramets with tree guards, and we’ve used both solid tubes and open mesh Netlon guards for this, with the latter being more effective. In other locations, small areas containing ramets have been protected with stock fencing, with a typical size of such an exclosure being about 3 metres by 6 metres. Deer are unlikely to jump into such a small, confined space, and these exclosures are readily erected by groups of volunteers, using recycled fencing materials, thereby keeping the costs to a minimum. In some situations we have also used small areas of deer fencing to protect aspen stands, or have planned the fence lines for larger exclosures to specifically include aspen stands.

Ramets that are protected from overgrazing are able to grow very quickly – much faster than a newly-planted tree seedling is able to do. This is because an individual ramet grows off the root system of a mature tree, and is able to draw on all the food resources produced by the tree. As a result, some ramets we’ve protected have grown by just over a metre in height in 10 weeks!

Our intention with this work is to ensure that successful regeneration takes place in as many stands as possible, and also to extend the aspen stands. In addition, another objective, where stands are located near one another, is to link them up to form larger contiguous areas – this is crucial in creating aspen stands that are large enough to support some of the associated species, which require a minimum habitat of 4.5 hectares of aspen woodland to live in.

In 1992, we began work on the propagation of aspen from root cuttings, using methods first developed by the Forestry Commission. Roots were initially collected from a few sites in Glen Affric, and propagation trials were on a small scale, until the technique had been mastered. We’ve subsequently developed a custom aspen propagation facility, which now consists of two polytunnels, a mist propagation unit and adjacent standing areas in our native Tree Nursery at Dundreggan. Using this facility, the production of young aspens from root cuttings has been increased to its current level of between 3,000 and 4,000 plants per year, making us the largest producer of aspens by this method in Scotland.

Roots are collected from aspen stands listed in our database, with any given stand being left for several years to recover before further roots are collected. The young aspens grown from the root cuttings are all tracked with regard to which parent stand they are derived from, and when they are planted out, this is done in groups containing representatives of at least six or seven parent stands. This not only provides genetic diversity within each planted group of aspens, but also, by statistical averages, should ensure that both male and female plants are represented in each planting. This, in turn, should help to facilitate pollination and seed production when the trees reach reproductive age. To ensure that the local provenance of aspen is maintained, only young trees grown from parent stands within a particular glen, such as Glen Affric for example, will be used for planting out in that glen.

Planting has been done to both establish new aspen stands and to enrich the clonal diversity of existing stands, by planting trees sourced from parent stands elsewhere in the same glen. Because aspen is one of the most palatable trees for herbivores such as red deer, all the aspens we’ve planted have been protected, either with individual tree guards, or inside fenced exclosures. The planting sites have generally been selected on the basis of exhibiting similar characteristics to the extant stands of aspen, with the caveat that these conditions may not be not be the preferential ones for aspen, but just where the species has been able to survive.

Our planting has also been targeted at linking up existing stands where possible, and in some cases, at creating a future habitat for European beavers, should further reintroductions of that species be approved in Scotland. At Dundreggan we have planted a substantial area of aspen in the riparian zone of the River Moriston, with a view to creating adequate habitat in future for the aspen-dependent invertebrates that require large areas of aspen to live in.

With aspen having been little known and studied in Scotland in the past, research into the ecology of the species and its associated organisms is an important component of our project. There are several different aspects to this, with one element consisting of research projects carried out by students from various universities in Scotland. Past projects have included one focusing on isozyme analysis, to ‘fingerprint’ the different aspen clones in a number of stands, and another documenting the extent of grazing damage by red deer on aspen ramets, the results of which we used to plan and implement protection measures for a number of aspen stands in Glen Affric.

At Dundreggan, our ongoing partnership with Plymouth University for research into the biodiversity in the canopy of the Caledonian Forest has included an ascent into the canopy of a large aspen tree, and this will be developed further in future. In our tree nursery, in partnership with the charity, Coille Alba, we are carrying out experimental work on grafting aspens, to encourage more regular flowering of the trees, and are planning to establish an aspen seed orchard, to facilitate greater seed production of the species.

We have a programme of biodiversity surveys at Dundreggan and other sites where we work, and this has produced a number of important discoveries regarding species associated with aspen. These include mapping of the distribution of the aspen bracket fungus, the first record in Scotland of the palisade sawfly that feeds on aspen trees, and the discovery of a fungus (Calathella eruciformis) that grows on dead aspen wood that had not been known from anywhere in the UK before.

