Leaf pigments are used by trees to capture photons from sunlight that drive the photosynthetic machinery that turns light energy into chemical energy. The key pigments used by trees are called chlorophyll A and chlorophyll B. While both pigments absorb photons from different wavelengths of light from throughout the spectrum, we see the light that they reflect as green in color. The pigments are high in nitrogen, phosphorus and other nutrients so at the end of the season they are reabsorbed by trees to use again next spring. As the chlorophyll is degraded by the tree for winter storage and future use, other pigments become dominant in the foliage. These pigments, primarily carotenoids (orange), xanthophylls (yellow), and anthocyanins (red), provide us with the brightly colored trees we associate with Autumn (Figure 1).
In Minnesota native red and pin oaks, red and sugar maples, and sumacs provide the best color shows in woodlands (Figure 2). Ash, birch, cottonwood, boxelder, silver maple, hackberry, elm, bur and white oaks may have some coloration but it is usually drab in comparison (Figure 3).
Some of the best autumn leaf color shows are actually in the Twin Cities. Many trees planted in the “urban forest” are cultivars of trees that have been selected and bred to exhibit fall color much more than their natural cousins. Autumn blaze maple is a cultivar that was produced by breeding silver and red maple to create a fast growing tree with bright fall colors (Figure 4). These trees have been widely planted throughout the Twin Cities. Other cultivars of maples, crabapple, pear, gingko and serviceberry have names that portend the fall color they can exhibit such as; Prairie Flame, Forest Rouge, Iroquois Beauty, Autumn Brilliance, Windover Gold, Fall Fiesta, and Prairie fire.
Foliage colors tend to be more dramatic in drought years, likely due to the production of secondary metabolites that provide trees with drought stress resistance. In years with plenty of moisture as we had in 2016, the color show tends not to be as dramatic.
When the pigments have been absorbed, deciduous trees will then drop the leaves and photosynthesis will stop for the season. The trees will continue to invest in root development and water and nutrient uptake up until the soil freezes. During this time the trees will also move photosythate that was collected this summer into storage. In oaks and maples, the photosynthate is stored in the root tissue as starch. In birches and many shrubs, the starch is held in the stem and trunk tissues.
Coniferous trees also go through the process of nutrient reabsorption from chlorophyll. In deciduous conifers like Tamarack it follows the same process as with deciduous broad leaf trees. Pine, spruce and fir trees that retain needles through the winter do shed needles (Figure 5). However, they don’t shed them all. The needles that are abscised are older needles from three to five years old that may be growing in shaded areas or from the trees’ perspective are not providing a photosynthetic benefit to the tree. Spruce, pine and fir that are under stress may lose more needles than healthy trees, due to the lack of energy they have to support the additional foliage.
Like the deciduous broad leaf trees, the roots of conifers are active far into the fall and early winter until the ground freezes. Unlike deciduous trees, the evergreen conifers must protect themselves from desiccation through the living needles that they are holding through the winter.
Dr. John Lloyd, Ph.D. of Plant Health Doctors, LLC contributes to the Ostvig Tree Care website blog. He is also available through Ostvig Tree Care to answer questions on tree care and plant health issues. Feel free to send any questions you may have about this blog or any other tree issues to firstname.lastname@example.org.