I know its a cliche to talk about autumn colour in October but as I’m sitting at my desk writing this
I am inspired by the bright blue sky and autumnal sunshine outside my window, illuminating the
buttery yellows and fiery reds of the countryside surrounding our house.
I’ve always been more interested in the growing of plants than the science side of things but one
of my favourite plant science lessons focused on the ‘how’ behind autumn colour. My tutor at the
time really sold it to me and though its doubtful I will be able to recreate in you the feeling of awe
he left in me, I’ll give it a shot as plants are truly fascinating and deserve our marvel.
Interestingly its not the cooler weather or frost that are responsible for autumn colour displays,
though these can have an affect on colour and intensity, but the shorter days. Or to be more
specific, the longer nights. Plants are sensitive to the amount of darkness in each day and in the
autumn as the days shorten, the cells between the base of the leaf and the stem begin to divide
rapidly creating a dry, corky layer of cells known as the abscission layer. The abscission layer
slowly begins to block the nutrient highway, causing a jam in both directions preventing the
downward movement of carbohydrates from the leaves to the stems and upward movement of
nutrients from the roots to the leaves.
Chlorophyll (the green pigment in leaves and stems) is constantly produced during the growing
season as it breaks down with exposure to sunlight. However, once the abscission layer is in
place (our nutrient roadblock) the production of chlorophyll slows and stops completely, revealing
the spectacular autumn colours that lie beneath. During the growing season, chlorophyll masks
the yellow and orange pigments (xanthophylls and carotenoids) that exist in the leaf meaning they
are only visible once the chlorophyll has shut up shop at the end of the season. However, the red
and purple pigments are usually not present year round (with the exception of purple leaved
plants such as Copper Beech and the Smoke Bush etc). Known as anthocyanins, they are
manufactured from sugars trapped in the leaves as the season comes to a close.
Over the course of the autumn, the cells in the abscission layer become even drier until eventually
the connections between cells become so weakened, the leaves fall from the tree. Some plants,
such as beech and hornbeam, are able to retain much of their leaves over winter. But the
coloured pigments will eventually break down in sunlight or frost leaving behind tannins, a
pigment which is, you guessed it, brown.
Environmental factors such as light levels, temperature and rainfall influence the quality of the
display. An early frost will cause the leaves to drop early as can a very dry growing season, which
can sometimes trigger the early formation of the abscission layer. However, a moist spring and
summer followed by a dry autumn consisting of warm sunny days and cool nights is the best
recipe for outstanding autumn colour. So start praying for that Indian summer if you want to get
the best show!