Trunkline Magazine (Louisville Zoo) December 2018 | Page 11

GARDEN TALK
TREES AND
WINTER DORMANCY
By Matt Lahm, Assistant Curator of Education
Cold weather is here with all its
barren trees and evergreens. Have
you ever wondered why some
trees and shrubs lose their leaves
in winter and others don’t? You’re
actually observing an adaptation
for winter survival! Those trees and
shrubs that do lose leaves are called
deciduous, meaning their leaves fall
off or shed seasonally or at a certain
stage of development in the life cy-
cle. Trees like pine trees and spruces
are called conifers or “evergreens”
because while they lose needles oc-
casionally, typically they don’t go all
at once. Did you know that “de-
ciduous” isn’t exclusively a term for
plants? Some animals experience a
deciduous adaptation including the
male white-tailed deer: their antlers
drop off every year around late
January and then regrow starting in
the spring!
Beginning in the fall, the criti-
cal elements plants need start to
change: the daylight is shorter, the
weather is colder and drier and
water begins to freeze. Plants can
sense these environmental changes!
These are also the signals for the
deciduous trees and shrubs to stop
producing food/sugars via photo-
synthesis, stop growing and begin
the process of going into dormancy
for the winter. Like those animals
that hibernate for the winter, dor-
mancy is a complex series of physi-
cal and physiological processes. This
means they are not dead but rather
they overwinter in a resting phase
with essential life processes continu-
ing at a minimal rate.
So, what’s going on inside of
these deciduous trees and shrubs?
One of the greatest misconceptions
is that the shorter days are a signal
to plants to enter dormancy. It’s ac-
tually the increase in the length of
the nighttime that’s critical, even in
the spring and summer! This "pho-
toperiodism" is instrumental in the
dormancy process. A longer period
of nighttime triggers the production
of a chemical in the tree’s cells for
the tree to enter a state of dorman-
cy, while shorter periods of night-
time produce chemicals that signal
the tree to grow.
When it comes to dormancy, tim-
ing is everything. If a plant enters
dormancy too soon, it loses the
opportunity to photosynthesize,
grow and store more energy before
settling in for the season.
However, if it waits too
long, it risks frost damage
and desiccation.
Once the process of
dormancy begins, the leaves
on these trees begin to turn
color and drop off, hence the
term “fall.” This festival of color
brings autumn to life before our
evergreen trees take center stage
during the winter season. As
you celebrate the end of
the year, take a mo-
ment to appreciate
the wonder and
beauty of the trees
around you.
Louisville Zoo Trunkline • Winter 2018 • 11