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How Leaves Fall
The process begins at the point
where the leaf attaches to the stem of a tree or
shrub. A barrier begins to form at this point as the
first step in a process horticulturists call leaf
abscission. On the stem side of the barrier, the
tissues harden and fill with corky, wax-like
compounds. The flow of water and soil elements into
the leaf stops, but for several weeks, the flow out
of the leaf, of sugars and other compounds useful to
the plant, continues through the vascular bundle,
the leaf’s veins. Eventually the leaf remains
connected to the tree only by this bundle of veins.
With the action of frost and wind these soon break
and the leaf falls to the ground. But what falls is
an empty shell, all of it’s contents that can be
reused have been pulled out and stored elsewhere in
the plant. Where the leaf was attached to the stem
is now an impermeable leaf scar, which will be
incorporated in the stem’s bark. Often right next to
the leaf scar on the stem are the buds that will
produce now shoots and leaves next spring.
How Fall Colors Emerge
Fall colors emerge as the plant
begins to seal off the leaf and pulls the reusable
contents of the leaf back into the plant. One of the
first steps is that the plant stops producing the
green photosynthetic pigments. As the existing
chlorophyll in the leaf breaks down, it is not
replaced. The green color of the leaf begins to fade
and other, mostly yellow, pigments that have always
been in the leaf begin to show through. For plants
whose leaves turn yellow in the fall the story is
just that simple. For plants that produce darker
orange, reddish, or purple colors the story is more
complex. As the leaf shuts down, these plants are
producing a red pigment inside the leaf. This
happens as small amounts of sugars that are trapped
in the leaf react with a group of otherwise
colorless compounds in the leaf in the presence of
sunlight. All leaves contain the basic yellow
pigments, but if a lot of these red compounds are
produced in the leaf as the leaf shuts down, the red
color dominates the yellow, and the leaf appears
red. If the red and yellow pigments are equally
concentrated, the leaf appears orange. It is
interesting to note that sunlight must strike the
leaf in order to form these bright red pigments,
which explains why red leaves will tend to be on the
top and outside layers of foliage, and the foliage
will tend to be orange and yellow farther into a
tree where the leaves are shaded.
Weather Affects Fall Color
The production of these red
pigments is greatly affected by fall weather
conditions. Cool nights with sunny days in early
fall make for good red pigment production. Low, but
not freezing temperatures help slow the movement of
sugars out of the leave at night. Sunny days help
produce sugars initially, and then drive the process
that combines the sugars with the otherwise
colorless leaf compounds to produce the red
pigments.
Why Leaves Eventually Turn
Brown
Not long after the leaf falls,
the once vibrant colors fade to a dull tan or a
light brown. This occurs as the yellow and red
pigments decay and their colors fade. What remains
in the leaf are several compounds that are slower to
decay that act as brown pigments. One of the most
common brown pigments is a common plant compounds
called tannins. Tannins and other similar compounds
are among the slowest to decay, giving a brown color
to leaves long after they have fallen from the tree.
Plants whose leaves turn brown before they fall
often contain large concentrations of these
compounds. Bur Oak is on tree whose leaves turn
brown in the fall because of high concentrations of
tannin.
Look for Fall Color
Everywhere
Bright fall coloration is not
limited to trees and shrubs. The same process occurs
in many herbaceous plants too. The native bunch
grasses that have been selected for use as
ornamental grasses all have outstanding fall color.
Indian Grass (orange), Big Bluestem (maroon), Little
Bluestem (maroon to red), and Switch Grass (yellow
to orange to reddish) are among the most striking.
It is worth noting that the period of fall color
generally lasts much longer on these plants than on
trees and shrubs. The fall color of both Big and
Little Bluestem holds well into the winter.
Here’s hoping you’ve been
enjoying the fall colors. |
