I’ve been a less frequent photographer of my willow tree on the banks of the River Wear over the summer, despite enjoying the sight of it in full leaf from a boat a couple of times a week. Its appearance hasn’t changed very obviously since about May, though the colour of its leaves soon lost that early spring freshness.

That is all about to change. In fact, the willow in my garden at home already has some yellow leaves which just goes to confirm my suspicions about the height of Shincliffe Bank which I cycle up on my way home from work every day!

Leaves on the tree I’ve been observing this year on the Wear haven’t really started to yellow yet, though many of the leaves have now got unsightly brown spots which show where they’ve been attacked by viral or fungal disease.  If you look carefully, you can just see the start of some yellow patches near the main leaf veins and those will soon grow.

Many of the trees surrounding my willow are already starting to turn, with the promise of glorious colours to come if we get some autumn sunshine.

The beeches and horse chestnuts are amongst the first to go, and the beech trees are already starting to look like they are on fire; each branch shades from bright orange at the tip, through shades of yellow, to green at the core of the tree. The leaves on the Virginia creeper covering my apple tree are now a vivid crimson, with just a hint of green along the leaf veins to show their previous colour.

Of course not every tree changes equally spectacularly. Many of our native trees go a beautiful yellow but don’t change to the reds and oranges we associated with fall colours in North America. This begs the question, why?

The yellow colour is easy to explain. Deciduous plants shed their leaves in autumn to avoid the unnecessary cost of maintaining a photosynthetic system which is expensive to run and inefficient at low winter temperatures and light levels – a process known as senescence. Before the leaves are shed, as much material as possible is recycled by the plant – broken down and reabsorbed into the perennial tissues of the trunk and branches. The light-absorbing pigment chlorophyll, responsible for the green colour of leaves, is one molecule recycled in this way. Chlorophyll, and its associated proteins, make up a significant proportion of each leaf’s assimilated carbon and nitrogen and the tree cannot afford to lose this. As the chlorophyll in a leaf is broken down, the green colour disappears, unmasking the yellow and orange colours of carotenoid accessory pigments (see More Weird and Wonderful Lichens) which were already present in the leaf to improve its photosynthetic efficiency.

The red colours sported by some trees are more difficult to explain as they require the new production of an additional type of pigment, anthocyanins, at a time when the leaf is basically dying. Why would a plant go to this trouble? For a long time it was believed that anthocyanins were simply a by-product of the process of senescence but their synthesis often precedes chlorophyll breakdown. Another theory is that the anthocyanins act as a deterrent to herbivores to stop damage to leaves whilst their nutrients are being reabsorbed. This doesn’t seem that convincing – the same pigments are responsible for the flower colours which help to attract animal pollinators and seed dispersers.

However a third suggestion, that anthocyanins reduce light capture by chloroplasts as chlorophyll is broken down, and thus reduce damage to leaf cells as they senesce, seems more plausible. After all, they have a similar role in protecting some young, expanding leaves

Young hazel leaf (Corylus avellana)

The protection offered by anthocyanins appears to enable leaves to reabsorb nitrogen more efficiently than they would otherwise be able to, during senescence.

Looking closely at my willow tree I was surprised to see that, even at this late stage in the growing season, it is still producing new young leaves. It seems a surprising indulgence, given that these leaves will senesce and drop in a week or two.

Less surprising were the new buds appearing where these leaves meet the stem.  These buds will protect next year’s delicate new leaves over the winter.  My year looking at the willow is close to coming full circle.