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TT(^34) September/October 2019
nins seem to be the key. Carotenoids
are always present in leaves in
roughly the same amounts, so their
effect on color shouldn’t vary much.
However anthocyanins, produced
mainly in the fall, do vary. Warm,
sunny days with cool but above-
freezing nights are best for anthocy-
anin pigments. The sunlight spurs the
production of sugars, but the gradual
collapse of the leaf veins keeps it in
the leaves. This extra sugar aids in
anthocyanin production.
Soil moisture directly affects
the overall well-being of the tree.
Drought conditions will slow all
processes and result in a diminution
of fall color. Some water-sensitive
species, such as yellow poplar, may
even drop their leaves early, with
little color to be seen. Excessive rain
in fall is also bad for color. It doesn’t
dilute the pigments, as is widely
believed, but rather is associated with
cloudy, sunless conditions that limit
sugar production. Sufficient rainfall
is essential for good fall color.
Temperature also plays a role in
color vividness. Cold springs will
delay the start of the growing season,
and may have a ripple effect, delay-
ing and lessening fall color. Exces-
sive heat in summer can take a toll
on sugar production. Abnormally
warm weather in fall will also throw
travel, the sooner fall
days become shorter.
Typically, temperatures
cool faster too. In north-
ern Alaska, fall colors
start in August. The
northern tier of Ameri-
can states can start to
see fall color in Septem-
ber (although warmer
coastal areas will be
later). The southernmost
areas where leaves still
change can see peak
color in November.
The importance of temperature is also shown by the effects of elevation. With
temperatures typically cooling as you go up, fall colors at higher elevations can ar-
rive weeks before foliage turns at lower levels. The general rule (officially Hopkins
Bioclimatic Law) is that for each 100-foot gain in elevation, fall colors will arrive
a day earlier than below. Higher elevations can show the contrast between colorful
deciduous trees and evergreen conifers. In addition to earlier color, mountain roads
also frequently offer sweeping vistas and less development. In fact, many roads
were designed with tree viewing in mind.
Since this whole leaf process is designed to occur before temperatures reach
32 degrees Fahrenheit, knowledge of when this typically occurs will give you a
general idea of when fall colors will peak. With freezing temperatures being critical
for agriculture, there are extensive climatological records on them. A database with
this information exists at The National Centers for Environmental Information.
Visit http://www.NCDC.NOAA.gov and search for “freeze/frost data” for details. This in-
cludes the average date of the first 32-degree temperature for thousands of stations
in the U.S.
But as we all know, some years fall colors are brilliant, and other years they’re
dull. This is where weather conditions come into play, most importantly tempera-
ture and soil moisture (which is a function of rainfall and evaporation). Anthocya-
Since this whole leaf
process is designed to
occur before temperatures
reach 32 degrees Fahrenheit,
knowledge of when this typically
occurs will give you a general idea
of when fall colors will peak.