leaves are like mythological characters:
they either live long and tranquil lives, when evergreen, or short and flamboyant ones, when deciduous.
evergreen leaves and needles,
are not always green as their name implies and they don't live forever; they live anywhere from 1 to 30 years depending on the species. when it is time for them to die they turn yellow and drop off. while some do this in the fall, like pines, others do it in the spring, like yews. during these times we receive many calls from concerned clients thinking that their trees are dying, but they are relieved when we tell them that this is quite normal.
deciduous leaves live for only one year and turn a variety of subtle and brilliant colors depending on the species. even though a certain tree or shrub has the reputation of having a particular fall coloring, each individual within that group can have a different expression due to genetic variables, environmental conditions or both.
let us now look at the mechanisms that cause the many varied colors. actually, leaves don't turn colors, they just reveal their true colors. during the growing season these colors are masked by the green pigment called chlorophyll. when chlorophyll starts to disappear in the fall because of shorter days and cooler temperatures, the previously hidden pigments of yellows, oranges and browns are revealed. the yellow pigment called xanthophyll along with chlorophyll are both present during leaf development, hence the light green color of spring leaves. this is the same pigment that gives color to yellow vegetables, fruits and flowers. the orange pigment called carotene arrives shortly after and is the same pigment that gives color to orange vegetables, fruit, flowers and salmon flesh. salmon that live in waters devoid of shrimp (which have carotene pigment in their shells) have white flesh. the brown color in leaves comes from tannins that are produced in the leaves as they mature. their bitter taste, is a deterrent to feeding insects and has other purposes.
as fall arrives, trees in the temperate zones prepare for the winter by battening down the hatches. this process is called leaf abcission... which involves the development of a cork-like layer of cells between the twig and the leaf stem. the water conducting tissues are the last to be cut off and this is one reason for the yearly differences in color displays. another reason for these differences is the amount of sunlight hours in conjunction with cool nights. this causes the living chlorophyll to continue manufacturing sugars with no place to go, changing the leaf sap to more acid solution. the results are chemicals called anthocyanins (greek for: antho- meaning flower and cyanin meaning blue), which are heavily pigmented with hues ranging from tangerine orange to deep blue-violet, depending on the acidity of the trees sap. when the sap is acid (sour) the bright reds are produced, when it is alkaline (sweet) the blue and purples result. trees growing in high acid soils traditionally have brighter colors. some tree leaves like the norway maple and willow keep their green color up to time they fall off. this is because their abscission layer permits the passage of some sap, and their chlorophyll continues manufacturing sugar almost until the leaves fall off. other trees, like some oak and beech, keep their leaves attached throughout winter. this is due to lack of water, individual expression, stress, disease or a combination of these.
then all of a sudden this crescendo of color ends with a puff of mother earth's cold clear breath... leaving behind an arboreal nudist colony to survive the winter.
a question for us to ponder is:
"because mother nature uses color to encourage reproduction in the higher life forms by either attracting a mate or a pollinator, why at this time of the year when plants and animals become dormant does she display this kaleidoscope of colors?" what are your thoughts?
robert p. d'ambrosio