Trees for Life is a founder member of the Highland Aspen Group, which brings together aspen enthusiasts and experts from throughout northern Scotland to share their knowledge, experiences and skills, and we contributed to the two Aspen Conferences, held in the Highlands in 2001 and 2008.

Aspen trees in Scotland rarely flower or set seed, nor do they readily grow from stem cuttings, so Tree for Life’s aspen propagation work has focused on a method using root cuttings.

As aspen naturally spread by sending new shoots, called ‘suckers’, up from their root system, we can encourage this suckering by bringing root cuttings into a polytunnel in early spring, and then harvest the shoots as they come up, rooting each cutting individually using a mist propagation unit. This method, despite a good success rate of usually between 80 and 95%, is very labour intensive; hence trees are expensive and not available in such large quantities as other species that are readily propagated from seed.

Another charity called Coille Alba, which leads the Aspen 2020 Project, one of whose main objectives is to increase the area of aspen woodland in Scotland, has been looking into methods of stimulating aspen to flower and produce seed inside a polytunnel and also working on creating a ‘seed orchard’ for aspen. Working in conjunction with Coille Alba this year we have been producing grafted aspen trees destined for a future ‘seed orchard’ or ‘gene bank’. Cutting material has been collected from the top of mature aspen trees, which are known to be aspen clones more willing to flower than others – this material will not itself root readily, but it can be grafted on to the top of another young aspen tree. Abbey, our Tree Nursery Assistant who did most of the grafting work, took small cuttings with about three buds and cut the end of the stem in a ‘V’ shape to carefully match the cut made into the side of the stem of the rootstock plant. The cutting is inserted into the stem of the rootstock and taped so that the cambium layers of the two stems are held tightly together, and waterproofed, until the plants unite. The shoots of the rootstock plant are carefully trimmed back to encourage the new shoot (scion) to grow instead. Despite some initial problems with removing the tape when the plants started growing, the grafting trial here has been a great success with 84% success rate, and the plants grew prolifically in the polytunnel, so much so that they had to be cut back before being moved outside for hardening up later in the year.

The grafting process is known to encourage prolific and early flowering in other species, and there has been some success with producing aspen seed from Coille Alba’s early trials, and this looks like it will increase year on year, so we plan to continue working together on this project in the coming season. If seed can be obtained from such orchards, it is hoped that trees propagated from them will be abundant and inexpensive, thereby dramatically increasing the availability of young aspen for native woodland planting schemes.

Since aspen rarely produces seed, alternative methods are required for its propagation…

Trees for Life has been working with the propagation of aspen from root cuttings since 1991 and we now have the capacity to produce up to 3,000 young trees each year in our special aspen propagation facility at our Tree Nursery. Our innovative techniques yield a 90% success rate in the rooting of cuttings and grow trees to plantable size in one year. Based on our experience, the instructions here provide a simple explanation of how to propagate aspen by this method.

