<lk a walk outside, even a step, and you’ll be hit by it. The crunch. The dance of gold and red. The leaves. All of a sudden your street is alive with the glow of golden hour all throughout October. But, what makes the maples wake up one morning and fling their beloved leaves to the wind?
The bright colors and crisp leaves of fall are merely trees’ way of conserving energy over the winter. Trees, like most other plants, obtain energy from sunlight via photosynthesis, which occurs in leaves. Leaves provide trees with lots of energy during long summer days, but cannot provide trees with enough energy to make up for the cost of maintaining them during winter. Therefore, leaves are shed in the fall, as days get shorter.
Right before leaves are senesced, leaves change color because of a shift in leaf pigments. During photosynthesis, pigments absorb sunlight, which is then converted to usable energy. Each pigment absorbs a certain range of light wavelengths and reflects other wavelengths. The color we see in a leaf or any object is the wavelength of light it reflects. The pigments in plant cells change over the course of the season due to changes in resource availability. Throughout the spring and summer, leaves are green due to a pigment called chlorophyll. Early season leaves also contain xanthophylls and carotenoids, which reflect yellow and orange colors. However, the yellow and orange pigments are masked by the large amount of chlorophyll.
In early fall, cold nights signal seasonal change to trees. Trees respond by building up of tissues between the leaf and branch, which blocks the transport of chlorophyll and other nutrients from the roots to the leaves as well as the transport of sugar from the leaves to the roots and branches. Without a continuous influx of chlorophyll, the bright yellow and orange colors of xanthophylls and carotenoids are visible. Furthermore, anthocyanins, another type of pigment, are produced from the additional sugar that is trapped in the leaves. Anthocyanins are responsible for the red and purple colors also visible in fall foliage.
This weekend get out in the Arb to enjoy the glowing zanthophylls, carotenoids, and anthocyanins first hand in our local boxelders, sugar maples and red maples!