While stopped in heavy Melbourne traffic recently, I noticed that what looked like a shadow under a row of spotted gums (Corymbia maculata) along a major road was actually a stain on the concrete kerb.

As a botanist, it caught my attention; biological stains always have an interesting story attached.

Stains like these – under many tree species, on your car after certain leaves have fallen on them, and on your timber deck after rain has washed leaves onto it – are from tannins leaching out of foliage.

Tannins are astringent and bitter-tasting chemicals found in many leaves; they’re what add flavour to red wines, chocolate and tea. Oak timber is high in tannins, and it’s the tannins in oak barrels that enrich the flavour of some wines.

So tannin stains on concrete, cars and decks may be unsightly, but that doesn’t mean tannins are unimportant.

When it rains, materials such as amino acids (the building blocks of proteins) and sugars can be washed or leached from leaves of trees. These contribute to the........

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