All summer long, we live in a sea of green. The forest, the bogs, the meadows are awash in greens — silvery, yellowish, dusky, bluish, greyish, even reddish — but all greens. I’m reminded of the story of the visiting symphony musician who took a walk in our forest, and came back thinking of all the green shades as a form of music — a symphony in green.
Technically, we see “green” because all those leafy surfaces reflect the green wavelengths of the sun’s light. Other wavelengths are used by the plants to power the process of photosynthesis, which links carbon dioxide with water to form carbohydrates, releasing oxygen as a by-product. All terrestrial and freshwater life and much of the life in the ocean depend directly or indirectly on plant carbohydrates for energy and on oxygen for respiration.
After a green summer, fall and winter come (and here I am thinking of a proper winter, with snow and frost). In addition to the effects of cold itself, cold weather often makes water less available, in some cases creating a degree of drought. Both cold and drought can create problems for plants. As fall proceeds, we can begin to see that plants in our forests have a variety of strategies for dealing with the cold and ice and the relative lack of available water: some endure the hard season, others have ways to avoid the difficulties of the season.
The spruces and hemlocks are “evergreens.” They hang on to their needles for years, replacing them gradually and in piecemeal fashion. They can carry out photosynthesis, at a low rate, even in winter. The sturdy, compact needles have relatively little surface area per volume, which conserves water, and they are covered in a waxy cuticle that helps protect against frostbite and water loss. Basically, they just tough it out through the cold season.
In contrast, the deciduous trees and shrubs (willow, cottonwood, alder, blueberry, devil’s club, etc.) drop their leaves, avoiding frost damage. Their leaves release lots of water in summer, so losing leaves is also a means of conserving water. There is a cost to this strategy, of course, because the plants have to produce new leaves in the following spring. If leaves are dropped below the plant, some nutrients may be recycled, but large amounts of stored carbohydrates and nitrogen are spent on new leaves. Nevertheless, deciduousness is a way to avoid many of the problems that winter can bring.
Within the forest, blankets and festoons of moss adorn trunks, branches, rocks and ground. These small plants have a different strategy. They are evergreen, but in dry or very cold times, they tend to shrivel up and wait for better times. When the rains come again, they take up water through the leaves and revive, almost magically, resuming life where they left off.
Among the herbaceous plants of the forest, we see that many of them turn brown and limp as fall progresses. Skunk cabbage leaves lie flat on the ground and start to decay quickly. Lady ferns are brown but still stand, droopingly, and most other ferns die back altogether. In short, they (like the deciduous trees) avoid most of winter’s effects and pay the cost of producing new leaves in spring.
In contrast to such species, there are several species in the understory that stay green all year long: Think of fern-leaf goldthread, the several species of wintergreen, and the so-called rattlesnake plantain (which is an orchid), and deer fern and licorice fern, for example. They all keep their green leaves, which become a bit more conspicuous in winter when other species have disappeared; even against a background of green mosses, their leaf shaped are distinctive. I have to wonder about the wintering strategies of these herbaceous species. The two ferns appear to have tough fronds that might work like the conifer needles, but how are the other herbaceous evergreen species protected from frost and desiccation?
In summary, some plants endure winter’s effects, some largely avoid them, either temporarily (mosses) or for the whole season (all the deciduous species). Although botanists can suggest ways in which the spruces and hemlocks manage to endure, I have found no information on how the evergreen herbs survive and thrive over a winter.
More unanswered questions! Nevertheless, I suggest that it is worth asking the questions, because ultimately they help us perceive and eventually understand the complexity of our forests.
• Mary F. Willson is a retired professor of ecology.
