How pretty normal stuff works: Leaves!

 
 

Caitríona Farrell, inspired by autumn’s vibrant yet vanishing hues, explains why nature transforms its leafy decor

The rich shades of terracotta, amber and burgundy mean that the season’s trends ablaze this autumn. It wouldn’t surprise me if fairy lights were invented to compensate for the lack of flamboyant colour ignited by autumn in a bid to allow a more graduated transition from autumn to winter.

102_0095Autumn really basks us in that golden warm glow despite bringing our days to a shorter close. Scattered around the place is some remaining evidence of last season’s bright styles. The planet’s fashion trend luckily is very predictable and this autumn gone by proved no different to any other. Earth’s biological cycle doesn’t fail to fuel our greenery with the over-familiar hints that signal autumn’s arrival.

The green leaves of the summer months saturated in the pigment of chlorophyll, reflects the underlying process of photosynthesis. The green plant is itself a molecular factory, fulfilling the important transformation of carbon dioxide and light into glucose, which serves as an excellent energy source for growth and storage. The removal of chlorophyll uncovers autumn’s spectrum of oranges, yellows, browns – even reds and purples – that leave any artist or onlooker in awe.

Leaves sweat through pores called stomata on the high surface area to exert a force of suction through the tree, and draw water from the ground in summer. Wintertime poses a different scenario: these leaves could cause the trees to dry out and die. As a natural defence mechanism, the trees need to shed the leaves – so nature allows for this with a process known as abscission.

In autumn many chemical changes arise and the abscission zone begins to swell, preventing the flow of nutrients from tree to leaf and vice-versa. Following this, the zone begins to tear and the leaf falls off or is carried by the wind. A protective layer seals the wound, preventing water evaporation and any entry of bugs.

The shorter days which stimulate the abscission process also initiate another process in the leaves of certain trees, to produce a group of chemicals called anthocyanins, which are deep red or purple in colour. The red colours are used to conceal the shades of yellow which attract aphids. Effectively trees which are more susceptible to aphids, or are native to habitats where aphids are a problem, are able to confuse their enemies and survive until another spring.

Evergreens have adapted, protecting their needle-like foliage from freezing by adopting waxy coatings and natural “antifreezes” – but broadleaf plants, like sugar maples, birches, and sumacs have no such protections, resulting in them shedding their leaves. Before they shed their leaves, the plants first try to retain important nutrients such as nitrogen and phosphorus.

Leaves contain carotenoids, a kind of natural pigment, which produce yellow, orange, and brown colours in plants. These carotenoids are always present but their colours are easily disguised by green chlorophyll, until autumn brings with it a shorter day and reduced temperature. Factors influencing nature’s rate and general operation such as soil moisture and weather ensure that no two autumns are identical.

Cooler temperatures, shorter days, and the changing angle of the sun’s rays upon the plant leaves are indicators for the plant to stop producing chlorophyll. The internal process being carried out in the leaves ceases, reducing the amount of green pigment. As the green pigment decreases, the other pigments play a more predominant role such as xanthophyll (a yellow pigment) and carotene (an orange pigment).

Observers have long noted, however, that the predominant colour found in autumn foliage in North America contains shades of red, while Scientists have long questioned why the main autumn leaf colour in North America is red and in Europe, yellow.

Of course the type of tree will come into play but this only slightly justifies this contrast. At Israel’s University of Haifa and at the University of Kuopio in Finland, however, two scientists have recently proposed a theory based on geography, insects, and climate change to explain this observance.

The two professors in Finland have constructed their research around the idea that evolution during the Ice Ages influenced leaf colour. According to the researchers, large portions of the world were covered by evergreen jungles. As the climate shifted from tropical to cold and the Ice Ages began, dry spells were common and deciduous tree species evolved.

Their reasoning is that North America and East Asia contain mountain chains that run north-south, so animals and insects migrated southwards as the glaciers advanced as a result of the Ice Age. In Europe, the mountain chains run east to west, and insects and animals could not possibly migrate to escape the ice age. Over time, the trees adapted to the insects, the red leafed tree surviving better in the United States.

In Europe, however, the lack of migration meant that trees that couldn’t survive the ice ages died, along with the insects dependent on them for survival. Therefore these trees maintained their common yellow colour in the autumn opposed to the predominant red colour visible in the U.S.

In other words, North American trees had adapted to repel insects and thrived to produce red leaf colours. In Europe, there was no need for the red to survive and the yellow emerged as predominant.

This theory is fairly recent, but it is intriguing that this evolutionary process may have in fact dictated the colour palette of our 21st century autumn. Trees are a monkey puzzle – a puzzling natural structure – and we still haven’t figured them out to their full extent. Leaves’ veins are effectively networks communicating with the surrounding ecosystem.

In the words of the famous poet Emily Brontë, “Every leaf speaks bliss to me, fluttering from the autumn tree.” Science voices itself in our global forest, each distinctive tone on each patterned leaf signifying change and the cycle’s next revolution.

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