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Early warning systems

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Sally Nex Sally Nex | 12:28 UK time, Tuesday, 9 November 2010

Have you been out to look at your trees just lately? It's a good time of year to do it: with the leaves off, all sorts of hitherto unsuspected problems show themselves. But a patrol of the apple and pear trees in my garden this weekend revealed another rather worrying development: the air around them isn't as clean as I thought.

You see, I had my camera with me and got distracted taking pretty pictures of the lichen growing on their trunks. The appearance of lichens, mosses and liverworts are a mild concern to gardeners, as they colonise plants which aren't growing as vigorously as they might be, and a downright annoyance to those with mossy lawns (though those into Japanese gardening tend towards mossy-lawn envy: gardeners are a hard-to-please bunch).

But look closer and you'll find a whole other garden in miniature, filled with beautiful and strange plants of ethereal silvery-green and shimmering filigree emerald.


Some lichens are incredibly rare: at Wakehurst Place in Sussex there's a whole garden dedicated to their conservation. All are resolutely wild, and though you can encourage them in the garden by creating ideal conditions for that mellow, aged look they bring, it's all but impossible to transplant or seed them yourself.

They're more than just a pretty leaf, though. They also act as early warning systems for air pollution.

Some lichens die out in dirty, nitrogen-rich air, whereas others enjoy a little extra nitrogen and thrive, which is why lichen populations are the focus for the on-going OPAL Air Survey, a 'citizen science' project which has just turned one year old. It's now gathered enough lichen records from gardeners and nature lovers all over the country to draw some interim conclusions about air pollution patterns in the UK.

They're remarkably optimistic: pollution-sensitive lichens are on their way back even in places like London and Birmingham, formerly 'lichen deserts' according to Natural History Museum lichenologist Pat Wolseley. Surprisingly, and rather depressingly, though, some of the worst results were in rural areas - attributed to high ammonia levels from intensive agriculture.  

And that's why, here in the rural south-west, my trees are covered in nitrogen-loving leafy Xanthoria and Physcia. Rather confusingly, there also seem to be tufts of Evernia – a nitrogen-sensitive type. But then science was never supposed to be easy.


I have intensively-farmed fields all around me: arable fields the other side of the hedge, a sheep field over the road, and more for miles beyond that. So that would explain it. I can't help being a little disappointed to discover I am living in a cloud of ammonia.

If you're interested to find out more about the air you breathe, here are the varieties of lichen to look out for in your garden:

Clean air:

  • Usnea: thread like branches, like tiny tufts of grey-green hair
  • Evernia: a little reminiscient of seaweed, with branched, grey-green lobes 
  • Hypogymnia: Greyish broad, leaf-like lobes lie flat on the branch

Polluted air:

  • Cushion anthoria: Yellow, with distinctive orange fruiting bodies like inside-out mushrooms
  • Leafy Xanthoria: Smooth yellow carpet spreading across branches: similar orange fruiting bodies
  • Physcia: Like Hypogymnia but not as flat: black-tipped whiskers on some lobes

    Sally Nex is a garden writer and blogger and part of the BBC Gardening team.


    • Comment number 1.

      Thank you for this fascinating article. I had some faint recollection that lichens indicated clean air, so when I recently took a picture of some amazing lichen forms on a row of old blackthorn trees near us ( when foraging for sloes, I supposed that they indicated how clear our country air is. Lovely to be able to confirm that the two types I saw were probably hypogymnia and usnia, so the farming around us is not too aggressive.

    • Comment number 2.

      @Sally Nex: Nice article, it's always good to see lichens being promoted! Where did you get your pollution information from? I'm fairly certain Evernia are pollution tolerant, as are Hypogymnia. Also, the main source of nitrogen for Xanthoria parietina is bird poo. So whilst increasing nitrogen in the water supply because of fertiliser use will likely affect riverside lichens, the reason (and most other people) have lots of Xanthoria everywhere is because birds perch (and poo) on those branches a lot. For the same reason, they're common on roof edges.

      @hillward: the two most prominent species in your photo are Evernia prunastrii (the one which is mistaken for Usnea), and I would guess Punctelia subrudecta (the one you guessed as Hypogymnia). Evernia prunastrii grows in quite a wide range of atmospheric pollution levels, and I'm not sure about the pollution indication of Punctelia. Unfortunately it doesn't look like there are any Usnea species there.

    • Comment number 3.

      Oh sorry one more correction, I notice you've identified the top photo as Physcia, but it is definitely not. It's a Parmelia of some sort I think, sulcata or saxatilis.

    • Comment number 4.

      @hillward: thanks! and good to see there's some clean country air left - do you live near fields?

      @Richard Smith: a lichenologist, I presume! I don't remotely pretend to be a lichen expert: just very interested. All my identifications are from the pictures given in the OPAL survey's field guide: you can have a look yourself here:

      Both Evernia and Hypogymnia are clearly indicated as being pollution sensitive. It is of course not exhaustive so I'm really interested to hear your suggestion that my 'Physcia' may actually be a parmelia - that wasn't even on the OPAL sheets so you can see why I might have missed it! Now - what does that say about the quality of my air I wonder?


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