Autumn colour

Earlier this year I had a mind to try and capture all the various shades of green in the park, while all the colours of the leaves were clean and fresh as a breath of spring air. Well, it didn’t happen and now it’s way too late as summer has come and gone and we had a bit of a drought. Many of the leaves were baked to a crisp and some trees dropped their leaves as thought it was autumn, in order to conserve water. It’s well into autumn now and the leaves have changed colour and are dropping faster than the park staff can sweep them up. Some trees seem to have dropped their leaves a little bit earlier – perhaps they’ve had enough after that hot spell – or maybe that’s my imagination.

There is a lot of colour in the park at the moment; the various shades of green have given way to yellows, oranges and reds. There’s still plenty of green left as some trees seem reluctant to change and may leave it to the last moment.

The reason for leaves changing colour is usually given a simple explanation. Leaves contain chlorophyll. Chlorophyll absorbs light mainly in the red and blue region of the spectrum. It absorbs this light energy for the process of photosynthesis. This leaves green light to be reflected from the leaves, so we see the leaves as green. There are, however, other coloured substances within the leaf such as carotenoids, flavonoids, xanthophylls and sometimes anthocyanins. These other colours are largely masked by the dominant chlorophyll. That is until autumn when the chlorophyll decomposes and useful nutrients from it are reabsorbed. The green colour of the leaf fades and the colour of the other substances is revealed. If that happens to be a lot of beta-carotene then the leaves will become bright orange, like a carrot. If the compound is predominantly a xanthophyll called lutein then the leaves will appear yellow. Some leaves also begin to make anthocyanin at this time. If they never had any anthocyanin before then this will start to make the leaves look bright red. This process depends on the presence of sugar in the leaves and is what makes sugar maples turn bright red.

Back to those shades of green. Most explanations of the changes to leaf colours start with a green leaf. No particular green. They then tell you that this is due to chlorophyll. But they don’t always tell you there’s more than one type of chlorophyll. Chlorophyll A makes leaves a blue-green colour. Chlorophyll B makes them yellow-green. Chlorophyll A is the primary agent in photosynthesis and makes up about 75% of a plant’s chlorophyll. Chlorophyll B makes up about 25%. Add the different colours into the mix, possibly in varying amounts in leaves of varying thicknesses and its possible to see how trees are going to end up with leaves of varying shades of green. It is also true to say that anthocyanin is not solely made during Autumn. It is present in some leaves all year round – copper beech has anthocyanin in it to make its leaves that purple colour. The green of the chlorophyll is masked this time and the two together make the leaf look that brownish purple rather than the bright red of Autumnal colours. The purple Field Maple, Acer Platinoides ‘Crimson King’, will also probably have anthocyanins in its leaves.

There are some trees that are well known for turning bright red in the autumn, when they start to produce anthocyanin. Trees such as the Sweet gum (Liquidambar styraciflua), the Pin oaks (Quercus palustris) and the Persian ironwood (Parrotia persica). There are two Sweet gum trees down by the basketball court in full colour at the moment. There is a Persian ironwood opposite the bowling green changing colour but it seems to be dropping its leaves no sooner than they have changed to red. There are several Pin oaks in the area between the two car parks and the tops of the trees are just starting to change to red. In that same area closer to the Avenue Road car park there is a large Red oak (Quercus rubra) which has completely changed to a reddish orange colour.

Several Ash trees have turned completely yellow, which is something I have never noticed before. Strangely other Ash trees are still green, sometimes right next to a yellow one. The Manna ash is also turning yellow – this is next to the small exit from the main car park. In that same triangular area there is a Honey locust tree (Gleditsia triacanthos). It started out yellow in Spring then turned green and now it is back to yellow. The Maidenhair tree (Ginkgo biloba) has also turned yellow – this is the one opposite the nursery entrance as the little one by the car park has now lost all its leaves.

Another splendid large, orange tree is the Norway maple (Acer platanoides) on the path below the bowling green.

The wind and rain and time is making inroads into the display of leaves and now is the time to get out and see these colourful sights before they are gone.

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