The Science of Autumn

Autumn is associated with pumpkins, brightly coloured leaves, branches laden with berries, mushrooms, and warm bonfires on chilly days. Although the changing of the seasons is something that we take for granted in temperate climates, the science behind autumn is fascinating and driven by astronomical, environmental, and physiological processes.

For many animals and plants, autumn is a time of preparing for winter, by reducing resource requirements, increasing food intake, or migrating to other areas. For other organisms such as fungi, autumn is a time for reproduction as their fruiting bodies appear and spread spores. It is one of the best times of the year to enjoy nature and explore how the organisms around us are adapted to our seasonal cycles.

The timing of autumn

The cycle of seasons is caused by the tilt in Earth’s axis and the orbit of the planet around the sun. There are two common classifications that define when autumn occurs, astronomical and meteorological. The astronomical definition of autumn is based on the position of the Earth in relation to the sun and runs from the autumn equinox until the winter solstice. The meteorological definition of autumn uses the Gregorian calendar and counts the months of September, October, and November as autumn. When we are considering autumn in terms of the natural world, however, we talk about the phenology of autumn and can use natural seasonal markers to define when autumn is occurring. Autumn is characterised by decreasing daylengths and cooling temperatures, and these have noticeable effects on the organisms around us.

Changes in leaf colour (tinting), leaf fall (abscission), and bird migration can all be used as markers of when autumn is occurring. In the UK autumn phenological markers are recorded for many tree species by the Nature’s Calendar citizen science project. The markers include ripe fruit, leaf tinting, falling leaves, bare trees, first flowers for ivy, appearance of fly agaric fungi, the last records of summer visitors such as swallows and house martins, and the first records of winter visiting thrushes (redwings and fieldfares). The focus of phenological studies has very much been on spring, however, so we do not currently have a clear picture of the mechanisms behind the timing of autumn events. The timing of these events is thought to be mostly an interaction between shorter daylength and reducing temperatures, although recent studies have indicated links between summer temperatures and the timing of autumn.

In recent years leaf tinting and leaf fall have been occurring later in many areas due to warmer late summer temperatures as a result of climate change. There is considerable research being conducted into this currently as the presence of leaves on trees is critical to how much carbon dioxide deciduous trees can absorb. Although we might expect that leaves being present for longer means that a tree can absorb more carbon dioxide, there is some evidence that leaves have a carbon saturation point and may in fact fall earlier. This means that we can expect an advance in the timing of autumn of 3-6 days this century instead of the predicted delay. The idea of a carbon saturation point in trees is concerning in terms of climate change mitigation, as previously it was predicted that warmer summers and later autumns would increase the carbon sequestration capability of trees. Our understanding is further complicated that shifting autumn phenology can have on the timing of spring, with one study finding that delayed autumn timing delayed spring events such as budburst. This acts as a counterpoint to the advance in spring phenology that occurs as a result of warmer winters, demonstrating how complex the balance between seasonal events is.

Autumnal processes in trees

The annual of cycle of deciduous trees involves producing leaves, flowers, pollination, fruiting, and then abscission (losing leaves), and dormancy. Trees must balance the time allocated to each of these phases in order to maximise the chances of successful pollination and seed dispersal, but still ensure their overwinter survival. Deciduous trees lose their leaves in winter because it is not worth the energetic cost of maintaining them when light levels drop and temperatures start to reach freezing point. Losing leaves also protects the tree from water loss during cold weather, and reduces the risk of cold, brittle branches snapping due to wind catching in the leaves.

The first visible sign of leaves preparing for abscission is that they turn yellow, orange, or red. Photosynthesis slows down, and many plants stop producing chlorophyll, revealing the hidden yellows of the carotenoids left behind. As the chlorophyll is degraded, the chemical components such as nitrogen are resorbed and moved to other tissues within the plant. The orange and yellow carotenoids are slower to degrade than the green chlorophyll but eventually fade to leave the brown skeleton of the dead leaf cells. Other trees and plants prepare by making red anthocyanins at the end of summer, turning the leaves red, pink, or purple and protecting the leaf from sun damage as it ages. Different conditions affect the vibrancy of the anthocyanin colours, with sunny, dry days producing more vivid colours as the sugar concentration is increased and more anthocyanins are produced.

Once the resorption of the leaf chemicals has been completed, the abscission process begins. Leaves are not simply blown off trees, but are excised at a specific point called the abscission layer. This layer of cells forms when the leaf grows in the spring and it is activated in the autumn by decreasing levels of the hormone auxin coming from the leaves into the tree as the chlorophyll degrades. Prior to detachment a protective, waterproof layer of cork cells is formed under the abscission zone. The cells in the abscission layer elongate as auxin decreases and sensitivity to another hormone called ethylene increases and the leaf is released. Ethylene is the gas that bananas produce that encourages ripening in other fruits.

