Where does it all start?
As with all life on our planet, everything starts with and exists because of our Sun.
It dictates our atmosphere, the temperature of the planet and the life that lives on it. For trees, things are no different and, cutting a very long story short, trees require sunlight to sustain themselves.
As the Earth rotates around the Sun, it does so on an angle of around 23 degrees. This tilt is fundamental to our seasons. Every year, the North Pole moves on a path where it tilts towards and then away from the Sun as the Earth moves around it. Consequently, more of the Sun's rays hit the Northern Hemisphere when the North pole tilts towards the Sun than when it's further away. More rays, greater, more consistent temperatures and we get Summer. But as it tilts away, the Northern Hemisphere receives less rays and so lower temperatures - aka Winter.
So now you know why were have seasons, let's take a closer look at trees.
From my secondary school biology lessons (and in no way as a result of Googling this 🤣) trees depend on the Sun's rays to generate Glucose - its energy source, discarding Oxygen as a 'waste product' into the atmosphere. Therefore, in order for it to grow it requires the Sun's rays to do so.
As you can see from the equation above, the tree relies upon photons from the Sun's rays ('light') to provide the mechanism for change, along with Chlorophyll.
Plants require warm temperatures and sunlight to produce chlorophyll and it's this that gives leaves their green colour.
What happens at the start of autumn?
Referring back to the diagram with the sun in it, as seen above, we see the tilt of the Earth starting to point away from the Sun as it progresses through Summer and into Autumn. And so, as we know, this is where temperatures also start to drop.
With colder temperatures, the amount of Chlorophyll produced begins to decrease and existing chlorophyll is slowly broken down, reducing the green colour of the leaves. But what about the yellow and red colour, where does that come from? Good question.
The yellow colour is caused by pigments that are always contained within each leaf but are masked by the green colour produced by chlorophyll.
As this subsides it reveals more yellow and this becomes more prevalent as the seasons change. These pigments come from chemicals called Carotenoids and Flavonoids. One such carotenoid compound is called Xanthophylls and it is this which is responsible for the yellow colour. As an aside, another type of xanthophylls, called Lutein is a compound that give sus the yellow colour in egg yolks!
Staying with a food theme for a moment. When leaves turn orange it's due to other carotenoids, specifically Beta-carotene. This absorbs green and blue light strongly whilst reflecting yellow and red light. Ultimately, this is what gives it its orange-like appearance and is the same compound found in carrots.
What's next?
Lastly, there's red.
As Autumn really kicks in, sugar concentration in the leaf increases due to the onset of Anthocyanin synthesis which produces Anthocyanin. Together with carotenoids, this gives the leaf its red appearance. Although the reason for this is still unclear, it's once thought that anthocyanin production might have protected the leaf (by deterring aphids) and delayed leaf fall. However, this is no longer considered to be the case.
The changing of leaf colours is called 'Tinting' and is a process which trees undergo for a few reasons. One is the chemical reason as described above, the second is to preserve itself for the impending winter. Trees store water in their trunks over the winter so they don't dry out and so by not photosynthesising it permits itself to do this rather than consuming it. If you study a tree over a number of seasons, you'll find that there's some years where the tree tints more than in others. You may also find that trees of a similar type tint more or less before dropping their leaves.
As trees consume sugar for energy which, as we've learnt, comes from the Sun, it cannot rely upon it having as much throughout the winter. So it undergoes this process to survive through the colder seasons. However, it also sheds its leaves so that it's more resistant to being blown over by winter storms that may occur.
What does this mean for photographers?
Another fine question.
Largely, this is all about aesthetics and predicting when we can get out for those beautiful Autumnal colours.
Essentially, the best scenario occurs when we get a long dry Summer followed by colder nights to allow the leaves to fully change from green to yellow, orange and finally red and having a shorter leaf dropping period
Or, more scientifically, a longer period of time for chlorophyll, carotenoids, and anthocyanins to remain in each leaf before breaking down.
Unfortunately, it's not been like that in the Lake District this year and some trees are already showing signs of dropping brown leaves instead. We've had a mild and very wet Autumn so far and so the leaf dropping period has been much slower meaning leaves are often completely brown before they fall. Sometimes, individual trees, and even single leaves, can change colour at different times, reducing the whole spectacle.
So keep an eye out for the weather around where you live, prey for warm days and colder nights (as I'm sure you do anyway) and keep your camera at the ready to capture those epic Autumnal shots we all aspire to get.
