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 of mushrooms, compost, rot and decay 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. 

Much of this takes place out in the open and it inevitably produces a variety of organic smells.

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 and held in the air.  

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 like cellulose, hemicellulose and lignin into their constituent parts.  

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.  

Some organisms do seem to make use of bad smells in ways that are comparable to the way flowers use scent to attract pollinators. 

The classic autumnal example of this is the very smelly and aptly named stinkhorn. 

Once your nose is switched onto the smell, however, it’s a sure way of finding the mushroom in undergrowth from several metres distance and often long before you can see it.  

Beware, however, of the rather rude appearance of the intact stinkhorn. Charles Darwin’s daughter, Etty, was apparently so offended by this mushroom that she used to spend autumn ‘sniffing’ them out so that she could destroy them before the morals of others could be offended.

Stinkhorns can be found in a wide range of woodland types including conifer plantations.  Here of course the exceedingly pleasant aromas of volatile conifer essential oils and pine resin can also diffuse in the atmosphere in the right conditions. These essential oils are thought to have a defensive function in protecting the trees from fungal and insect attack.  

In Japan, where there are some particularly strongly aromatic conifer species, these pine needle scents are associated with the health-promoting attributes known to come from ‘forest bathing’, Shinrin-yoku or walking slowly through the woods. Further evidence is required to determine exactly what produces the health benefits of visiting woodland, but there is a large body of evidence which indicates spending time in woodlands is a great way of relieving stress and improving mental health and wellbeing. It would be great to think that the smell of pine resin is good for you too.

Next time you’re out for an autumnal walk, do use your nose to enjoy the full spectrum of delights that autumn has to offer.

Some ‘must sniff’ smells of autumn:

1. The stinkhorn – look out for the smell of this rude fungus in the undergrowth of many woods and scrubby areas.
2. The earthy aroma of the leaf litter in a damp woodland. 
3. The flesh of other fresh mushrooms in the woods or fields (don’t eat unless you can identify the species and are sure it’s safe).
4. The deep composting litter of a bracken bed.
5.  The amazing smell of piles of rotting seaweed on the seashore.
6. Put your head into a hollow tree trunk (be careful not to get stuck) and take a deep earthy breath.