Lyn Baldwin is an associate professor in the Department of Biological Sciences at Thompson Rivers University in Kamloops. But she is more than a botanist; she is an award-winning teacher who focuses her scholarly work on reframing the natural environment in a way that inspires her students to have a deeper understanding of the natural world, often through drawing. In fact, she has been known to present at Tourism conferences and offers a niche workshop on creating journal entries that provide a visual memory of a trip or experience. 

She is also an accomplished non-fiction essayist and artist. (Her blog can be found here.) So when I wanted to know more about why the leaves in the Okanagan create such a breathtaking autumn landscape, I went to the expert. 

The three Cs of leaf function

Lyn explained that leaves are green because of chlorophyll, which gives plants their green colour and absorbs light energy. The chloroplast work to convert the light energy into sugars that can be used by cells - chlorophyll and the chloroplast function together to allow for photosynthesis to occur and absorbs sunlight. The carotenoids are responsible for the yellow and orange pigments in the plant, which become apparent when the leaves change colour.

As temperatures change and decline, the leaves  of deciduous plants begin to die. These leaves have huge amounts of nutrients, so there is an exchange occurring where the nutrients from the leaves are being sucked back into the tree. As these nutrients are being returned to the tree, its unmasking the carotenoids, showing off the colours. 

Colour across the spectrum

While carotenoids also help show the red pigments in trees, specific types of tree leaves will turn red, but not all. Sugar maples are common in the Thompson Okanagan and turn red, and that’s because they have anthocyanins, which are the blue, red and purple pigment found in 70 per cent of plants. There are two popular hypotheses about why leaves turn red: Red as an umbrella, and red as an alarm. 

The theory that leaves turn red as an umbrella says that as the nutrients are being reabsorbed from the leaves into the tree, chlorophyll is being dismantled, and the red pigment creates a more efficient reabsorption. The theory that leaves turn red as an alarm is because in the fall many trees are attacked by insects laying their eggs; therefore, the red pigment of the leaves warns the insect that this tree is well protected and that it shouldn’t bother laying its eggs. It’s the ecological equivalent of a skull and crossbones.  You can read the scientific literature here. 

Pretty as a picture

One of the most beautiful parts of autumn is seeing the leaves change colour. There are specific areas in the world that are well-known for fall foliage, including the eastern United States and Canada. But the Thompson Okanagan is made up mostly coniferous trees, like pine, spruce, fir and cedar. Most coniferous plants don’t change colour during the fall. (you can tell the difference between coniferous and deciduous--trees that lose their leaves after they change colour--because they have needles instead of leaves and use cones instead of flowers to spread their seeds) And yet, some of the most spectacular fall scenery can be seen in the Okanagan. We have maples that change from green to yellow to orange, and we have Golden larches, which are deciduous conifers--a very special type of tree. 

So now you know why a tree changes colour in the fall - it is preparing itself for winter, where it lies dormant and waits for spring! And, you’ll be pleased to know that the Nature Conservancy of Canada now says to leave the leaves on the ground!