I never really thought of leaves as being particularly dynamic – they grow of course, which I love to watch in the spring, and they look beautiful in their autumn colours which, once I have come to terms with the end of summer, I love to see. But between spring and autumn, I imagined they were in a sort of stasis as they generated the energy the tree needed to keep alive and grow. I thought that leaves were at the whim of the seasons, and their end was determined solely by the onset of the winter months. It turns out the tree is in far more control of its own destiny than I thought. From the moment the leaf begins to grow, its autumnal fate is sealed. Built into its structure is a self-destruct button, a time bomb gradually ticking to a beat controlled by the tree itself.
It’s an ingenious system. As the tiny new leaf begins to grow, a layer of cells (the abscission layer) develops at the base of the stalk holding the leaf to its twig. And here these cells stay, quiet and seemingly innocuous, held in a state of inactivity by hormones (auxins if we are being precise) produced by the growing leaf. Over the spring, the leaf develops, producing the green pigment chlorophyll in abundance, using the chlorophyll to capture the suns energy. This energy is used to convert raw materials into sugars which surge from the leaf into the tree. Along with them travel the leaf-produced auxins, keeping the cells in the abscission layer suppressed. But almost as soon as the leaf is fully grown, the balance is already tipping – the leaf is bringing about its own death.
Trees are able to sense a loss of light – even a reduction in day length of half an hour is perceptible – so very soon after the summer solstice in June, the tree is beginning to react. Slowly it cuts down on the production of chlorophyll – there is no point wasting energy on creating something that cannot be used. The leaf begins to age, to senesce. As days move into summer, the chlorophyll production slows, then stops, and as we move closer to autumn, the remaining chlorophyll begins to break down into colourless components. As much of the useful parts of the leaf as possible is taken back down into the tree to be stored in the roots over the winter, but for me, there is a different treasure too. As the green pigment disappears, the yellow of the carotene pigments, hidden until now, are revealed – the amber hues of the leaves turn the trees to gold.
The shortening day length is not only reducing chlorophyll production; auxin production by the leaf also reduces. The control keeping the abscission layer inactive begins to diminish. The cells start to elongate, become corky. They choke off the vessels which bring water to the leaf, but also those that remove sugar. As the sugars build up in the leaf, they convert to other pigments (anthocyanins), so creating the reds and browns we see in the autumn leaves. The cells in the abscission layer don’t stop there – they start to produce enzymes which break down their cell walls. Eventually, the cells begin to fracture and soon, a gust of wind or the weight of the leaf itself breaks apart the layer as the leaf irrevocably detaches from its twig. It leaves barely a trace, no open wound, and in this way, moisture is preserved within the tree for the winter.
I find the logic of science so satisfactory. This process of leaf loss explains why a cold, but bright and dry autumn is best for autumn colour. The cold speeds up the break down of cholorophyll, so exposing the yellow colours, and the sunny days encourage any chlorophyll remaining trapped in the leaf to create sugars, so leading to the creation of anthocyanins and the red leaf colours. Dry weather means any sugars in the leaf are more concentrated hence are more likely to convert to anthocyanins making the reds and browns even more pronounced. I think most people would go for the bright, dry autumns regardless, but here is another reason to hope for good autumn weather.
So, back to the leaf’s built in self-destruct button. Clearly leaves are created by the tree as a temporary structure. Why would the tree do this? Why invest so much into a structure only to dispose of it? How is this a good evolutionary strategy? There are a number of reasons.
A very big part of the explanation is self-protection. A leaf is perfectly adapted for capturing energy from the sun, but it is thin and full of water. When temperatures drop, a leaf will freeze, the leaf’s cells will rupture and the tree will lose everything in the leaf. But if the tree controls when the leaf falls, many of the useful nutrients can be reclaimed from the leaf before it is lost. There is a second aspect to self-protection too: leaves have a large surface area. If they remained on the tree in the winter when the boughs and trunk were cold and brittle, they would provide a huge resistance to the higher winds of winter storms, causing branches to break off or trunks to snap. And any snow fall caught by the leaves could cause trees to break under the weight. Actively losing leaves in autumn reduces the likelihood of storm damage.
There are other benefits to the autumn leaf loss. Leaves, that by the end of the summer are insect eaten, diseased or decayed, are cleared away and the tree has a fresh start the following spring. The tree can save its energy for the spring by remaining dormant through the winter; reducing the tree to its toughest parts increases its chance of survival. And even those parts of the leaf that do fall to the ground, that the tree is unable to reabsorb when the leaf is still attached, are not really wasted. They decompose and the nutrients are recycled, ready to be taken up by the roots next year.
So the beauty of the autumn woodland is a mere by-product of a clever and highly coordinated plan, followed in order that a tree can survive the winter. Yet to stand in a field, with cascades of golden leaves whirling down all around, or to stand on a hilltop surveying the myriad shades of russet and honey is memorable way beyond the ordinary. The science is enlightening and fascinating, but sometimes, just to sit and see the beauty around is uplifting too. A balance in all things…