Why do leaves change colors in the fall? Why don’t they turn blue? Will the changing of the leaves get later each year due to climate change? The mysteries of autumn are solved here.
It’s all about
the pigmentsFirst of all, why do leaves change colors when fall arrives? They do it because chlorophyll, the pigment that gives leaves their green color, breaks down and reveals other colors that were already inside the leaf all along.
Chlorophyll is the prime player in photosynthesis, the process of changing sunlight energy into sugars that nourish the tree. The powerful chlorophyll reflects green, which hides other colors.
“Each leaf just has buckets of green,” said John Seiler, a Virginia Tech tree physiology specialist and a nationally renowned tree expert.
When longer nights and cooler temperatures of autumn arrive, the leaves stop making food, which causes the chlorophyll to break down and be absorbed.
“That reveals the oranges and yellows” and other colors, Seiler said. Cool autumn nights and sunny days can spark more reds, he added.
“Sunny, bright days really trigger red pigment to form,” Seiler said. “In fact, if you ever see a leaf that was tucked under another leaf, it won’t develop any red” because it wasn’t exposed to the sunshine.
Not feeling blueIf the loss of green in leaves allows the reds, yellows and reds to emerge, why don’t we see blue leaves, too?
“The simple answer is that we don’t know why,” Seiler said.
Blue pigment is rare in nature. Some studies have found that less than 10% of plants have blue flowers.
Plants that do appear blue might actually be showing a mix of pigments and natural light that replicates blue, according to a study done by Florida International University.
So, it’s red, white and blue for Fourth of July, and red, yellow and orange for fall.
Leaves leaving later?
With annual average temperatures warming in recent decades, could it be possible that the turning of the leaves will happen much later in future falls?
Seiler doesn’t believe so. That’s because the shorter days of autumn have the biggest impact on when leaves change color.
“The whole climate thing is not going to move it much because it’s tied to day length,” Seiler said.
Some years, though, leaves do change later than usual due to warm temperatures, especially years when nighttime low temperatures are above average. Warmer temperatures can delay the breakdown of chlorophyll.
Eventually, the shorter days and longer nights will trigger the color change in the leaves. Some years that might be mid-October, other years a few weeks later.
“But we’re not looking at leaves changing in December,” Seiler said.
Drought of colorful leaves?
This summer was fairly dry throughout Southwest Virginia, but Seiler thinks that the region received enough sporadic rain, which included passing tropical systems, to ensure a vibrant fall.
“Overall, we got enough precipitation scattered around,” Seiler said. “If we keep the moisture levels up, and have some sunny bright days and cool nights, we should be good.”
Of course, one freezing night or one tropical system with high winds could erase fall’s colors.
“The leaves could look great,” Seiler said, “then everything gets blasted off [by wind] and everyone will say it was a bad year.”
Peak peeping potential
Even though autumn begins this Wednesday, the leaves in our region won’t reach peak autumn color for another two to three weeks.
The interactive Fall Foliage Prediction Map (smokymountains.com/fall-foliage-map) predicts that leaves in the Roanoke and New River valleys will be at their colorful peak sometime between Oct. 11 and 18. Counties farther west could be changing a week earlier than that, according to the map’s tool that allows users to check specific dates.
The Virginia Tourism Corporation’s website includes a fall foliage report (bit.ly/2XjElKE) that should be updated soon.
Enjoy the leaves, peeps.