> Fall Foliage: Why Do Leaves Change Color?


New England is known for its exquisite show of color during
the autumn season. “Leaf peepers” come from all over the globe just to witness
the phenomenon, and it’s no wonder as there are only five regions in the world
to experience the changing of the leaves. We are still about 3-4 weeks out from
forecasted peak foliage, but the extremities of trees are already beginning to
turn shades of red, orange and gold. But what causes this beautiful display?

During the growing season, leaves are the largest
contributors to a tree’s growth. It is in the leaf where chlorophyll absorbs
energy from the sun, transforming carbon dioxide and water into carbohydrates
which fuel growth in a process known as photosynthesis. Chlorophyll is also the
chemical which gives leaves their green color! But there are secondary pigments
known as carotenoids and flavonoids in leaves which are not visible through the
chlorophyll—until…
As the temperature begins to drop and the length of daylight
shortens, photosynthesis slows down and then stops altogether. This causes a
breakdown in the chlorophyll, and the green pigment fades away, revealing a
myriad of brilliant autumn colors! Some trees undergo a second chemical process
which creates pigments known as anthocyanins; these tend to be red and purple
in color. Anthocyanins are thought to be produced as a form of protection,
helping the tree to recover nutrients in the leaves before the next growing
season.

See larger version.

Weather conditions can also impact the brilliance of fall
foliage. A warm, wet spring, an average summer, or a fall with warm sunny days
and cool nights present the best conditions for a spectacular autumn. Severe
frosts will kill the leaves early causing them to turn brown and drop, and a
warm, wet period during the fall will lower the intensity of colors. Droughts
can delay foliage by weeks. So, as many things in life and nature, the
conditions must be “just right”!
Most trees shed their leaves to conserve energy for the
winter months. A layer of cells called the separation layer develops at the base
of the leaf stem which severs the connection to the tree. The tree closes off
the cut, and leaves will either fall of their own accord or be blown off by the
wind. This process contributes to the breakdown in chlorophyll as the flow of
nutrients is cut off.
Isn’t science awesome? Happy Fall!