The science of autumn: from September to November

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September: Autumn’s palette begins to show

The stunning hues underpinning the autumn’s arrival can be attributed to the infamous green pigment chlorophyll, and its breakdown during the autumn months.

During the spring and summer months, when daily sunlight hours are at their highest, chlorophyll utilises this abundance of light through photosynthesis: the process by which light is absorbed by chlorophyll-containing reaction centres to convert carbon dioxide and water into carbohydrates. These carbohydrates are then used in a variety of vital metabolic pathways, such as amino acid and lipid biosynthesis. Chlorophyll is the dominant pigment in many plant tissues, and since it scarcely absorbs light wavelengths in the green region (~550 nm) relative to red and blue light, leaves appear green. That is, until the onset of autumn, characterised by the appearance of vibrant yellows and oranges where green foliage once was.

As the availability of light decreases, so does the extent of photosynthesis. Consequently, leaves stop synthesizing chlorophyll, and any that remains is broken down. Such plant senescence is triggered by a decrease in light, alongside a complex system of internal hormonal signals. Catabolism of chlorophyll allows for the recycling and remobilisation of nitrogen so that essential nitrogenous compounds, e.g. proteins and DNA, can be synthesized by the plant.

With the dominant green pigment removed from plant tissues, other pigments, including orange carotenes, yellow xanthophylls and red anthocyanins, become visible. Given the considerable genetic diversity between one species of tree and another, the relative proportions of each pigment are variable, leading to the impressive array of autumnal colours readily observable in nature.

Other pigments, including orange carotenes, yellow xanthophylls, and red anthocyanins, become visible

October: Winter preparations begin

Leaves are not the only aspect of nature undergoing changes throughout autumn.

As September comes to an end and October begins, animals begin to employ a variety of strategies in order to ensure survival in the coming winter months. A commonly seen behaviour is migration, where birds travel to warmer regions with more reliable food sources during the colder months. Some species carry out annual trips of several thousands of kilometers: the arctic tern migrates an astonishing distance of around 30000 km from the Arctic Circle to the Antarctic circle every year.

The way in which birds are able to sense the Earth’s magnetic field and use it for navigation has remained a mystery for many years, however researchers are now beginning to put forward theories to unravel the mechanism. The cryptochrome radical pair hypothesis proposes that, within a light-sensitive protein contained in the bird’s retina, a pair of unpaired electrons (radicals) switches between two quantum states, the triplet and singlet states, following the absorption of light. In the presence of a magnetic field, the radical pair spends more time in one of these states relative to the other – this change in balance between spin state lifetimes is thought to form the basis of magnetoreception.

Some animals, such as moles, enter hibernation in mid-late autumn. In preparation for this feat, many hibernators will hunt vigorously and consume as much food as possible, through a process known as hyperphagia. Remarkably, hibernating animals manage to endure several months without food by lowering their body temperatures and metabolic activity to the absolute minimum required for survival. These measures function to conserve energy during the coldest months, when food is scarce.

Other animals respond to the change in seasons through physical adaptations, such as the growth of thicker fur or even the change in fur colour, as seen with weasels and ptarmigans – these animals all shed their brown fur coat for a white one, which allows them to camouflage in snowy areas, avoiding predation. Interestingly, it has also been theorised that the white fur acts as enhanced insulation: since the fur lacks pigments, it has an increased capacity to hold air inside the individual hairs.

White fur acts as enhanced insulation: since the fur lacks pigments, it has an increased capacity to hold air inside the hairs

November: Wildlife braces for a temperature drop as days get shorter

As autumn draws to a close, leaves begin to fall from their trees. This is a clever energy-conserving strategy employed by deciduous trees – since leaves are no longer gathering light through chlorophyll to be used in photosynthesis, it is no longer energetically sustainable for them to be present. Triggered by the decrease in sunlight and lack of chlorophyl, ethylene and auxin work antagonistically to bring about leaf abscission, saving the tree both energy and water. These leaves do not go to waste: fallen leaves contain valuable organic material and trace minerals that enrich soil fertility, boosting the potential for new plant growth.

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