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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 , as they colonise plants which aren't growing as vigorously as they might be, and a downright annoyance to (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 . All are resolutely wild, and though you can encourage them in the garden by 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 , 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 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.ÌýÌý

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 lobesÌý
• 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 ³ÉÈË¿ìÊÖ Gardening team.