How Plants Survive Winter
There were a couple of reasons that I got a doctorate degree in botany. Most importantly I could study plants in the nice part of the day and sleep in all morning. If you are interested in salamanders or birds, you have to get up at some ridiculous hour such as “early.” Secondly, plants don’t move. They kindly stay right where they are and let you uncover their secrets. But this same quality that let me be lazy means plants have to cope with winter by adaptations and do not simply go south with the warblers.
An understanding of plant winter adaptations can be best achieved by thinking about the plant’s yearly life cycle. Let’s imagine the sugar maple trees on a Vermont hill in August. One could not ask for a more deep and complete green. But by mid-October that same leaf is red or orange or yellow or all three. Like this:
The Green Mountains of Vermont (the state name actually means “green mountain”) turn the deep tans, yellows, and reds as plants undergo a change in advance of winter. What is the biology behind this change?
If we were to take some leaves back to the back to the lab and analyze them chemically we would find some interesting things. The green of these leaves is largely due to two kinds of chlorophyll molecules, call chlorophyll A and chlorophyll B. (All plants have these two type of chlorophyll, but red algae like nori for example have chlorophyll A and D. Brown algae, like the kelps, have A and C). All chlorophylls have one job, to capture the energy of sunlight. One characteristic of chlorophylls A and B is that they absorb red and blue light and reflect green light, so we see most leaves as green.
In addition to chlorophyll though, leaves have other pigments. For example there is a class of pigments called carotenoids. These pigments absorb blue light and mostly reflect the orange and reds to us. We see this color in many flowers such as the many flowers that make up a sunflower. (One sunflower head is actually made up of many flowers, the same can be said for a dandelion.) A second class of pigment is the anthocyanins. These chemicals absorb blue light and reflect red. When we see a Red Delicious apple or a bee balm flower, the red is because of the presence of anthocyanins.
Our sugar maples on that Vermont hill have chemical systems (the phytochromes) that monitor day length, and at some point in September, a sugar maple begins to mobilize the chlorophyll out of their leaves. This is a crucial adaptation, because the tree will not have to make all this chlorophyll all over again in the spring. The chlorophyll (often chemically altered) goes down to the roots and stays there all winter.
Chlorophyll vanishes from the leaves but it does so before the carotenoid and anthocyanins do. Leaves don’t “turn color,” colors that are there are “revealed.” The yellows, reds, and oranges of these secondary pigments are always there but we don’t see this until the chlorophylls are gone.
By the end of November even the secondary pigments have left the leaf. Our sugar maples also send a lot of water from their leaves and trunks back down into the ground too. There, protected from the most severe cold by the insulating ground, the water and chemicals wait. The dry, pigment-free leaf then falls from the tree. The remaining chemical reenter the ecosystem when they decompose.
As the world turns, the plant’s phytochrome system alerts the tree that spring ha arrived, and all the components of the pigments and all the water come back up the tree trunk.
What happens with the sugar maple and most other plants from temperate or tundra climes is that they cope with winter by moving their precious resources into the ground, much like some animals do via hibernation in winter or storing food in the ground. Then, everything is brought back up again in the spring.
Daniel H. Franck is director of science for K12. He developed the scope and sequence for all science courses for grades 3-12. Dr. Franck has also worked for Holt, Rinehart, Winston, Harcourt, Scholastic, Inc., and Discovery Channel, among others, in developing science textbooks as well as multimedia products for students from kindergarten through high school. He was part of a team of educational specialists that visited the nation of South Africa under the auspices of the U.S. Agency for International Development helping that nation's biology teachers restructure their national science curriculum. Dr. Franck has a Ph.D. in botany from the University of California, Berkeley and has been a professor of botany at the University of Wisconsin.