Why Do Leaves Change Color in the Fall?

The brilliant reds, oranges, and yellows we see on trees in the fall are actually part of trees' defense mechanisms against the oncoming winter. Learn more in this episode of BrainStuff, based on this article: https://science.howstuffworks.com/life/botany/leaves-turn-red.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to brainstuft, a production of iHeart Radio, Hey, brain Stuff,
Flour and vog obam here enthusiastic snowball fights in winter
or blazy summer days at the beach. Notwithstanding, autumn might
be the most beloved season of the mall. Every year,
people in temperate climates watch in awe as the trees

(00:22):
begin their magical transformation from green powerhouses to veritable Morgese
boards of brilliant yellows, fiery oranges, and vibrant reds. It's
even a tourist attraction. The northeastern United States, for example,
hosts millions of leaf peepers from all over the world
who come to observe nature's fine art gallery. Myths about

(00:43):
why this occurs stretch back through time, from the hot
and O'shannee to the Algonquin to the Wye Dot. A
few Native American people's legends feature a wayward bear, sometimes
represented by ursa Major or the Big Dipper, which returns
to the sky every autumn. The legends say that the
bear was injured in a hunt or a fight, and
that's blood tends the leaves red each year. Much more recently,

(01:07):
researchers have sussed out the science behind these transformations, involving
photosynthesis and pigments. For a large part of the year,
leaves are a tree's workhorses, constantly converting carbon dioxide, water,
and sunlight into energy in the process called photosynthesis. The
key to this process is the pigment chlorophyll, which is
what gives leaves their green color for much of the year.

(01:30):
But while chlorophyll is the star of the show, it
has some help in the form of other pigments like
caroteene and xanthophill. Xanto is Greek for yellow, and Caroteene
is named for carrots because it's what gives them and
other things their orange color. These two pigments are always
present in leaves and help absorb sunlight, which they transfer

(01:51):
to chlorophyll for photosynthesis to occur. As summer nears its
end and days get shorter, the decreased amount of sunlight
incites trees to prepare for a sort of hibernation. The
leaves won't be able to continue photosynthesizing during winter due
to the dry air and lack of light, so the
tree does two things. First, it forms a separation layer

(02:14):
made of cork like cells at the base of each leaf,
to seal it off from the tree the second it
stops producing chlorophyll, since it won't need this pigment until
the days start to Lengthen once again in the spring,
with chlorophyll out of the picture, the yellow and orange
pigments get a chance to shine. The red hues which
come from pigments called anthocyanins, are slightly more complicated. Whereas

(02:39):
all trees contain chlorophyll, carotene, and xanthophill, and not all
of them produce anthocyanins, even the ones that do have
anti synons only produce it under certain conditions. Remember that
layer of cork like cells at the base of the leaf.
Its purpose is to protect the tree during the colder winter,
conserving energy and preventing the tree from drying out. And

(03:00):
when the separation layer is complete, the leaves fall off
of the tree. But before the leaves fall off and
the tree closes up shop, it wants to pull in
as much sugar and nutrients as possible from its leaves,
which is where the antisyonin comes into play. Although scientists
offer several different possible reasons for why some trees produce anthocyanins,

(03:21):
the prevailing theory is that these pigments protect the leaves
from excess sunlight and enable the trees to recover any
last remaining nutrients. The reason you'll see more vibrant reds
during some years is that stressors like an overabundance of
sunlight and dry weather increase the sugar concentration in tree sap,
triggering the tree to release more anthocyanins in a lastic

(03:42):
effort to gather up enough energy to get through the winter.
In addition, near freezing weather, low nutrient levels, and other
plant stressors seem to trigger increased levels of antisynins. So
if it's been especially rainy and overcast, you won't see
much red foliage. Without bright sunlight, the trees don't need
the added protection that the red pigments provide, so they

(04:04):
don't bother producing them. If autumn just isn't the same
for you without that splash of red, hopefully the weather
will collaborate. Today's episode is based on the article why
do Leaves change color in the Fall? On how stuff
works dot Com written by Jennifer Horton. Brain Stuff is
production of I Heart Radio and partnership with how stuff

(04:26):
works dot Com, and it's produced by Tyler Klang. For
more podcasts from My heart Radio, visit the heart Radio app,
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