Fall is a season of mixed emotions. Fall brings joy in seeing the beautiful, bright flowers of autumn including the yellow goldenrods and colourful asters. It brings a tinge of melancholy as we get the final glance at our warblers and other birds as they head south for the coming winter. The rich reds, yellows and oranges in the leaves can’t help to bring a smile to your face and a sense of awe in your heart.

Fall leaf colour has always fascinated me. As a kid I always wondered why leaves changed colour to red and orange in the fall. The thing that I never really questioned was why they were green in the first place. I learned in elementary school that leaves were green because they had chlorophyll in them. I accepted this at face value and didn’t question it. What I really should have asked is why chlorophyll is green. It doesn’t make a lot of sense when you first think about it. Chlorophyll absorbs light from the sun to make food. The colour that we see an object as, is the colour that is reflected and not absorbed by the object. Therefore, chlorophyll doesn’t absorb green light for use in photosynthesis.

This doesn’t really make sense considering green light makes up a decent chunk in the middle of the visible spectrum. Why wouldn’t plants want to use this band of light? One day while I was doing some research on trees I came across a fascinating sentence – “Trees are green because long ago the seas were purple.” Thankfully the book went on to explain this statement. It turns out that the ancients seas were filled with a type of photosynthetic bacteria call Halobacterium halobium (which can still be found in extremely salty environments).

Because H. halobium was dominant in the seas, the water looked purple. To photosynthesize, this bacterium used light in the green spectrum. As environmental conditions changed, the population of H. halobium decreased and allowed for the evolution of new microbes that used chlorophyll for photosynthesis. Since these microbes evolved deeper in the water below the H. halobium the green light was absorbed before it reached these new microbes. This left only the light on both the blue and red ends of the visual spectrum to use. These microbes eventually took over and evolved into the plants that we see today that absorb blue and red light and reflecting the green light.

So that is where the green colour comes from. What about the reds, yellows and oranges? In fact, the yellows and oranges are there all the time. Their colour is produce by other pigments in the leaf called carotenoids (the same pigments that give carrots and squashes their colour). When the chlorophyll starts to break down in the fall the yellows and oranges are revealed. The carotenoids help to expand the colours of light that can be used for photosynthesis (the smaller peak in the absorption spectrum image).

The red/purple colour is different, they are from compounds called anthrocyanins. These compounds are made in the leaf in the fall to act as a kind of sunscreen to block harmful UV light (trees can get sunburned too). During the summer, the leaves have other compounds that perform this job, but they break down in the fall along with the chlorophyll. The fascinating thing about anthrocyanins is that they are different colours depending on the pH of the leaf. In more acidic leaves like maples the anthrocyanins appear red, in more basic leaves like ash they become more purple in colour.

As a final note, it is time of year for me to say goodbye for another year. My contract finishes at the end of this week. I’ve had yet another amazing summer thanks in the most part to a great summer staff and all the wonderful visitors who came to the Visitor Centre and programs this summer. I hope the rest of the fall and the winter treat you all well and I’ll hopefully see you next summer. If you have any questions about autumn leaf colour or anything else nature related please email them to me at kipling.campbell@ontario.ca. If you have questions after Friday, September 30th send them to laura.penner@ontario.ca.

Colourfully Yours,
Kipling