Wisconsin’s autumn countryside is quite a sight to behold with its oranges, yellows, and reds, but why does this color-change phenomenon happen?
Broad-leafed trees need to shed their leaves for the winter so the trees don't lose moisture through them, and because otherwise, the leaves would freeze.
The color-change is just a part of that leaf-dropping process.
When the days grow shorter in the autumn, trees stop producing chlorophyll.
Leaves are green because of chlorophyll, but there are always other pigments present in the leaves, too.
Once the chlorophyll is gone, orange and yellow pigments will show through.
"We also have trees like red maple and sumac that turn that real brilliant crimson red--that's actually a new pigment that's produced in the fall,” says Dr. Paul Whitaker, Associate Professor of biological sciences at the University of Wisconsin-Marathon County.
In both cases, the tree absorbs all the nutrients it can from the leaf, forms a scab-like boundary between itself and the stem of the leaf, and then the leaf falls off.
While the yearly lack of daylight is the main signal for trees to change color, other things, such as frosts, can help trigger the change as well.
Some species of tree, like the red maple and the sumac, naturally turn color earlier than others.
If a tree is under some type of stress, that stress can also cause an early color-change.
The stress could be from any of several factors including construction, disease, insects, root damage, soil compaction, or drought.
"I think the rain that we got in August probably kept our trees greener longer than they would have been if we hadn't gotten that rain. I think it came just at the point where the trees were making that decision to start dropping their leaves," says Dr. Whitaker.
The brilliance of red in leaves is also influenced by the weather.
“The leaf that’s starting to change colors is still making sugar during the daytime by photosynthesis and storing them in the leaf and then typically those would leave at night,” says Dr. Whitaker.
However, when nights get chilly, the sugar sap can’t move from the leaf to the tree very well, so a chemical is produced to help the tree get those nutrients before the leaf drops.
That chemical gives the leaves the brilliant red color.
So, sunny days (in which lots of photosynthesis can happen) followed by chilly nights will cause some trees to turn a very bright shade of red.