The fabulous world of nature's own 'Christmas tree decorations'
From old men’s beards and string-of-sausages lichens to redwings, fieldfares and even ‘fairies’, Scotland’s trees play host to a wealth of natural decorations
Traditionally, we decorate a Christmas tree with tinsel and ornaments, making a tree bright and cheerful at a time of year when at a quick glance, our native species mostly seem dull and bare of leaves. However, if you look closely, you might be surprised just how full of life our trees are, even in the depths of winter.
Tinsel draping the branches of our Christmas trees may be the first festive decoration you think of. Visit many of our woods, and you will see trees draped in natural tinsel: lichens. In the winter, with the leaves off the trees, these fascinating organisms have their chance to shine.
Lichens are interesting because they are not a single organism but a symbiotic association between a fungus and an alga, and/or a cyanobacterium, all living together!
They range from the fine yellow-green strands of the Usnea species, variously known as old men’s beards, fishbone lichens, and string of sausages, to large leafy lichens such as the beautiful emerald-green lungwort (Lobaria pulmonaria), which is indicative of clean air and ancient forests.
A close-up look at the tree trunks will reveal the diverse mosaic of encrusting lichens such as script lichens that form thin white crusts over the bark with their black fruit-bodies which resemble curly writing. If you are out at night, some of these species even glow under an ultraviolet torch.
Trees support thousands of species
As a child, I had a favourite Christmas tree decoration, a red mouse! I always took great care in positioning my mouse, working out exactly where in the tree it would live. In real life our trees support many different organisms, providing food and a place to live and breed.
Across the whole of the UK, our oak trees support 2,300 species, our ash trees 953, and our Scots pine trees 1,589. Some of the species found on these trees are only found on these trees and no other species: 326 on oak, 44 on ash, and 227 on Scots pine.
I used to hide decorations in the most difficult places for my parents to find after Christmas. Similarly, many species love the nooks and crannies in trees. Old ones with lots of crevices and holes act as a home to many distinct species, which is why older trees, often with some dead wood, provide more biodiversity benefits.
Dead wood is particularly important: 40 per cent of the species that use oak trees and 50 per cent of the species that use Scots pine use the dead wood.
READ MORE: How Scotland's forests can deliver future prosperity with higher wages, affordable homes and more
Birds attracted by natural baubles
While even I did not decorate the soil below our Christmas tree, in our woods there are thousands of organisms living in the soil. These include a host of fungi, called mycorrhizal fungi, living in a close relationship with the tree, enabling it to access soil nutrients such as nitrogen while the tree provides the fungi with sugars. Modern DNA techniques are now allowing us to uncover much of this hidden diversity but many of these organisms have not even been named.
We often have baubles on our Christmas trees, and our native trees have their own versions in the form of berries, seeds, nuts, and cones.
Thrushes such as redwings and fieldfares fly thousands of kilometres from continental Europe to overwinter in the UK, where they eat rowan and holy berries. Next time you see these thrushes eating the berries, just stop and wonder at how far they might have flown to join us in our winter feast.
The golden-brown baubles of acorns provide food for 97 species across the UK and our Scots pine cones provide food for 65 species.
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Trees helped by fairies
For most families, the key decision in decorating the tree is what to put on top. While our native trees will not be sporting a Father Christmas, angels, or stars in their canopies, they might have fairies. Well, fairy flies (actually parasitic wasps) which help keep other insect populations in check by feeding on their eggs and thereby limiting things like caterpillars that eat the tree’s leaves.
In addition, even in winter, you may find a range of species high up in the treetops, for example a crossbill eating cone seeds or great spotted woodpeckers and treecreepers, amongst others, looking for insects in the bark.
At Christmas, we are encouraged to think of the homeless and to provide support for them. But we can also think about the biodiversity that is becoming homeless as our tree species are increasingly hit by climate change and novel pests and diseases. In recent decades, there has been a dramatic increase in the number of new diseases threatening our trees.
Those species that only use one tree species or a small suite of tree species are particularly at risk. So, how can you help limit the risk of our tree-associated biodiversity becoming homeless? Try to buy “Grown in Britain” or plants that have been certified as “Plant Healthy” as this will reduce the risk of accidentally moving diseases around the globe.
If you are lucky enough to have a holiday abroad, do not bring plants back home. Clean your shoes, prams, and bikes etc before moving between different woodlands. All these actions will reduce the risk accidently moving a disease around, which would impact not just the trees but all those species living in them.
So, as you enjoy the beauty of your decorated Christmas tree, do not forget to look outside and enjoy the beauty of our trees and to look closely at them to see what decorations (species) are hiding away in the tree, even in the winter.
Dr Ruth Mitchell is a plant and soil ecologist at The James Hutton Institute
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