How to predict fall color

As I drive around the county, I’m seeing a patchwork of reds and yellows dotted among the green. While various parts of the country are known for their vibrant displays of fall colors, the areas surrounding the Appalachian Mountains are lauded as one of the most beautiful. People are drawn from all over the world to witness the stunning change of scenery. Thus, predicting color intensity and timing is also economically interesting, as fall colors are a huge tourist draw, worth millions of dollars in revenues every year.

So, what is the cause for the color change? Ultimately, the reason leaves change color in autumn is based on plant biology and phenology – a scientific term that describes changes in a plant or animal’s life cycle based on environmental cues. When it comes down to it folks, we’re looking at deciduous leaf senescence, plant genetics (the trees that are present) and the weather.

Few deciduous trees and lots of conifers will yield a primarily green display in autumn. Thus, in the northeast the abundance of pines and hemlocks somewhat reduce the color intensity, although the presence of some conifers provides a nice contrast to the bright colors.

European forests lack many species that produce “good color,” especially the reds, which again comes down to plant genetics. Just because a tree is deciduous does not mean that it will yield red fall color. For example, hickories and tulip poplars are celebrated for their reliable orange and gold fall colors, while our black gums, dogwoods, sourwoods, and some oaks are known for their striking hues of red.

So then, how does this change happen? It just so happens that certain leaf cells have a special feature: pigment-containing chloroplasts that enable them to produce energy and their own food through photosynthesis. These chloroplasts contain different pigments, which are what gives a leaf its color.

Chlorophyll (green) is the most common type of pigment, but there are also xanthophylls (yellow), cartenoids (yellow, orange), and anthocyanins (red). The chlorophylls usually hide the other pigments, except when autumn comes along and chlorophyll begins to break down. This phenomenon is the hallmark for leaf senescence, a process of closing down, reallocating resources, and sealing off a leaf.

Why sugars are trapped in fall leaves, and why chlorophyll disappears in them, is due to something called the “abscission layer.” This layer of cork-like cells develops between branches and leaf stems in response to longer nights and cooler temperatures, and seals off the flow of nutrients between leaf and stems, causing chlorophyll to not be replaced.  Thus, the green pigment seen all summer disappears during early to mid-fall, leaving the yellow and orange pigments to be seen, and red pigments to be produced.
   
With time, even these colored pigments break down, leaving brown ones called “tannins.” Again, the specific colors produced, in what amounts, and when in relation to other plants, is due to the programmed genetics of a particular plant.

But, it’s not all up to the plant. Weather also plays a hand in the timing and intensity of each season’s color change. A prolonged lack of rainfall over the summer months may cause trees to lose their leaves or start color production prematurely. The end result is a reduction of color during the peak of the season. Adequate summer rains promote good tree health, leaf retention and, therefore, color production during autumn.

But, we can’t blame everything on a dry summer. The right weather during autumn also has a role in promoting a color change in deciduous trees. The red pigments (anthocyanins) require sunlight for production, and are enhanced by cold and sunny days. Rainy and windy weather during early fall can knock leaves down prematurely and thereby shorten the color display at its peak.
      
Cooler temperatures in late summer and plenty of sunshine, often leads to brighter colors sooner.  This is particularly true if the sealing layer on leaf stems has started forming.  Cool is good, but too cold (as freezing or below) can be bad, killing leaves early.  So, ideal conditions for fall color would involve a moist growing season early, dry late summer and early fall, with sunny warm days and cool nights during the latter.

Simple enough, right?

So, now that you’ve had a crash course in autumn leaf color development, while you’re touring through areas with brilliant fall colors, I hope you enjoy them and now have a better appreciation of what all goes into making this this yearly art show happen.

Have questions about what trees you can plant that will showcase brilliant fall colors? Give us a call at the Extension office!