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October Nature Notes

The colours of autumn

autumn leaves
You might think that leaf fall and the autumn colours are a consequence of the colder, rougher weather of this time of the year, and that leaves drop because of damage to the plants. Actually the plant itself causes the action in preparation for winter. Leaves are for forming food, but low winter temperatures slow the process of photosynthesis down so drastically that there is little gain in the winter months. Leaves become liabilities since they allow water loss (which can only be replaced with difficulty from frozen ground), and leaf damage can be a point of entry for disease organisms. In consequence cell growth at the base of the leaf forms a layer that gradually cuts off the leaf from the plant. In this zone new cells are weakly connected, but on the plant side are already waterproofed, like bark, so no opportunity for infection arises when finally the veins break and the leaf drops free. Before it goes, useful minerals are extracted, and even waste substances dumped into the leaf, and our beautiful autumn colour emerges as the plant deconstructs its chloroplasts. The yellow colour (due to xanthophylls) and the orange colour (due to carotenoids) are the pigments left behind in the chloroplasts, once the green has gone. The reds and purples come from new pigments (anthocyanins) formed in the cells as their chemistry alters (like the ripening process in fruit).

The wild Acers
Our native maple is the Field Maple (Acer campestre). Generally it has smaller leaves than the other two species that grow along the line. The leaf is simple with 5 main lobes which are smooth and rounded. The leaves of the Norway Maple (Acer plantanoides) are usually much larger and grander and give the impression of being much spikier because the margins back from the tips are concave. As its name suggests it isn’t native and was probably introduced into Britain in the 17th century.
Just how long the Sycamore (Acer pseudoplantanus) has lived with us is a matter of debate. It too was introduced, probably in the fifteenth century, although some wood has been found in the remains of Roman buildings. In this case the saw-edged leaf margin makes it easy to recognise.

norway maple sycamore leaf field maple

Leaves of the three different species, roughly in proportion to each other.

The autumn bird migration
We can sometimes make the false assumption that the birds we see in our gardens in the summer are the same birds we see in the winter. Many of our common garden birds also move south and west within Britain in the winter to find a generally milder climate. Many are replaced by birds from the continent. Recently one ringed blackbird which summered in Norfolk was found the next two winters in Devon, returning to Norfolk each year to breed. This demonstrates the importance of ringing in better understanding the behaviour of our birds.
All the same the last of our summer migrants have now departed to be replaced by birds from Scandinavia and central Europe. Our song thrushes and mistle thrushes are joined by two other members of the thrush family in the winter months, as redwings (Tardus iliacus) and fieldfares (Tardus pilaris) migrate south to Britain once frozen fields deny them worms and berry crops are exhausted, leaving them hungry. They fly during the night in large flocks, and can be heard calling to each other in flight. Both can be found in considerable numbers on the Nickey Line in some years, feeding on the many berry trees which border it.