There is a little more to it than just Jack Frost. The factors that influence fall colour can include: the growth habit of trees, light, colour, pigments in the leaves, leaf types and the influence of weather as summer ends and fall begins.
Fall colour is most noticeable in trees that shed their leaves each fall however evergreen trees have subtle colour change and can also shed some needles (needles more than two years old) in the fall.
I used to get calls every fall from concerned tree owners who were seeing fall needle browning and fall needle drop. This is generally not a concern. If the needles are falling off the tips of branches you have cause for concern however moderate needle loss from down the branch or inside the tree is normal.
On the prairies, warm summers with rainfall give way to a contrasting cool/cold season when trees refuse to grow. Fall colour occurs in the transition period between the warm weather and the cold weather. So you want me to tell you something you don't know right?
The changing colour that we see is a result in a change in the pigments within the leaf and what light waves are being absorbed verses the ones that are being reflected. Hmmmmmmmmmmm?
The four pigments that are known to cause colour include: chlorophylls (green), carotenoids (yellow), anthocyanins (reds) and tannins (browns). As summer moves to fall and growth conditions (day length and cooler temperatures) are less favourable to producing chlorophyll, which is the green colour of leaves, the green begins to fade and change to yellow. This is the carotenoid (the same pigment that colours carrots orange) starting be in larger concentrations than the chlorophyll. Good to know right? Bored? But wait, I haven't got to the anthocyanin part yet.
Short days and cool temperatures trigger the leaves into the ripening process that trigger colour change.
At the same time, the leaf attachment to the twig becomes sealed and brittle and wind and the weight of the leaf itself seals its final fate. It falls. You rake.
The anthocyanins (reds, pinks, purples) develop before the leaf totally falls off but at a time when the leaf attachment is becoming brittle and no longer allows tree sugars through but traps them in the leaf. Good soil fertility can deepen the red colour. Check out the amur maples on the way into North Town by the railroad overpass. Its one of our best red fall colour trees.
The tannins are just brown and not real exciting to talk about especially when compared to talking about the anthocyanins . I know, I know, keep calm.
Weather conditions can influence the intensity of colour. Warm bright fall days and cool nights (not freezing) seem to intensify and prolong fall colour. If we are plagued with grey rainy days at this time, colour intensity is reduced and the length of a colourful fall is shortened.
So there it is folks. Now you know. Maybe I'll tell you why a tomato is a fruit some day or better yet how come strawberry/rhubarb pie is not a fruit pie at all. Wait for it wait for it.
