Why Don't Evergreen Trees Lose Their Needles?

Evergreens' needles are their leaves, so why don't they change color and drop in the fall? Learn how evergreen leaves work in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff production of iHeart Radio, Hey brain
Stuff Lauren vogebam here. Evergreen trees seem to thrive during
the colder months, steadfastly holding onto their brightly colored, needle
like leaves as temperatures plummet, bringing welcome color and contrast
to snowy landscapes. On the other hand, the wide and

(00:22):
flat leaves of their deciduous counterparts transformed into brilliant shades
of red, orange and gold during the chili and dry
days of fall and early winter, only to eventually shed
Their branches will remain barren until spring. So why are evergreens,
which include many, but not all, conniferous trees such as pines, furs, junipers,
and spruces immune to seasonal leaf changes in shedding, while

(00:45):
species of deciduous trees like aspens, birch, cherry, oak, and
maple aren't. It mostly comes down to a little thing
you might have learned about in biology class called photosynthesis.
Both deciduous and evergreen trees use energy from the sun
and water from the ground to turn carbon dioxide in
the air into food. The difference is that shorter days

(01:07):
brings less sun during the fall and winter, and that
translates into less energy, so deciduous trees bearing broad flat
leaves go dormant or into a sort of hibernation mode,
and shed their leaves to help them conserve water and
stay alive until spring arrives and new leaves grow from
new buds. Evergreens, meanwhile, boast needles that are actually tightly

(01:27):
rolled leaves, and the low surface area of this shape
helps the needles conserve water throughout the fallen winter. A
waxy coating on the needles also helps keep water from
evaporating compared with broad, delicate leaves. These dense, waxy needles
resist cold and stay moist, which keeps them green throughout
the winter. But evergreens actually do shed their needles, just

(01:49):
not all at once. As needles age, they turn yellow
brown or a reddish tan and will fall from the tree,
but usually only a handful at a time. The trees
are constantly growing new needles, so they can stay green
year round. Evergreens greater leaf longevity means they can survive
in environments that don't work for their deciduous cousins. In fact,
evergreens can be found on every continent except Antarctica. The

(02:12):
bottom line is that Although those colorful deciduous trees attract
their share of seasonal leaf peepers, trees that stay green
year round add character to people's yards during the fall
and winter, offer shelter to birds and small animals when
the winter is cold, and many species are dense enough
to serve as wind breaks and privacy screens. Today's episode

(02:35):
was written by Windy Bowman and produced by Tyler Clang.
The brain Stuff is production of iHeart Radious How Stuff Works.
For more in this and lots of other evergreen topics,
visit our home planet, how stuff Works dot com. And
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