November 7, 2022 at 12:26 p.m.
How nature prepares for winter
Different species use different strategies
How nature prepares for winter
Annuals die each year, overwintering as seeds. Perennials, on the other hand, have a different strategy. They have branches and leaves that die back, but their root system continues to live on underground. Trees, on the other hand, must survive the winter above ground.
In the summer, Finton said, trees put out leaves to pull carbon dioxide from the air and convert it to sugar in a process called photosynthesis. Leaves, however, are vulnerable to freezing temperatures.
Trees lose water from the tiny holes on the bottom of their leaves and also take in carbon dioxide. When the ground is frozen, however, they can no longer take up additional water from the soil. If the leaves were kept on the tree throughout the winter, the tree would dry out and die.
Leaves also provide nutrients. These nutrients can be broken down and absorbed back into the trunk, and the roots as the leaves decompose after falling. It would be a waste, then, to keep those nutrients in the leaves only to see them freeze and fall off later, rather than helping to build the vitality of the tree. Trees intentionally and deliberately shut down for the winter, Finton said.
Trees also form buds in preparation for the next growing season. The buds are formed on the trees at this time of the year. They can even be seen as the leaves fall off the tree in the fall. Buds protect the leaves, twigs and flowers from the cold, letting them grow the next spring.
Each leaf is attached below each bud. This area, called a leaf scar, forms a layer that allows the leaf to separate from the tree and fall to the ground in preparation for the winter.
Leaf color change is contributed, primarily, by photoperiod, or day length. This means the change in color is not affected by climate change as other things may be. This triggers the breakdown of chlorophyll, which is responsible for photosynthesis. The green pigment is broken down, and the nutrients are reabsorbed into the tree. Beneath the green, Finton said, is a yellow pigment. When conditions are right, however, some species produce a red pigment, which brings about those vibrant colors.
Conifers, too, change color and drop their leaves every year, he said. They are still called "evergreen," however, because they only lose the leaves they produced three years ago. The leaves produced in the current and preceding year, are retained by the tree. These leaves have a narrow surface area and a waxy coating, unlike deciduous leaves, helping them live through the winter months on the tree.
Finton then turned to the ideal fall colors. These colors, he said, result from very specific conditions. Ample rain throughout the summer and into the fall, followed by bright, crisp days with temperatures in the 60s and nights dipping just to above freezing bring about those beautiful colors. This allows the leaves to produce more sugars, meaning more red pigments to bring about the red and orange colors of fall.
Climate change, he said, has an influence on timing and amount of color. In drought years, color changing can be accelerated when heavy rain events causing wet conditions can have the opposite effect.
The phenomenon of fall color, Finton said, can have personal, economic and ecological impacts. Economically, as those in the Northwoods know, fall colors bring tourists and sight seers here in Wisconsin. This has an economic impact on our area.
As far as ecological impacts, he said, colors are perhaps not completely indicative of forest health. However, he said he does take them as an indicator of how well the forests are thriving or struggling, or how resilient they are.
On a personal level, people are inspired by the colors of fall and are reminded of the benefits humans receive from trees, he said. Trees pull carbon from the atmosphere and store it for centuries. They filter drinking water. They provide habitat for thousands of animals species, and also provide us with a sense of health and well-being.
Finton said we lose approximately 5,000 acres of forests per year to development. The viability of our forests are also being challenged by things such as climate change, habitat loss, pests and invasive species. However, he said, humans can help forests because forests are resilient.
Reducing carbon emissions is one of those things under the control of people, he said. Keeping native forests intact, too, can help keep forests healthy and resilient over the years.
Rene Wendell of the Nature Conservancy was the second presenter looking at how nature prepares for winter. He did so from an animal perspective. Animals, he said can use several different strategies such as hibernation. The most well-known hibernator is the bear.
Diapause is another strategy. It means undergoing a period of suspended development. This is the case for some butterflies or moths, for instance.
Bromation is another strategy to survive winter, Wendell said. This is used by many reptiles who may get their energy from an outside source such as the sun. This is the process where the body's metabolism slows down to the point of using only a small amount of energy to keep the animal alive.
The wood frog, he said, freezes almost completely solid. The wood frog is an ectotherm.
"This animal is absolutely crazy," he said. "It's about as close to death as you can get. It's heart completely stops." The frog works to remove as much water as possible from its extremities, pulling it in to its body. It then floods its entire system with glucose, which acts as a type of antifreeze. As it warms up, the frog starts to thaw out and its heart starts beating again. Wendell recommended finding a YouTube video of the phenomenon.
Chickadees lower their body temperature at night as a survival method when it gets colder. When temperatures dip a great deal, they do into a state of controlled hypothermia. They can use up to 10% of their body fat overnight. The next day, they seek out food, seeds, to put that body weight back on.
Chickadees can store up to 1,000 seeds per day, Wendell said, as they get ready for the cold winter month. To remember where they put all of their seeds earlier in the year, the part of their brain responsible for memory actually grows to 30% larger in the winter than it was in the summer.
Wendell also spoke about painted turtles. They spend months under the ice in the water. Water temperature under the ice, he said, is relatively stable, unlike air temperature. They lower their metabolism by up to 95% and burrow into the mud.
"So how do they breathe under water, right?" He asked. "This is crazy, right? So, right now they're breathing air, right? But underwater they absorb oxygen directly through their skin." They can also take oxygen in through their mouth and through their hind end.
There is a good deal of dissolved oxygen in colder water, but because the turtle has slowed their metabolism so much, they are not burning fat.
"What they do is the convert energy and they break down glycogen, but there is a lot of lactic acid in their system, right?" In order to balance their body chemistry, then, he said, they actually borrow nutrients from their shell and their skeleton.
Lastly, Wendell said he could not have a hibernation discussion without bears. They are not true hibernators, however, they go into a torpor. This means they sleep for three or four days, then wake up. If its warm enough out, they may even go outside and look for food. He said they do not decrease their metabolism a great deal. Instead, they live off of the calories they have stored in fat.
The urine they make while in this state is recycled in their body, he said. The nitrogen made during this process is used to build protein. This means the bear can still come out in spring and be the muscular animal they need to be.
Chipmunks, he said, are similar to bears in that they have a torpor. Instead of storing fat in their bodies, though, they store their food underground and sleep on top of their food pile.
The full presentation from the Nature Conservancy was being recorded and will be showcased on the YouTube channel at some time in the future.
Beckie Gaskill may be reached via email at [email protected].
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