Ever since I was a tiny little kid growing up in the Alps, I was told
that frost was causing leaves to change colors in the Fall and I
suspect still many of my contemporaries and even younger inhabitants of
my remote valley of origin have maintained that belief.
My big
sister, who is 9 year my elder, not only has lived through the German
occupation of France, but has seen and learned many things during her
long life. Yet, since she stayed in my hometown all of her life, she
still subscribes to the popular belief that that deciduous trees change
colors because the frost makes them.
The other day as we were
having our regular phone conversation, I challenged her belief by saying
that this change in color was because the photosynthesis. Naturally,
she disagreed vehemently and forced me to go into some basic research to
build up my argument.
I found out that both of our perspectives are
partially correct, but the primary reason for deciduous trees changing
color in the fall is actually due to a combination of factors related to
photosynthesis, which as you can guess, made me feel very good.
Still,
to be fair, my sister's observation about frost is partially accurate.
Frost can indeed contribute to the browning of leaves, as it can damage
the cells and cause them to die, however, it's not the primary reason
for the color change. My understanding about photosynthesis is closer to
the mark. As the days get shorter and temperatures drop in the fall,
trees begin to prepare for winter dormancy.
Gradually their
photosynthetic activity is reduced, which is the process by which they
use sunlight to produce food. As photosynthesis slows down, the
production of chlorophyll, the green pigment that gives leaves their
color, also decreases.
When chlorophyll production declines, other
pigments that were present in the leaves all along become more visible.
These pigments include carotenoids (which give leaves their yellow and
orange hues) and anthocyanins (which give leaves their red and purple
colors).
The combination of these pigments creates the stunning
fall foliage we see. So, while frost can play a role in the browning of
leaves, the primary reason for the color change is the reduction in
photosynthesis and the visibility of other pigments. Both my sister and I
have valid observations, but I’d (humbly) conclude that I won that
family debate!