By the time you read this, we will have passed the Autumnal Equinox that arrived on Tuesday, September 22nd. Did you know that Equinox comes from the latin meaning equal night? During the equinox, the sun crosses the celestial equinox creating days and nights of equal length followed by days of shorter daylight. The sun rises and sets due east and west, respectively. The sun will set before 7 pm from now until the middle of February.
As the days become shorter and the temperatures become cooler, a chemical change takes place within Wisconsin’s deciduous (which means falling off at maturity) tree leaves. In the summer, these leaves use the process of photosynthesis – when chlorophyll (green pigment in leaves) absorbs sunlight’s energy to transform water and carbon dioxide into starches and sugars – to fuel the tree’s growth and release oxygen.
As the hours of sunlight decline and cooler weather comes, trees respond to these changes by making less and less chlorophyll until they stop making it. This allows the carotenoids (yellow and orange pigments) existing within the leaves to show through.
Researchers have found some tree species have leaves that produce red anthocyanins pigments at this time of year. These pigments create leaves of bright oranges and reds, and deep purple colors. The brilliance of anthocyanins each year is weather dependent. During warm, sunny autumn days, trees still produce sugars but the cooler nights prevent these sugars from moving into the branches and trunk. Anthocyanins allow these trees to recover nutrients found in the leaves before the leaves fall off.
In the fall, you can identify trees by the colors the leaves change to.
Dogwoods and sumacs – reds/purples
Birches – bright yellows
Aspen/poplars – golden yellows
Sugar maples – orange-reds
Red maples – bright scarlett
Hickories – golden bronze
Oaks – reds, russet browns
Larches/tamaracks – golden yellows
Also during this time, a special layer of cells is created at the point where the leaves attach to the branches. These special cells gradually cut the tissues that hold the leaves to the trees, while the trees seal the cut by creating leaf scars when the leaves fall off the trees. Trees lose their leaves because the leaves will not be able to survive the cold. Oak trees are an exception – they hang onto their leaves through the winter!
