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All about algae on trees

Most of us have likely spotted mosses and lichens growing on trees at some point.  They can have vivid colours, and are especially visible during the winter months when most deciduous trees have lost their vibrant colours.  But have you spotted their shyer cousins, the algae?

You probably associate algae with large water bodies, but some species are terrestrial. In the UK, we have many varieties that grow on trees, often visible as a green or red film on tree bark.


Red algae come from a group of algae called Trentepohlia. These species of algae appear on trees (and some other outdoor surfaces) in quite bright colours, ranging from orange through to a rusty red. The orange colour is caused by the pigment in the algae, which is a carotenoid – the same thing that makes carrots orange.

Where to find algae

Like lichens, algae are sensitive to moisture. They are therefore more likely to be found on rough, textured bark, where rain gets trapped and an ideal damp habitat is created. It’s predominantly associated with oak, ash and beech, and is more common in the south of the UK.

Red algae on the trunk of an ash tree (Photo: blickwinkel / Alamy)
Red algae on the trunk of an ash tree (Photo: blickwinkel / Alamy)

Can algae help with navigation?

It’s suggested that you can navigate by looking at the growth of algae, mosses and some lichens on trees, but preference for moisture supersedes the power of the sun. Northern-facing tree surfaces are likely more damp, so algae growth can be an indication of direction, but if you need to use this as a form of navigation always walk around the whole tree to make sure the algae is only growing on one side, indicating where sunlight isn’t reaching the surface. Also, be aware that fertiliser drift from farmland can cause artificial blooms of algae on terrestrial land, just as it can in marine environments.

Algae doesn’t harm trees. It’s just using them as a surface to grown on, and they may appear by themselves or as part of a lichen. Why not go on an algae hunt and share your photos with us? Email them to us or share them with us on Twitter or Facebook.

Can you find algae on trees in your area?

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