1. Root Collection
  • The best time to collect roots is from February to May on frost free days. The earlier the better as this will allow time for shoots to develop and cuttings to be taken before summer.
  • Locate suitable roots near mature trees: check exposed rock and banks of streams and lochs where roots may be uncovered; lines of suckers will indicate the location of underground roots and wind blown trees will provide an abundance of propagation material. Collect roots of 1-3 cm in diameter.
  • Expose the root and follow in both directions until it becomes too big or too small.
  • Detach the root and cut into sections of approximately 30 cm in length.
  • Place in a plastic bag, with damp moss if they are to be stored for more than a day or two.
  • Label carefully with site name or reference number and date collected.
  • Severed roots left in the ground will be stimulated to produce suckers, benefiting the site, but over-collection may be harmful.
  • Leave the site as undisturbed as possible, replacing soil, surface vegetation etc.
  • Record activity so that the site is not stressed by over-collection in subsequent years. Several years should be allowed between root collections, particularly at smaller sites.
  • Put the roots in boxes of soil immediately or store in a cool, dark place.
  • Roots packed with damp moss may be stored for several weeks if collected outside the growing season, or several days otherwise.
2. Treatment of Roots
  • Keep roots in the shade wrapped in polythene to prevent drying out until they can be planted.
  • Plant in boxes which will accommodate approximately 10 root sections of about 30 cm in length.
  • Cover with compost or composted bark to a depth of about 5 cm – this planting medium is to keep the roots moist, it does not need to provide nutrients as the parent roots use their own reserves of energy.
  • Place boxes in a polytunnel, without heat, and water thoroughly whenever the compost starts drying out underneath the surface. Keep the polytunnel doors closed in cool weather, and then open to provide increasing ventilation as temperatures rise during spring and summer.
  • Suckers will begin to appear after 4 – 6 weeks and, if they are harvested regularly for cuttings, new suckers will continue to emerge from some of the roots for up to 12 weeks. The root sections are very unpredictable, however, and it is likely that some roots will produce plenty of suckers, whilst others produce few or none at all. Record data, noting the most productive clones, for future reference.
  • The root sections run out of energy to produce any more suckers in late summer or autumn and at this stage they should be discarded, as fresh roots will be needed for the following season. Empty the boxes out, brush clean and leave to dry out over winter.
3. Preparation of Cuttings
  • When the suckers reach 5 – 7 cm in height, cut off individually with a sharp knife or scalpel near to where they join the parent root. Take care not to damage younger suckers below the surface of the compost, as several often grow in a cluster at one point. Leaving the suckers to grow larger before harvesting may exhaust the parent roots’ energy faster and reduce the number of suckers produced.
  • If cuttings are prepared in hot and sunny weather, they should be placed out of direct sunlight as soon as they are cut, and kept shaded throughout preparation.
  • Remove lower leaves from the cutting and re-cut if necessary to provide a clean cut below a leaf node. Dip the cuttings into a rooting compound – either a synthetic one with hormone and fungicide such as ‘Strike’ or an organic seaweed rooting powder.
  • Plant the cuttings, using a dibber, into trays of compost mixed with perlite. Perlite improves both the aeration and water-holding ability of the compost, which is ideal for rooting cuttings. 25% perlite by volume is sufficient, but the percentage can be increased up to 50% if required.
  • We have used a peat-based tree and shrub compost, a bark-based and a coir compost. All allowed adequate rooting, although the bark compost was probably slightly less satisfactory and peat based composts are less desirable for environmental reasons.
  • Label all trays with the date and source of cuttings.
  • There is no need to water the cuttings, the trays are simply placed immediately into the misting unit.
4. Care of Cuttings in the Misting Unit
  • The misting unit automatically produces intermittent mist to increase humidity and to cool the leaves of the cuttings, preventing excessive moisture loss at a time when they have no roots to draw up more water. A soil heating cable in the base of the unit provides warmth from below to encourage the formation of roots.
  • When the electronic leaf sensor in the misting unit dries out, there is a break in the electric current between its two electrodes, so it can detect when the leaves of the cuttings may have dried out – this allows the misting unit to respond to the needs of the cuttings at different times of the day, etc.
  • There are also dials to manually control the length of the mist burst, the delay period before misting begins and the sensitivity of the leaf. If the mist is on for too long or too often, the cuttings and the compost become too wet and black rot may develop on the stems, causing them to topple over – this appears to be a particular problem in cool, damp weather in summer. Increasing the amount of shade over the unit, especially over the newest cuttings and in very sunny weather, allows you to decrease the amount of misting without detriment to the cuttings and this helps prevent the stem rot occurring. We have found it best to turn the misting unit down to almost the minimum settings when it is cool or damp, and then turn it back up to allow more misting in hot, sunny weather.
  • Cuttings usually form roots within 2 or 3 weeks, after which time they should be removed from the misting unit.
5. Hardening Off and Potting On Cuttings
  • When the cuttings come out of the misting unit, they need to be gently acclimatised to outdoor conditions by placing them in a cold frame and using heavy shading to protect them when the sun is strong; this also retains some humidity around the plants.
  • After a further 2 or 3 weeks, there are usually plenty of white roots showing beneath the trays of cuttings, and at this stage they can now be potted into individual pots.
  • We have found that both peat-based and coir composts give excellent results when used for potting but, as before, the bark-based compost was far less satisfactory.
  • Use fairly small pots (half litre size) as this limits the amount of compost needed and means the trees are not too heavy to carry when planting out. However, root growth is often vigorous and if the pots are placed on the ground, the trees will rapidly root through the pots, causing a real problem for when they need to moved. It is better to stand the pots up on benches or racks so that the roots are ‘air-pruned’ and kept inside the pots. Careful attention to watering will be required with this method – watering will be necessary every day during hot weather, to prevent the plants becoming stressed.
  • Plants that get stressed are vulnerable to the fungal disease Venturia, which causes the leaves and ends of the stems to turn black and die back. Strong plants will recover from the disease, but it could be fatal or prevent further growth on small, weak trees. (The fungus persists over winter in the dead stems so these should be cut out to remove the source of infection for the following year.)
  • Trees grown from cuttings taken in spring are generally ready to plant out by autumn or the following spring. Trees from cuttings taken later on during summer may need to be kept on in the nursery for a further growing season before planting out.
  • We have been experimenting with the use of ‘Rootrainers’ which produce trees with lots of straight roots in long, thin ‘plugs’ – ideal for planting out. Provided that the rooting success rate is high, they save on compost and also on time because the cuttings are not potted up. Cuttings are placed directly into the Rootrainer cells in the misting unit. After hardening off outside, the Rootrainer trays are lifted up off the ground onto racks to ‘air-prune’ the roots and ensure they stay inside the container. We have been pleased with the results using Rootrainers, but in dry weather it is difficult to keep the trees moist enough as there is so little compost and such dense foliage. Extra fertiliser is also probably needed as each tree has such a small nutrient reserve in its compost plug.
Hardwood cuttings
  • Hardwood cuttings of aspen only root readily when they are taken from extremely vigorous wood of the previous year’s growth. (For example. we were able to collect such material from a site where some big aspen trees had been felled and their root systems had then sent up large numbers of extremely vigorous suckers, often up to a metre in height within one growing season.)
  • Collect the material in early spring, e.g. late March, and trim to approximately 20 cm in length, cutting just below a bud at the base of the cutting and just above a bud at the top of the cutting.
  • Dip the base of the cuttings into a hormone rooting powder – although this step may not be absolutely necessary.
  • Plant the cuttings deeply (two thirds of each cutting below ground) into well drained soil or compost, and do not allow to dry out.
  • The cuttings can produce strong new shoots throughout spring and summer and can be large enough to plant out after one growing season.

