Nature's Brew Brings Out Fall's Beautiful Palette

Tag : pigments

There are three major factors that alert trees that the season ischanging: length of night, pigments in the leaves and weather. Ofthose three, the amount of light (or photoperiod) is the keyfactor. Other aspects, such as species and genetic variation, alsoplay a role.
All those factors together make for spectacular color changes. Butsome of those yellows, reds and oranges actually are there all thetime -- we just can't see them.
Trees are far more active than you might realize. They workconstantly during the growing season to produce carbohydrates fromlight, water and carbon dioxide. Light comes from the sun, waterfrom the earth and carbon dioxide from the air.
In the process, called photosynthesis, trees release oxygen andwater into the atmosphere. All parts of the tree are dynamicallycommunicating with one another. They are constantly interpretingand reacting to changing conditions in the atmosphere and in thesoil. They know how much water they require and how much they'regetting, and adjust their fluids and nutrients to the weather. Thatincludes knowing how much light they're getting and makingnecessary adjustments as it increases or diminishes.
In the spring and summer, leaves are green because they are full ofchlorophyll, used in photosynthesis. The diminishing of sunlighttriggers a number of changes in the way trees function. One ofthese is the development of a corky membrane between the branch andthe leaf stem, which reduces the amount of nutrients getting intothe leaves. Reduced nutrients mean reduced production ofchlorophyll. That's why the green color disappears.
As chlorophyll production slows in the fall, other pigments in theleaves begin to show. Among them are carotenoids, which areattached to the cell membranes and give leaves yellow, russet orbrown colors.
Reds, purples and bright oranges come from anthocyanins, which sometrees produce to help extract all the nutrients from dying leaves.Anthocyanins are produced only in the fall and in response to lightand excess sugar in the leaf cells. These pigments are sensitive tothe pH levels of the cell sap. Acidic sap produces reds; alkalinesap produces shades that are more purple.
The impact of weather on leaf color is related to temperature andrainfall. If the days are dry and bright and the nights dry andcool, that encourages the production of anthocyanins, making thedisplay more intense.
The mix of pigments in each type of tree governs the colors. Sugarmaples become a brilliant yellowish red. Oaks turn red, russet orbrown. Dogwoods turn deep red. Hickories are a brilliant yellow;aspens are famously golden, as are birch and yellow poplar. Sweetgums can be deep red or a mixture of yellow and deep red. Chestnutsdisplay a wide range of colors from yellow to red to purple,sometimes on the same leaf.
Deciduous trees don't hold onto their leaves all year because theywould lose more water through them than they would gain from theground. Rainfall might be scarce or come in the form of snow, andthe ground might be frozen. The corky membrane shuts off each leaf,and they all fall to the ground.
The spring and early summer this year were most generous with rain,but August and September were dry. This must bode well for amagnificent display because this season has been beautiful.