On an autumnal visit around the forest at the Dundreggan Rewinding Centre, our friendly guide spoke of the wonder of trees, including the extraordinary photosynthetic properties of Aspen bark. The next day, on tree recording excursion for the Woodland Trust's Ancient Tree Inventory, in a special clump of very large Aspen, we hit the jackpot and found ‘green gold’!
Aspens in full autumn colours
Leaning old Aspens
One of my favourite native trees here in Scotland is the Aspen, Populus tremula. With its gleaming pale branches and tembling leaves this is a beautiful species. Ecologically, Aspen are considered a keystone woodland species, as a documented host to at least 100 species of fungi and a further 130 species of lichens, and they are also a firm favourite of Beaver. Their distribution is limited, having suffered heavily from deforestation and grazing pressure, and now limited to steep ground and inaccessible cleuchs and cliffs where the sheep and deer cannot reach. So it is wonderful to know that there are lots of people working hard to redress this imbalance, with great conservation work being carried out across Scotland by organisations such as Affric Highlands, Allerdale Rewinding, Borders Forest Trust, and Trees for Life.
“This important physiological aspect of bark photosynthesis [as a] contribution to the carbohydrate supply of the bark when phloem transport is minimal and when deciduous trees have lost their leaves.”
Aspen naturally form clones, reproducing asexually by suckering and rarely flower. Trees for Life have now made a major breakthrough for the future biodiversity of Scotland's forests through a pioneering technique to induce Aspen to flower. The resultant seed will help with future reforesting as well as ensuring greater genetic diversity across Scotland where they rarely if ever flower.
For a species living in tough places and long winters, any adaptations to lower light levels will be a bonus. One such adaptation is photosynthetic bark and Aspen are one of a few species of trees that are able to to do this. Known as corticular photosynthesis, the trees have a green, chlorophyll-containing layer beneath a thin outer bark. This layer is able process starlight into energy, converting sunlight, water, and carbon dioxide into sugars and oxygen. For trees growing in tough sites, this additional ability to make sugars will be of great benefit during the winter.
Green gold - the actively photosynthetic bark of a mature Aspen
Lots of the more mature trees there also have fungi around their root bases and as we measured trees and took photos we discovered the UK Champion tree was a champion no longer…only a wide stump covered in fungi remained. It reminded us that Aspen are not a long-lived species as an individual, some may get to 100 years but remember, clonal colonies can be very long lived indeed, like the world-famous Pando which scientists have estimated to be around 10,000 years old! For now, one of the nearby trees here may now hold the title, or could the true champion be growing quietly somewhere in another valley? Let the hunt continue…
