On our travels over the long weekend, I noticed the leaves of some of the small trees and shrubs, especially Douglas maple, pin cherry and red-osier dogwood, were already starting to change colour.
Visitors to Canada from warmer countries are often fascinated that many of our trees drop their leaves and wonder why. The leaves of deciduous trees and shrubs are just not designed to withstand our harsh northern winter conditions. Unlike the conifer needles, deciduous leaves are very thin and not protected by a thick “skin” so the living cells will freeze and die easily.
Other folks are curious why some tree leaves turn bright colours and others do not. The process of getting ready to drop their leaves, or “senescence”, in deciduous trees and shrubs is preparation for going into dormancy and starts well before the leaves drop.
As soon as the day length starts to shorten, a row of cells called the abscission layer slowly starts to form at the leaf stem base gradually cutting off water to the leaf cells. At the same time, the amount of green chlorophyll diminishes so the leaves cannot support photosynthesis at a rate that will sustain life.
Leaf nutrients such as chlorophyll, proteins and other cell contents start to break down, and are recycled as they are moved to the bark (or stem) or storage organs underground for the winter. The following spring these nutrients are then used to feed new growth.
As the chlorophyll disappears, other pigments are revealed in the leaf cells. For example, aspen leaves in good conditions turn a brilliant yellow colour in the fall due to the presence of lots of carotenoid pigments.
The cells of other plants such as maples and cherries, contain anthocyanin pigments, which are produced during senescence. Anthocyanins combined with carotenoids, and even a little chlorophyll, can lead to orange colours. The presence of more anthocyanins results in red colouration in leaves, especially in colder weather.
Alders on the other hand tend to stay green longer and eventually go brown due to the accumulation of tannins (waste products from cell contents) in the leaves.
Eventually, the abscission layer closes over completely blocking the passage of water to the leaf followed shortly by leaf fall.
This year we are seeing evidence of drought senescence in the valley.
Some of the aspen and cottonwood tree leaves are turning a dull brown and have entered senescence earlier than normal. Early senescence can also result from other stresses such as poorly drained soils and the lack of oxygen around the roots or lack of nitrogen.
Both higher and lower temperatures than normal can bring on early senescence as can prior infestation with pests such as the aspen leaf miner.
Talking of leaf miners, it was such a joy to see green aspen leaves this year instead of the silvery grey that we have been seeing throughout the long leaf miner epidemic.
