The red leaves of autumn

My favourite tree – click on the picture to read my blog post on Liquidambar styraciflua – red queen of the fall

In Britain autumn leaves are usually considered to be more golden than red, but there are exceptions and I have quite a few of them in my own garden.

But why in temperate latitudes do autumn leaves change colour and fall at all?

The green leaves are the plant’s food factories, taking water from the soil, carbon dioxide from the air and energy from sunlight and turning them into glucose sugar and oxygen with the help of a green chemical called chlorophyll. This is called photosynthesis.

When the days get shorter in autumn it’s a signal to the plant that there is not going to be enough sunlight – and in some regions not enough water, either – to carry on producing glucose. The leaves would also be a net drain on resources as they lose water and carbon dioxide. So the little factories are abandoned (and eventually cast off). The green chlorophyll is withdrawn and we see the colours of other substances that were maybe there all along.

Hedge of beech (Fagus sylvaticus) on the turn, the chlorophyll withdrawing...

Euonymus alatus in the garden starts to turn red in October 2010

Euonymus alatus at the end of October - no wonder it's called the fire bush...

A brown colour, such as sometimes seen in oak leaves, may come from tannins. Tannins are sometimes considered waste products but apparently they are useful to the tree as they give a bitter taste to leaves and stop animals eating them.

Some of the brown in autumn leaves of oak (Quercus robur) comes from tannins - these were pictured on a recent Taff Trail walk

As well as green chlorophyll, some leaves contain pigments called carotenoids, which give a yellow colour, and anthocyanins, which give a red or purple colour.

Autumn leaves of yellow-flowering deciduous Rhododendron luteum (formerly called Azalea ponticum)

Some plants, such as Cotinus coggygria and Acer japonica Dissectum Atropurureum, have more anthocyanins than chlorophyll all year round, so have permanently dark red or purple leaves.

An interesting shape as well as colour – Cotinus coggygria in the garden in October 2010

All these pigments tend to remain in autumn when the chlorophyll fades. We see anthocyanins particularly in maples (Acer), sweetgums (Liquidambar), Sassafras, sumach (Rhus) and scarlet oak (Quercus coccinea).

Small, flat-growing red Acer japonica Dissectum Atropurureum

A different red in the leaves of Acer japonica dissectum atropurpurea at the end of October...

The related Acer japonica dissectum does not have as much anthocyanin as the atropurpurea type...

A combination of sunlight and the chilly nights of autumn causes the pigment to turn red. The brightness of the colour is also thanks to these trees’ acidic sap. Anthocyanins are purplish to blue in more alkaline cell conditions.

Hence the wonderful colours of New England and Canada.

In the UK my favourite place for autumn colour is Westonbirt Arboretum in Gloucestershire. But look around and you will see autumn’s glory all around, even in the city…

Attractive Quercus coccinea in front of Cardiff Central railway station in October 2010

Leaves of Quercus coccinea in front of Cardiff Central railway station in October 2010

And if you haven’t been there yet, visit my blog post on Liquidambar styraciflua – red queen of the fall

More Liquidambar (sweetgum) leaves at the end of October 2010

Red sweetgum and blue skies at the end of October 2010