 

Seed
  • Trees for Life does not have any direct experience of growing aspen from seed ourselves, as we have not been able to collect seed to grow. However, if you are able to obtain seed, then from research, we would suggest using a similar method to growing willows from seed. A method for growing willows from seed is outlined below, with which we have had some limited success:
  • The timing of the seed collection is crucial in order to collect the seed when it is ready but before it blows away. Collect catkins when the white down just starts to appear (April or May for aspen). Leave the catkins to “fluff-up” for a couple of days in the warm, e.g. on a windowsill in the sun.
  • Separate the seeds from the white down – this can be done by placing the catkins in a container with holes in the base just a bit larger than the size of the seeds. Agitate the catkins by spinning a piece of stiff wire in an electric drill and the down will be left in the container whilst the seeds fall through the holes in the base.
  • Sow the seeds immediately as they are likely to lose their viability rapidly if stored.
  • Seedlings are likely to be very susceptible to heat, wet, drought and damping-off fungi. Therefore seeds should be sown very thinly on the surface of moist compost and kept shaded from strong sun, protected from rain and misted with a hand-mister to prevent drying out.
  • Germination is likely to occur rapidly, within a few days of sowing. Seedlings will need to be very slowly acclimatised to normal outdoor conditions.
  • If sown thinly enough, seedlings can be left in their seed tray for the first growing season, and then pricked out into pots or a nursery bed the following spring.

 

Micro-propagation

This is a technique which can yield enormous numbers of young plants from small amounts of plant material but must be undertaken in sterile conditions. It involves taking small slivers of the undifferentiated tissue contained within the growing tip of the plant (apical meristematic tissue) and growing it on in a culture medium. This can then be repeatedly subdivided, cloning many genetically identical plants. Due to the delicacy of the operation and the requirement for sterile equipment and conditions, this process is only practical in a laboratory.

 

Division of roots

If only small numbers of plants are required, the easiest method is to dig up some roots in March, which already have young suckers emerging from them and transplant them in the desired location. Root sections should be 2-3 cms in diameter and 40-60 cms in length, to provide sufficient reserves for a new plant to grow, with suckers of 15-60 cms. in height. These could be rescued from an unprotected site where suckers are being browsed and should become established quite quickly.