In addition to abscission, deciduous trees prepare for dormancy by slowing and halting their growth before the temperatures drop too far. They develop dormant freeze-resistant buds and the tree undergoes a rapid cold acclimation process to enable it to enter endodormancy and be prepared for freezing temperatures. Trees undergo changes at the cellular level to enable them to avoid or tolerate freezing within their tissues, using techniques called extracellular freezing or supercooling. Extracellular freezing works by allowing some layers of the cellular structure to freeze and protect the living tissues. Supercooling allows the temperature of water within cells to drop far below 0°C without freezing and is based on the principle that water needs a seed particle such as dust for ice to form at 0°C, otherwise it can reach temperatures as low as -40°C without freezing.

Autumnal processes in animals

For most animals, autumn is a time when they increase food consumption to help them survive the colder months. Animals such as hedgehogs and dormice pile on extra weight to help them survive hibernation, taking advantage of the seasonal glut of nuts and berries. The overwinter survival of many species is linked to their weight in autumn, and in some cases even to specific food types. For example the overwinter survival of juvenile and adult great tits has been linked to the abundance of beech mast. Other non-hibernating species such as squirrels and jays gather nuts and acorns and cache them in extensive food stores. Jays can hide as many as 5000 acorns in autumn but do not retrieve all of them, making them one of the key sources of broadleaf woodland regeneration. Many mammals such as foxes and rabbits grow thicker fur during autumn as preparation for freezing temperatures, and bird species use the food abundance of late summer and early autumn to conduct their annual post-breeding feather moult.

Food availability also drives many of our insectivorous bird species to migrate away from the UK in the autumn, flying thousands of miles to warmer climates in Europe and Africa. The autumn return migration to Africa actually starts in late July, but species such as blackcaps, whitethroats, and willow warblers migrate later, benefiting from the late summer glut in food. Before migration all species will fatten up, storing fat on their body in a cavity known as the furcular pit, using these reserves to help them cross barriers such as the Sahara Desert. There are many duck and wader species that make the reverse migration, flying to the UK for the winter from colder climates such as Greenland and Siberia. Here they form vast flocks on our estuaries where there is a plentiful supply of food, particularly on the east side of the country. The UK is one of the most important international overwintering sites for waders and waterfowl and seeing them return in the autumn is one of the great pleasures of the birdwatching year. We also have an influx of thrushes from Scandinavia and even common birds such as blackbirds and starlings move across from the continent to benefit from our milder climate and abundant food.

For some animal species, autumn is a time of reproduction, with the deer rut being one of the natural highlights of the season. The males compete for mating rights from the end of September until November, and the females are pregnant overwinter, giving birth in the spring when there is greater food availability. Bats have a similar strategy, forming mating colonies in late summer or autumn, stretching the pregnancy over the winter. Grey seals give birth during the autumn having spent the summer fattening up on booming fish stocks. The females come ashore on northern and western rocky coasts and spend 2-3 weeks feeding their rich milk to the pups. Autumn is also spawning season for some fish species such as sea trout and salmon, who swim upstream to their birthplace in the summer to lay eggs.

Autumnal processes in fungi

Autumn is the best time of year to see the mushrooms of many of our fungi species, with the warm, damp conditions being perfect for the dispersal of spores. Mushrooms are the fruiting body of a much larger fungal organism buried in the soil, which is composed of fine filaments called hyphae. This hidden network forms the growing part of the fungus called the mycelium and it is where the mushrooms are produced from. 

Mushrooms start as a nodule called a primordium on the mycelium which grows into a round egg-like structure made of interwoven hyphae. The mushroom develops inside this cottony layer, known as the universal veil, which tears as the mushroom grows and may form scales or a cup called a volva at the base of the stalk. A second layer of tissue, the partial veil, may cover the gills and, as this breaks it forms a ring or annulus around the stalk. Not all mushrooms develop within a universal veil and many species such as puffballs, jellies and earthstars do not have stalks.

Once grown the mushrooms release spores from their gills or pores, actively catapulting them into the air. They are then most commonly carried by the wind and can disperse over vast distances. The spores then recombine in the ground to produce more hyphae that form more mycelium. 

The timing of mushroom fruiting in autumn is predominantly determined by temperature, with warmer temperatures leading to a later fruiting date. Higher rainfall and more dense vegetation cover can also delay the appearance of mushrooms in the autumn. If climate change results in warmer, wetter summers, this could create ideal temperatures for fungi in the UK and lead to increasing numbers of mushrooms in autumn. The UK autumnal mushroom season has more than doubled in length from 33 days in the 1950s to 75 days now, and this could even lead to year-round fruiting in some species. With the increase in fungal tree species, this is not necessarily good news for our trees. Edible species to look out for in autumn include chanterelles, boletes, and hedgehog fungus.

In spite of the cooler temperatures, wetter weather, and lack of wildflowers, autumn is one of the best times to get out and enjoy our fantastic countryside. The abundance of berries, nuts, and fruits is not just good for wildlife, but also for foragers, and making crab apple jelly, elderberry syrup, or sloe gin to see us through the colder months reconnects us with the seasonal cycles of nature around us.

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