During the active growing season the leaves of deciduous trees are various shades of green. This is due to the production of a pigment called chlorophyll. It’s function is to capture solar rays and to use the energy to make food from water and carbon dioxide (photosynthesis). It is turned into simple sugars and carbohydrates and then the by products are released as oxygen (lucky us).
As decreasing daylight and temperature signal the onset of winter the tree begins the process of slowing down production of chlorophyll. As the level is reduced, the other chemicals become noticeable even though they are present year round. These main pigments are called cartenoids and anthocyanins.
Carotenoids are present in leaves the whole year round. They produce the yellow to orange colours. (like carrots and daffodils). Anthocyanins are responsible for the colours red and purple. They are responsible for the red coloration of berries and fruit. As chlorophyll levels decreases they become more noticeable. You will also notice in the Spring when some leaves are unfolding – they can be a different color than they are later in the growing season. Anthocycanins are also affected by soil pH. Leaves with both will be more orange in the fall. Tannins make the leaves of oak trees look brown in the fall. If we experience a slow cooling down with few frost days, the colours last longer and will be more spectacular. Well irrigated trees also produce a much better fall display.
The other thing that is happening as the days get shorter and temperatures drop is the tree creates an absission zone – a corky barrier between the tree and the leaves. It stops the flow of water and nutrients. Once this is done, the leaf will ‘fall’ off.
Take time to drive around the City/Town. Notice all the amazing colours. You will notice some turn and drop leaves earlier in the season than others. If you want shade as long as possible you wound avoid Green Ash since they leaf out later and drop their leaves sooner than many other deciduous trees. If you spot one you like, take a photo and jot down the address. This will be your resource for tree shopping next season. Enjoy it while it lasts.