There’s nothing quite as good at bringing back old memories as an evocative smell. Autumn is one of those times of year when evocative scents are everywhere.

As the weather cools and the days shorten, the smells of places like woodlands seem to reflect and resonate perfectly with the season.

Damp, earthy aromas dominate as the leaves drop and the ecological ‘tidying up’ process of decomposition gets underway. This is the season for earthworms, woodlice and fungi, and the many other species that earn their living as ‘nature’s recyclers’ and cleansers. Billions of individual organisms from thousands of different species, all working to extract the last bit of energy and nutrition by eating and digesting the dead tissues of animal and plants whose lives have ended as winter approaches.

Nature abhors waste and the tireless collective efforts of bacteria, fungi and invertebrates mean that every possible scrap of valuable nutrient and energy is removed from dead tissues for exploitation and reuse. This is the essential process of decomposition, and without it we would soon be buried under a large heap of dead matter.

A smell is simply a volatile chemical or suite of chemicals that are released into the air where the sensory receptors in our mouths and noses can detect them.

In spring and summer, the atmosphere can be filled with a blend of scents like those from flowers. We tend to be overloaded with scents at this time of year because organisms often produce aromas in quantity, and the energy provided by warm weather enables scent to disperse more rapidly and causes a larger quantity of the chemicals to be released.

In autumn, the temperature is cooler and can hold less scent, plus airborne chemicals diffuse less rapidly in still air. Scents can become dominated by the many and often highly complex organic chemicals produced by decay. The rotting activities of bacteria and fungi may produce volatile chemicals that combine to produce the evocative mix of earthy background aromas that we experience in damp places like woodlands.

Some are the ‘mushroomy’ smells that come from the fungal ‘fruiting’ bodies and their mycelium and hyphal threads which ramify through the soil; but other smells appear, as far as we know, to be the incidental by-products of the decay and rotting processes.

These aromas are produced as enzymes break down dead plant matter.

The smells of some fungi can be so distinctive that they can even assist with their identification. For example, the bright scarlet sickener brittlegill mushroom, Russula emetica, has a distinctly fruity smell when cut. Two of the related ochre-coloured brittlegills can only be told apart in the field by their smell. One has an oily, rancid smell (Russula foetens) whilst the other (Russula grata) smells of bitter almonds or marzipan. A sulphurous, coal-gas smell separates the yellow knight from the sulphur knight.

The carcasses of dead animals can be especially smelly as bacteria, beetles and fungi consume their flesh. Many species like the black sexton and burying beetles can detect the smell of death from long distances and will quickly arrive to feed and/or lay their eggs in the rotting flesh. Gruesome!

Next week I'll write about a fungus which makes use of bad smells like flowers use scent to attract pollinators: the aptly named stinkhorn!