Have you ever wondered — or maybe been asked by an inquisitive four-year-old — why do all the leaves change colour at this time of the year?
We take a little nature walk and brush up on our school science. Why do leaves change their colour in autumn? Why are some years more colourful than others? And why do trees let go of their leaves in winter?
Why do leaves change their colour in autumn?
The first thing to know is that leaf colour comes from pigments — natural substances made by leaf cells.
Their job is to help the plant capture sunlight and make food.
There are three main pigments that colour leaves: chlorophyll (green), carotenes (yellow and orange), and anthocyanins (red and pink).
During spring and summer, when sunlight is plentiful, trees make lots of chlorophyll. This green pigment masks the other colours, giving leaves their familiar deep green summer hue.
But as the days shorten and the air cools, trees begin to slow down they stop producing chlorophyll and start to break it down instead — storing it away for the winter.
This is because it takes a lot of energy to make chlorophyll, so this process helps the tree save strength for the months ahead.
As the green fades, the other pigments begin to shine through. Carotenes bring soft yellows and warm oranges. Anthocyanins give rise to deeper reds and pinks, while also protecting leaves from sun damage and hungry insects.
Why are some years more colourful than others?
The depth of autumn colour depends on the right mix of weather and chemistry.
Cold nights help chlorophyll break down, revealing yellow tones. If temperatures stay just above freezing, anthocyanins thrive — turning leaves a brilliant red.
Dry weather concentrates sugars in the leaves, deepening red hues even more.
Bright sunny days allow the last traces of chlorophyll to keep working a little longer, increasing sugar levels and enhancing the final display.
And when all these conditions line up — cold nights, dry air, and bright days — the result can be spectacular.
Why do trees let go of their leaves in Winter?
Winter brings short days, cold air, more wind and little sunlight. For the tree keeping leaves alive would take too much energy. So instead, trees stop growing and enter a kind of rest.They seal off each leaf stem, preserving their moisture and strength.
Also bare branches bend and flex more easily in winter storms, letting the wind pass through.
What might look like loss is, in fact, resilience and preservation — an act of preparation for dark cold stormy Winter season ahead.
